Infrared-Spectroscopy

Infrared (IR) investigations can be made on a sample of reactant previously heated to a known extent of reaction (a) and studied in the form of a mull or in an alkali halide disc. An alternative approach is to incorporate the reactant substance in a compact alkali halide disc [287] which is intermittently withdrawn from the reaction vessel for infrared measurements at appropriate intervals. Heated sample holders [288,289] permit repetitive scanning of the spectrum or continuous monitoring of a peak of interest during decomposition. 

Hisatsune and co-workers [290—299] have made extensive kinetic studies of the decomposition of various ions in alkali halide discs. Widths and frequencies of IR absorption bands are an indication of the extent to which a reactant ion forms a solid solution with the matrix halide. Sodium acetate was much less soluble in KBr than in KI but the activation energy for acetate breakdown in the latter matrix was the larger [297]. Shifts in frequency, indicating changes in symmetry, have been reported for oxalate [294] and formate [300] ions dispersed in KBr. 

The thermal decomposition reactions of KN3, T1N3, and AgN3 have been studied in the corresponding halide matrices [301]. The formation of NCCT from trapped C02 was described and labelling with ISN established that only a single end-N atom of the azide ion was involved in NCO formation. The photodecomposition of PbN6 and the effects of dopants have been followed [302] by the changes produced in the near and the far infrared. 

Infrared radiation causes excitation of the quantized molecular vibration states. Atoms in a diatomic molecule, e.g. H—H and H—Cl, vibrate in only one way they move, as though attached by a coiled 

Molecules with more than two atoms have, in addition, continuously changing bond angles. These bending modes are indicated in Fig. 12-1. 

Between 42 and 24 THz (1400 and 800cm ) there are many peaks which are difficult to interpret. However, this range, called the fingerprint region, is useful for determining whether compounds are identical. (It is virtually impossible for two different organic compounds to have the same ir spectrum, because of the large number of peaks in the spectrum.) 

Problem 12.12 How do the following factors affect absorption frequencies Use data in Tables 12-1 and 12-2. (a) For C—H stretch, the hybrid orbitals used by C. b) Bond strength i.e., change in bond multiplicity, (c) Change in mass of one of the bonded atoms e.g., O—H versus O—D. (d) Stretching versus bending, (e) H-bonding of OH.  

1450-1600 (s) 0=C bond in aromatic ring (usually shows several peaks) 

Infrared radiation causes excitation of the quantized molecular vibration states. Atoms in a diatomic molecule, e.g. H—H and H—Cl, vibrate in only one way they move, as though attached by a coiled spring, toward and away from each other. This mode is called bond stretching. Triatomic molecules, such as CO2 (0=C=0), possess two different stretching modes. In the symmetrical stretch, each O moves away from the C at the same time. In the antisymmetrical stretch, one O moves toward the C while the other O moves away. 

IR spectroscopy is another vibrational spectroscopic method used to determine the conformational structure of proteins and polypeptides. NIR spectroscopy provides little interpretable protein structural information because the broad bandwidth produces a severe overlap of most of the bands in the NIR spectra. As a consequence, the use of NIR is limited to basic structure determination in food components, and it has not been used for in-depth study of protein structural changes in muscle foods. On the other hand, mid-IR spectroscopy is a powerful tool for investigating the conformation of the polypeptide backbone of proteins. For these reasons, this chapter focuses on mid-IR spectroscopy. In mid-IR spectroscopy, the frequencies of amide I and amide II bands are very sensitive to protein conformation in food [18, 33, 34] and therefore to protein structural changes produced by thermal treatment [22]. 

The infrared spectra of all complexes showed a shift of the asymmetric and symmetric acetate stretch to higher frequencies (respective to free acetate 

when infrared radiation of a particular frequency is passed through a sample containing molecular species, it may or may not be absorbed. If all frequencies are passed through, some will be absorbed to varying degrees, depending on the molecular species involved. For example, a typical spectrum of transmittance (%) versus wave number (cm 1) (wave number = 1/wavelength) 

There are two types of spectrometers that one can use to generate such spectra.12 One uses a monochromator to evaluate each frequency in turn. The second uses a Michelson interferometer to examine all frequencies simultaneously, and then a Fourier transform to display the spectrum. The advantage of the latter approach is its greater sensitivity, and the speed with which it can produce a spectrum. 

Regardless of how it is obtained, the spectrum can be used to make quantitative estimates of the concentration of molecular species in thin films. Using the Beer-Lambert Law,12 we can write simply 

A = absorbance = log10 lQ/l lQ = incident radiation I = Transmitted radiation E = extinction coefficient L = path length C = concentration 

The extinction coefficient is a constant for one substance and one frequency. Then a measurement of A gives a resulting value for C. 

The IR-spectroscopy yields detailed information on the surface covering of the material under test. Therefore, it constitutes a method complementary to those analytical techniques chiefly characterizing the bulk properties. 

Infrared spectroscopy resembles Raman spectroscopy in that it provides information on the vibrational and rotational energy levels of a species, but it differs from the latter technique in that it is based on studying the light transmitted through a medium after absorption and not that scattered hy it (see Section 2.11.2). 

The techniques of Raman and IR spectroscopy are generally considered complementary in the gas and solid phases because some of the species under study may reveal themselves in only one of the techniques. Nevertheless, it must be stressed that Raman scattering is not affected by an aqueous medium, whereas the strong absorption in the infrared shown by water proves to be a troublesome interfering factor in the study of aqueous solutions by the IR method. 

Infrared spectroscopy is of course a standard technique for the characterization of compounds in the current context of solid materials. Because it is based on the measurement of the vibrational modes generally of bonded atoms, with absorptions usually in the range of400 000 cm it is primarily a tool for investigating molecular properties rather than solid state properties. 

A number of experimental alternatives to traditional IR transmission spectroscopy are suitable for overcoming some of these complicating experimental factors. In the technique of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (Hartauer et al. 1992 Neville et al. 1992) the sample is dispersed in a matrix of powdered alkali halide, a procedure which is less likely to lead to polymorphic transformations or loss of solvent than the more aggressive grinding necessary for mull preparation or pressure required to make a pellet (Roston et al. 1993). For these reasons, Threlfall (1995) suggests that DRIFTS should be the method of choice for the initial IR examination of polymorphs. He has also discussed the possible use of attenuated total reflection (ATR) methods in the examination of polymorphs and provided a comparison and discussion of the results obtained on sulphathiazole polymorphs from spectra run on KBr disks, Nujol mulls and ATR. 

In spite of many of the potential experimental pitfalls and difficulties (which should be viewed here as caveats rather than as deterrents), IR spectroscopy is still one of the simplest and most widely and routinely employed analytical tools in the study and characterization of polymorphs. Some other modifications, developments and hyphenated techniques are worthy of note here, since they often considerably enhance the potential of the technique while reducing the drawbacks. Perhaps the most obvious of these is the combination of microscopy with FTIR spectroscopy for visual examination and spectral characterization of small areas in heterogeneous samples or identity and analysis of the spatial distribution of components of mixtures (e.g. pharmaceutical formulations) (Messerschmidt and Harthcock 1988). 

Just as polarized light enhances the utility of optical microscopy in the study of polymorphic systems, so polarization can be used in conjunction with IR spectroscopy. As we shall show later in greater detail (see Section 6.3.2) polarized spectroscopic methods provide detailed information on the directional properties which distinguish the spectral features of polymorphs. Thus, for instance, the directional properties of a polymorphic transformation of fatty acids (Kaneko et al. 1994a-c) and inferences about the differences in packing modes (Yano 1993) have been investigated with polarized IR methods. 

As with other analytical techniques previously widely used for polymorph characterization (i.e. polymorph identity), IR spectroscopy is being increasingly employed as a technique for quantitative analysis (e.g. polymorphic purity) (Aldridge et al. 1996 Blanco et al. 2000 Bugay 2001 Stephenson et al. 2001 Patel et al. 2001). 

Infrared spectroscopy is a technique based on the vibrations of atoms of a molecule. In order to get a spectrum, the sample is placed in a sample holder and then an infrared ray is passed through the sample. The signals or peaks that can be appreciated in an IR spectmm correspond to the energy absorbed by the sample at specific frequencies that depend on the molecule s structure. To detect a signal, molecules must change their electric dipole during irradiation, which implies the generation of specific movements between atoms and chemical bonds [4]. 

Even for simple molecules, there will be many vibrational signals. A simple molecule can generate a complex spectrum. In a polymer, the repeating unit represents the simple molecule that will be generating a pool of signals or bands. Bands of vibrations associated with the presence of characteristic functional groups are called skeletal vibrations, and these skeletal vibrations are likely to constitute a pattern or fingerprint of the molecule as a whole. 

Out-of-plane bending or twisting In-plane bending or rocking 

Most organic molecules (including polymers) show absorption bands from the interaction between the IR radiation and the atoms in a chemical bond in the mid-IR region. Most IR studies are related to the analysis of vibrations in the mid-IR region, but near- and far-IR regions also provide important information about certain materials. 

The mid-IR spectrum (4000-400 cm ) can be divided into four main regions, and the nature of a group frequency may generally be determined by the region in which it is located. In Table 16.2, the fundamental vibrations of some common chemical bonds in the mid region are presented and the wavelength and region for each vibration are included. 

Infrared spectroscopy is a useful tool for molecular structural studies, identification, and quantitative analyses of materials. The advantage of this technique lies in its wide applicability to various problems in both the condensed phase and gaseous state. As described in the later chapters of this book, infrared spectroscopy is used in chemical, environmental, life, materials, pharmaceutical, and surface sciences, as well as in many technological applications. The purpose of this book is to provide readers with a practical guide to the experimental aspects of this versatile method. 

In this chapter, introductory explanations are given on an infrared absorption spectrum and related basic subjects, which readers should understand before reading the later chapters, on the assumption that the readers have no preliminary knowledge of infrared spectroscopy. 

Introduction to Experimental Infrared Spectroscopy Fundamentals and Practical Methods, First Edition. Edited by Mitsuo Tasumi and Akira Sakamoto. 

It is usually possible to observe an infrared absorption spectrum from any material except metals, regardless of whether the sample is in the gaseous, liquid, or solid state. This advantage makes infrared spectroscopy a most useful tool, utilized for many purposes in various fields. 

Measurements of infrared spectra are mostly done for liquid and solid samples. In the visible absorption speetra of liquids and solids, only one or two broad bands are typically observed but infrared absorption spectra show at least several, often many relatively sharp absorption bands. Most organic compounds have a significant number of infrared absorption bands. This difference between the visible and infrared absorption spectra is due to the different origins for the two kinds of spectra. Visible absorption is associated with the states of electrons in a molecule. By contrast, infrared absorptions arise from the vibrational states of atoms in a molecule. In other words, the visible absorption spectmm is an electronic spectrum and the infrared spectrum is a vibrational spectmm. Vibrational motions of atoms in a molecule are called molecular vibrations. 

Infrared spectroscopy is useful for determining the presence and identity of functional groups. These spectra measure the frequency of bending and stretching of bonds where the bond dipole changes with the movement. The stretching vibrations of double and triple bonds in alkenes and alkynes 

Color absorbed Violet Blue Green Yellow Orange Red 

Color observed Yellow Orange Red Violet Blue Green 

UV/visible spectroscopy involves absorption of electromagnetic radiation exciting electronic transitions between bonding (or nonbonding) and antibonding molecular orbitals. The most important transitions are n to % and % to i  

Absorption is governed by the Beer-Lambert law, which relates absorption to concentration, path length, and the extinction coefficient e, which is a property of the molecule. 

Xmax- the wavelength for the maximum absorption is measured in nanometers, and both it and e increase with conjugation in molecules. 

1 Infrared Spectroscopy. Infrared (IR) spectroscopy is probably the one instrumental technique that has been applied most often to the study of catalysts in general and to acidity in particular. IR spectroscopy can be used to detect Brpnsted acidity in zeolites directly by measurement of the OH stretching bands and to ascertain structural information based on the abundance of various OH bands and in the metal framework region. 

Additionally, it can be used to provide information about the presence of Lewis sites through the use of probe adsorbents such as pyridine. In general, IR studies can be categorized into studies of lattice structure/hydroxyl groups, and studies of the interaction of adsorbed molecules. 

Perhaps more instructive than studies dealing solely with the OH region of the IR spectrum are those studies in which a probe molecule is used either to quantify the Br0nsted sites, or to qualify the type of site as to Brpnsted or Lewis. To this end numerous studies have been conducted. Pyridine is typically the adsorbent of choice as it is well known to exhibit characteristic IR bands at approximately 1450 and 1540 cm corresponding to Lewis and Brpnsted sites, respectively. Additionally, one can measure the disappearance of the acidic OH stretch bands with pyridine addition to identify the bands corresponding to acidic hydroxyl groups. 

Studies using IR of adsorbed pyridine are numerous and, like most other zeolite studies, have concentrated on H-ZSM-5 and zeolites. The majority of 

Other adsorbents have been used in an effort to measure the acid strength of the sites or eliminate diffusion limitations. Kubelkova et al. used low temperature adsorption of CO on H-ZSM-5, H-Y, NaH-Y, and various AlPO sieves to measure the shift in the acidic OH stretching frequency upon CO adsorption. The authors argue that this shift is related to the proton affinity of the zeolites and thus to the Brpnsted acid strength. Tvaruzkova et al. used d3-acetonitrile to characterize both the Brpnsted and Lewis acidity of a number of zeolites. Using the band intensities and the frequency of the C-N band they obtained relative concentrations and strengths of the various acid sites. 

Infrared (IR) spectroscopy is one of the most versatile and cost-effective instrumental analytical methods available to the analytical chemist. The traditional attractions of infrared are well known  

In this chapter we will first introduce the basics of the subject—in terms 

Infrared (IR) spectroscopy is an analytical method to determine the presence of functional groups and unveil the bonding 

The vibrational spectroscopy family is the most versatile in forensic chemistry. Unlike UV/VIS techniques, IR spectroscopy can provide unambiguous identification of isolated compounds, albeit after meticulous sample preparation. IR techniques are valued for drug analysis, fiber characterization, and other applications in which such identifications are essential. Applications of IR techniques 

The infrared method most students are familiar with is absorption spectroscopy spanning the mid-IR range. The infrared region starts just above the visible one ( 770 nm) and extends to approximately 3000 nm, which, by convention, is reported as a frequency in units of reciprocal centimeters (cm ) rather than in nanometers. The total IK range runs from about 13,000 cm to 4000 cm and czm be divided into the near IR, from 780 nm to 2500 nm (12,82(MOOO cm ) flie mid-IR (4000 to 400 cm ) and the far IR (400-10 cm ). Ail three regions can be exploited in forensic work, but midrange techniques dominate. 

Several recent advances in instrument design have enhanced the applicability of mid-IR techniques. The first and most important was the replacement of dispersive IR with Fourier transform spectrometers (FT lk) (see figure 5.28). Dispersive instruments utilized an infrared energy source and a monochromator such as a grating 

Advances in FTIR and in computer-processing power have fundamentally changed infrared spectroscopy and its forensic applications. Infrared microscopy and microspectrophotometiy, aUbut impossible with dispersive instruments, are becoming common in forensic laboratories. Finally, simplified sample preparation techniques allow for quicker analyses, always an advantage in busy forensic laboratories. The rest of this section smnmarizes the prevalent modes of FTIR. 

Diffuse Reflectance-. Diffuse reflectance, or diffuse reflectance infrared Fourier transform spectrometry (DRIFTS), exploits surface interactions and reflection. Although samples still are mixed with KBr, there is no need to press the mixture into a clear pellet. The technique is also more sensitive than traditional dispersive IR and requires less material than a pellet. The sample is mixed with KBr to create a dilute solid solution of reasonably uniform small crystal size. The mixture is placed in a small cup, and the IR beam is directed onto the surface, where 

X - my Absorption Spectroscopy and Large Angle X-ray Scattering of Grignard Coni ounds 

In order to apply infrared (IR) spectroscopic methods and techniques to certain applications (e.g. for the investigation of surface processes or some material characteristics in CMP), some general explanations and remarks have to be given in advance. 

IR spectroscopy is an analytic method based on the absorption of IR radiation by vibrational excitation of lattices, surface groups, molecules, etc. in each physical condition. The absorptions are always associated with a change in the dipole moment of the molecule/material. Consequently, vibrational and/or rotational modes of molecules. 

IR spectrometers generally consist of four main functional elements  

Novel developments using micro-opto-electro-mechanical systems as moving mirrors in the interferometers (Kenda et al., 2011) bridge the gap between classical FTIR setups and microsystems. They target the miniaturization of the spectrometers. 

With respect to the practical part of this chapter a short excursion into the theory and application of FTIR sampling techniques is recommended. More details concerning the sampling techniques are found in special literature (Chalmers and Griffiths, 2002 Giinzler and Heise, 2003 Karge and Geidel, 2(X)4 Zaera, 2012). 

Traditionally, IR spectroscopy has been one of the most popular physical methods in the polymer-characterization laboratory since it is useful in the elucidation of structures and the identification of organic and inorganic systems ahke. The quantitative analysis of samples down to picogram quantifies is straightforward for systems for which the spectra of the pure compounds are available. Yet, the most attractive advantage of the method is the potential for a rapid multicomponent analysis to be carried out from a single measurement (spectrum), once the methodology has been calibrated. 

IR spectrometers are compact, rugged, and relatively inexpensive. The user does not have to be a highly trained individual in order to operate the instruments or interpret the spectra. IR-specfroscopic analysis can be carried out on gases, liquids. 

FT IR has emerged as one of the most important analyfic tools for noncontact and nondestructive evaluation/analysis of polymers, irrespective of their type, nature. 

Raman spectroscopy can be used in the same fashion as IR for identification using catalogs of Raman spectra in a database. 

One of the most useful applications of FITR is the determination of the conformation of polymer chains. This is a result of the high sensitivity of the IR spectra to rotations around chemical groups. This is illustrated in Fig. 6.2, which shows the IR spectrum of the trans and gauche structures for the ethylene glycol portion of the polyethylene terephthalate (PET) chain. From these spectra, it is possible to determine the conformational composition for PET as a function of crystallization, annealing, and processing conditions. 

Several forms of infrared spectroscopy are in use, as illustrated in Fig. 8.4. The most common form of the technique is transmission infrared spectroscopy. In this case the sample consists typically of 10-100 mg of catalyst, pressed into a self-supporting disk of approximately 1 cm2 and a few tenths of a millimeter in thickness. Transmission IR can be applied if the bulk of the catalyst absorbs weakly. This is usually the 

A great advantage of infrared spectroscopy is that the technique can be used to study catalysts in situ. Several cells for in situ investigations have been described in the literature [4, 5]. The critical point is the construction of infrared-transparent windows that withstand high temperatures and pressures. 

In the diffuse reflectance mode, samples can be measured as loose powders, with the advantages that not only is the tedious preparation of wafers unnecessary but also diffusion limitations associated with tightly pressed samples are avoided. Diffuse reflectance is also the indicated technique for strongly scattering or absorbing particles. The often-used acronyms DRIFT or DRIFTS stand for diffuse reflectance infrared Fourier transform spectroscopy. The diffusely scattered radiation is collected by an ellipsoidal mirror and focussed on the detector. The infrared absorption spectrum is described the Kubelka-Munk function  

K is the absorption coefficient, a function of the frequency v S is the scattering coefficient 

If the scattering coefficient does not depend on the infrared frequency, the Kubelka-Munk function transforms the measured spectrum RJ V) into the absorption spectrum K v). In situ cells for DRIFT studies of catalysts have been described [10] and are commercially available. 

CHAPTER 11 Alkenes Infrared Spectroscopy and Mass Spectrometry 

The remaining sections in this chapter deal with two additional methods for the determination of the structures of organic compounds infrared (IR) spectroscopy and mass spectrometry (MS). IR spectroscopy is a very useful tool because it is capable of detecting the characteristic bonds of many functional groups through their absorption of infrared light. IR spectroscopy measures the vibrational excitation of atoms around the bonds that connect them. The positions of the absorption lines associated with this excitation depend on the types of functional groups present, and the IR spectrum as a whole displays a pattern unique for each individual substance. 

Absorption of infrared light causes molecular vibrations 

Stronger bonds give higher-frequency (v) IR absorptions  

More polar bonds give more intense (7) IR absorptions  

In contrast to Raman scattering, the absorption of infrared (IR) radiation is a first-order process, and in principle a surface or an interface can generate a sufficiently strong signal to yield good IR spectra [6]. However, most solvents, in particular water, absorb strongly in the infrared. There is no special surface enhancement effect, and the signal from the interface must be separated from that of the bulk of the solution. 

To minimize absorption from the solution, optical thin layer cells have been designed. The working electrode has the shape of a disc, and is mounted closely behind an IR-transparent window. For experiments in aqueous solutions the intervening layer is about 0.2 to 2 ftm thick. Since the solution layer in front of the working electrode is thin, its resistance is high this increases the time required for double-layer charging - time constants of the order of a few milliseconds or longer are common - and may create problems with a nonuniform potential distribution. 

Two different techniques have been devised to separate the interfacial and the bulk signal. In the first one IR spectra are recorded at two different electrode potentials, and subsequently subtracted. In this way the signal from the bulk is eliminated because it is independent of potential. The recorded spectra are usually presented in the form  

The other technique utilizes the different surface sensitivity for s-and p-polarized light - the former has its polarization vector perpendicular, the latter parallel to the plane of incidence. Due to the different 

Electrochemical infrared spectroscopy can be used on all kinds of electrodes and for all substances that are IR active. It is particularly useful for the identification of reaction intermediates, and has been used extensively for the elucidation of the mechanisms of technologically important reactions. A case in point is the oxidation of methanol on platinum, where linearly bonded = C = O (i.e., CO bonded to one Pt atom) has been identified as an intermediate Figs. 15.7 and 15.8 show EMIRS [6c] and IRRAS [8] spectra of this species. Near 2070 cm-1 the EMIRS spectrum shows the typical form produced by a peak that shifts with potential. This shift can be followed in the IRRAS spectrum 

The most common features of pyrrolizine derivatives in the infrared are the carbonyl bands of 3.ff-pyrrolizin-3-ones at approximately 1740 cmand iH-pyrrolizin-l-ones at 1690 cm . 

An exhaustive assignment of the fundamental vibrations of pyrrole is found in Table 39. A number of computational papers have also appeared on predictions and reassignments of these vibrations 88JPC1739,93KGS65i . However, many of these bands are indistinguishable in the conventional infrared spectrum. Pyrrole shows five major bands in the infrared at approximately 3600, 

and 1025 cm , and a broad band centered at 730 cm . Additionally, several small bands are visible in the range 1400-1600 cm . iV-Substitution usually reduces the intensity of the 3600 cm band, while heightening the bands between 900 and 1500 cm . The band at 730 cm is absent in tetrasubstituted pyrroles. 

Pyrroles and their Benzo Derivatives Structure Table 37 IR data (cm ) for 2//-pyrroles. 

Infrared data for several isoindole derivatives is included in Table 41. Probable assignments are as follows, the band near 800 cm should be assigned to the out-of-plane vibrations of the C—H 

Before the advent of NMR spectroscopy, infrared (IR) spectroscopy was the instrumental method most often applied to determine the stmcture of organic compounds. Although NMR spectroscopy, in general, tells us more about the structure of an unknown compound, IR still retains an important place in the chemist s inventory of spectroscopic methods because of its usefulness in identifying the presence of certsm functional groups within a molecule. 

FIGURE 13.25 Stretching and bending vibrations of a methylene unit. 

Like NMR spectrometers, some IR spectrometers operate in a continuous-sweep mode, whereas others employ pulse Fourier-transform (FT-IR) technology. All the IR spectra in this text were obtained on an FT-IR instrument. 

Infrared Absorption Frequencies of Some Common Structural Units 

All of the calculated vibrational frequencies given on Learning By Modeling are too high. For example, the C=C stretching frequency of 1-hexene observed at 1640 cm is calculated to be at 1857 cm 

For certain samples, in particular fossil fuels, highly overlapped infrared spectra are obtained. The overlap can be reduced by special methods that artificially reduce the 

Infrared bands show a bandwidth intensity according to a Lorentz curve 

Yi refers to the spectral width, v° refers to the frequency at maximum absorption. The function in Eq. (20.16) can be regarded as a convolution of 

The argument of the function G is a length because the wave number has the dimension cm If we divide the function G(x) by the exponential term at the right-hand side of Eq. (20.17), then the contribution of the bandwidth will be filtered off. The approximate bandwidth should be known initially. The back transformation of a so-modified function should be a 5-function, or a series of 5-functions, in the 

In gas chromatography coupled with mass spectrometry a procedure has been given, how to separate the inadequate resolved peaks [7,8]. It is remarkable that the mass spectra of the individual peaks that are overlapping need not to be known. Such a procedure is only possible, because in a mass spectrum of an individual compound there is so much information inside. The mass spectrum in a gas chromatographic peak is, for each mass number, the sum of the concentration of each individual compound, weighted by the relative intensity of the respective mass number and the relative amount of the respective compound. In the procedure, a normalized mass spectrum is constructed as a vector with the relative intensities of the mass numbers. Here it is necessary that the number of the mass numbers constituting a compound is equal for each compound, i.e., all the vectors should have the same dimension. Because of the formalism of the matrix multiplication, the overlapped spectrum is the product of the matrix of the spectra and the concentration vector. The overlapped spectrum is also addressed as matrix of the observable intensities 

The most common application of infrared spectroscopy in catalysis is to identify adsorbed species and to study the way in which these species are chemisorbed on the surface of the catalyst. Sometimes the infrared spectra of adsorbed probe molecules such as CO and NO give valuable information on the adsorption site that are present on a catalyst. We will first summarize the theory behind infrared absorption [29]. 

Molecules possess discrete levels of rotational and vibrational energy. Transitions between vibrational levels occur by absorption of photons with frequencies v in the infrared range (wavelength 1-1000 micrometer, wave numbers 10000-10 cm-1, energy differences 1240-1.24 meV). The C-O stretch vibration, for example, is at 2143 cm-1. For small deviations of the atoms in a vibrating diatomic molecule from their equilibrium positions, the potential energy V(r) can be approximated by that of the harmonic oscillator  

The harmonic approximation is only valid for small deviations of the atoms from their equilibrium positions. The most obvious shortcoming of the harmonic potential is that the bond between two atoms cannot break. With physically more realistic potentials, such as the Lennard-Jones or the Morse potential, the energy levels are no longer equally spaced and vibrational transitions with An 1 are no longer forbidden. Such transitions are called overtones. The overtone of gaseous CO at 4260 cm-1 (slightly less than 2 x 2143 = 4286 cm-1) is an example. 

The simple harmonic oscillator picture of a vibrating molecule has important implications. First, knowing the frequency, one can immediately calculate the force constant of the bond. Note from eqn. (6) that k, as coefficient of r2, corresponds to the curvature of the interatomic potential and not primarily to its depth, the bond energy. However, as the depth and the curvature of a potential usually change hand in hand, it is often allowed to take the infrared frequency as an indicator for the strength of the bond. Second, isotopic substitution can be useful in the assignment of frequencies to bonds in adsorbed species, because frequency shifts due to isotopic substitution (of for example D for H in adsorbed ethylene, or OD for OH in methanol) can be predicted directly. 

Molecules in the gas phase have rotational freedom, and the vibrational transitions are accompanied by rotational transitions. For a rigid rotor which vibrates as a harmonic oscillator the expression for the available energy levels is  

We first discuss two early and classic examples of the applications of IR spectroscopy for the identification of hydride and/or CO ligands in organ-ometallic complexes. We then discuss a few examples to illustrate how the probable structures of organometallic complexes that may be present in a catalytic system can be suggested on the basis of in situ IR data. 

Complexes 3.2 and 2.59 were mentioned earlier. The former was isolated as a solid from the reaction of dihydrogen with 2.58. The latter, as mentioned earlier, is a precatalyst for hydroformylation, and was also made using Wilkinson s complex. 

The IR spectra of 3.2 both in the solid state and in solution show two broad Rh-H stretching frequencies in the region -2000 cm. It is important to note that had the structure been trans, due to symmetry 

As we will see in Chapter 4, in the iridium-catalyzed methanol carbon-ylation to acetic acid, 3.6 is one of the active catalytic intermediates. Complex 3.7 is also a catalytic intermediate, but in the by-product of the water gas shift reaction. 

By in situ IR spectroscopy, the carbonyl bands of both 3.6 and 3.7 can be observed. Furthermore, when carbonylation is carried out under conditions that favor 3.7 to be the resting state of the catalyst, the carbonyl bands of 3.6 are gradually replaced by that of 3.7. Note that the fac structural isomer of 3.6 may exist under certain conditions, but for the time being we ignore this point. 

Of all the physical techniques, infrared (IR) spectroscopy gives the most valuable information about the constitution of organic materials. Indeed, qualitative information about specific structural and functional elements can often be deduced even though the spectra are too complex for individual compound analysis. With regard to quantitative evaluation of the constituents of coal by infrared 

In addition, one of the issues as it relates to coal science is the absence of specific standard test methods that can be applied to the investigation of coal properties by infrared spectroscopy as well as by other spectroscopic methods. There are, however, test methods that are applicable to the infrared analytical technique that should be followed when the method is applied to coal analysis. 

Infrared absorption is one of three standard test methods for sulfur in the analysis sample of coal and coke using high-temperature tube furnace combustion methods (ASTM D-4239). Determination of sulfur is, by definition, part of the ultimate analysis of coal (Chapter 4), but sulfur analysis by the infrared method is also used to serve a number of interests evaluation of coal preparation, evaluation of potential sulfur emissions from coal combustion or conversion processes, and evaluation of the coal quality in relation to contract specifications, as well as other scientific purposes. Infrared analysis provides a reliable, rapid method for determining the concentration of sulfur in coal and is especially applicable when results must be obtained rapidly for the successful completion of industrial, beneficiation, trade, or other evaluations. 

While thermal methods for the determination of polymer transitionspredominate, IR spectroscopy has been nsed to provide information on temperatnre transitions. 

Varob yev and Vettegren [81] used IR spectroscopy to determine temperatnre transitions in PC. Thermal transitions in PC were determined from the concentration variations of the residual solvent or plasticiser. 

Structural transitions and relaxation phenomena of PC have been followed by plotting the absorbance at 8.13 pm (stretching vibration of C-O-C groups) and 10.64 pm against temperature [82]. 

This chapter describes the uses of infrared (IR) spectroscopy in organic chemistry. By the end of the chapter you should  

CHAPTER 12 Structure Determination Mass Spectrometry and Infrared Spectroscopy 

Why does an organic molecule absorb some wavelengths of IR radiation but not others All molecules have a certain amount of energy and are in constant motion. Their bonds stretch and contract, atoms wag back and forth, and other molecular vibrations occur. Following are some of the kinds of allowed vibrations  

The amount of energy a molecule contains is not continuously variable but is quantized. That is, a molecule can stretch or bend only at specific frequencies corresponding to specific energy levels. Take bond-stretching, for example. Although we usually7 speak of bond lengths as if they were fixed, the numbers 

Equation (10.2) can be used to determine the concentration of a compound in a solution if the value of K is known for that compound. Chemical bonds, such as C-C, C-F, etc., absorb different amounts of infrared energy over various wavelengths. Absorption patterns vary from sharp to broad for different bonds. Peak IR absorption wavelength (wave number) is a characteristic of chemical bonds. Absorption over a range of wavelengths called the infrared spectrum is a fingerprint characteristic of an organic material. Qualitative identification can be achieved 

Infrared spectra can also be obtained by reflecting the IR beam on the surface of a sample. This technique is applied when it is not possible to obtain an IR spectrum by a transmission technique. Attenuated total reflectance (ATR) also known as ATIR (attenuated total internal reflectance) is based on multiple internal reflectance of the IR beam on the sample surface using a high refractive index crystal (e.g.. 

Modern infrared spectrometers use Fourier Transform for the calculation of results. The method is called Fourier Transform Infrared Spectroscopy, abbreviated FTIR. 

An infrared spectrum is a plot of percent radiation absorbed versus the frequency of the incident radiation given in wavenumbers (cm ) or in wave length ( xm). A variation of this method, diffuse reflectance spectroscopy, is used for samples with poor transmittance, e.g. cubic hematite crystals. Increased resolution and sensitivity as well as more rapid collection of data is provided by Fourier-transform-IR (FTIR), which averages a large number of spectra. Another IR technique makes use of attenuated total reflectance FTIR (ATR-FTIR) often using a cylindrical internal reflectance cell (CIR) (e.g. Tejedor-Tejedor Anderson, 1986). ATR enables wet systems and adsorbing species to be studied in situ. 

The absorption bands of goethite arise, as do those of the other FeOOH polymorphs, from Fe-OH and Fe-O vibrations. There are 36 possible Fe-O vibrations and 12 hydroxyl vibrations. Of these, 12 Fe-O and 5 hydroxyl vibrations (all B type) are infrared active, although not all of these are observed experimentally (Table 7.2). The same bands are detected whether the sample is examined by transmission, diffuse reflec- 

1) A comprehensive list of the vibration modes for a range of oxyanions is given in table 1 of Suarez et al. 1999. 

5 Fe-O symm. stretch parallel to a Fe-O antisyrrrm. stretch parallel to c sh shoulder 

An intense band due to the bulk hydroxyl stretch is observed at 3140 cm . Two far less intense bands at 3660 and 3484 cm can be attributed to the surface hydroxyl groups. In general, these bands can only be detected on an evacuated surface. The most reasonable assignment for these bands appears to be that of Russell et al. (1974) who found that the band at 3660 cm disappeared completely when the surface was phosphated and so attributed it to the singly coordinated OH groups the band at 3484 cm was not replaceable by adsorbed phosphate and was considered to consist of contributions from the doubly and triply coordinated OH groups. 

The purpose of this book is to introduce the reader to the fundamental concepts of Fourier Transform Infrared (FTIR) spectroscopy. The discussion assumes no previous background in FTIR, but a familiarity with the basic concepts of chemistry and physics will be helpful in understanding this text. This book teaches the basics of FTIR to those new to the field, and will serve as an excellent reference guide for experienced users. All terms shown in italics will be defined in the glossary at the end of the book. 

The standard format of an IR spectrum is transmittance [%T versus wavenumber [cm ]. According to lUPAC recommendations the values of the wavenumber axis decrease towards its right-hand end. The features of an IR spectrum (number of infrared absorption bands, their intensities and their shapes) are directly related to the molecular structure of a compound. The IR spectrum is a unique physical property of an individual compound, it is its molecular fingerprint. 

The IR region comprises fundamental vibrations of bound atoms. Whenever such bound atoms vibrate, they absorb infrared energy, i. e. they exhibit IR absorption bands. The condition for a normal vibration j to be IR active is a change in molecular dipole moment p during vibration  

Complications in evaluation of IR spectra are the overlapping of individual bands and the appearance of additional bands, e. g. overtone and combination bands, which may be caused by anharmonicity of some vibrations. In the NIR region, aU bands are overtone or combination bands. They are always weaker in intensity than the corresponding fundamental bands. Originally considered as a drawback, the weak intensity of the NIR bands turned out to be the background for the large success of NIR spectroscopy in process analysis. 

The intensities of the bands in pure components and in mixtures are proportional to the concentrations of the components. The relation between measured intensities and concentration is expressed in the Lambert—Beer law (Eq. (11)). Thus it is possible to carry out quantitative investigations by methods based on band heights or preferably by methods based on integrated intensities. Both single component analysis and multicomponent analysis by multivariate methods (see Chapter 13) can be performed. 

Even though NMR spectroscopy is a powerful tool for deducing structures, it is usually supplemented by other spectroscopic methods that provide additional structural information. One of the more important of these is infrared (IR) spectroscopy. 

Particular types of bonds usually stretch within certain rather narrow frequency ranges. IR spectroscopy is particularly useful for determining the types of bonds that are present in a molecule. Table 12.4 gives the ranges of stretching frequencies for some bonds commonly found in organic molecules. 

The IR spectrum of a compound can easily be obtained in a few minutes. A small sample of the compound is placed in an instrument with an IR radiation source. The spectrometer automatically scans the amount of radiation that passes through the sample over a given frequency range and records on a chart the percentage of radiation that is transmitted. Radiation absorbed by the molecule appears as a band in the spectrum. 

Infrared (IR) spectroscopy is used to determine the types of bonds present in a moiecuie. 

The wavenumber of an iR frequency is defined as the number of waves per centimeter. 

Selected physical properties such as spectroscopy and magnetic chemistry reveal useful data on the general skeletal arrangement, bond strength, energy, and valency of metal Tu-complexes. In this chapter some of the details of infrared spectroscopy (IR), nuclear magnetic resonance (NMR), mass spectra, Mossbauer spectroscopy, magnetic susceptibility, and oxidation state are discussed in terms of the characterizations of metal 7i-complexes. 

While of definite value in characterization and structural elucidation, IR does not have the wide range of applicability associated with NMR spectroscopy, which is discussed subsequently. The chief reason for this limitation is the complexity of IR spectra and the resulting difficulty in interpretation. However, in relatively simple species, especially those involving olefins and the cyclopentadienyl group, this tool has proven useful in the assignment of bonding modes of the organic moiety. Some of the 7i-systems subjected to IR studies will now be considered in more detail. 

Olefin Ti-Complexes Ethylene and Mono-olefin (2 7C-System) 

In Table 4-2 the Pt-olefin stretching frequencies of various monoolefin complexes are listed. This vibration characteristically appears between 410 and 380 cm  

On the basis of Av (C=C), the metal-olefin bond strengths appear to vary in this homologous series in the order W Cr Mo. This observation is in agreement with the order of the M—C force constants determined for the Group VI metal hexacarbonyls and also compares with the somewhat greater stability observed for tungsten complexes of simple monoolefins as contrasted with those of the other Group VI metals. 

Lin and Hsu [1.85] described the determination of residual moisture in protein pharmaceuticals in sealed glass vials by near-infrared (NIR) spectroscopy. Five proteins were studied recombinant humanized monoclonal antibody (rhuMAb) E25, rhuM-Ab HER2, rhuMAb CDlla, TNKase and rt-PA. Higher moisture contents (RM) were obtained by adding appropriate amounts of MilliQ water to the wall of the vial in a 

Product RM by KF (%) RM byTC (%) Content of water in vial space (mg/vial) 

TG-profile thermogravimetric profile by a given course of temperatures. 

MD = 0 °C- Pc = 008 mbar. Tsh,SD = 40 °C Below dW data calculated from DR measurements during secondary drying as a function of drying time. ( ) Mean dW of all three runs bars, minimum and maximum of all calculated dW 

Influence ofVial Stoppers on the Residual Moisture Content 

Early studies on PLLA mainly focused on the identification of characteristic bands to investigate the polymer crystallinity. Since Fourier transform infrared (FUR) spectroscopy is sensitive to the conformation and local molecular environment, this technique has also been used to elucidate the structure of the crystalline polymers. More recently, research on PLLA surface characterization using FTIR has been an object of interest. This section is divided into three parts structural analysis, surface characterization, and crystallization studies. 

FIGURE 8.3 Infrared spectra of poly(L-lactic acid)s PLA 100 (semicrystalline), PLA 100am (amorphous), PLA 50i (isotactic), PLA 50a (atactic), PLA 50s (syndiotactic), and PLA complex (stereocomplex). (—) Band sensitive to the tacticity. Adapted from Ref. 34 with permission from Elsevier. 

FIGURE 8.4 Infrared spectra of semicrystalline 98 2 (L D)poly (lactic acid) [36]. 

The C-0 stretching modes of the ester group appear at 1225 cm and the C-O-C asymmetric mode appears at 1090 cm  

The IR region of the electromagnetic spectrum extends from 10 to 20000cm Because of instmmental and functional reasons this region is divided into near-, mid-, and far-infrared spectroscopy. 

Mid-infrared (MIR) spectroscopy MIR spectroscopy (frequency range 2.5-25nm or 4000 00 cm ) is a popular technique for identification assays (chemical identity) of dmg substances in pharmacopoeias. The fact that different solid-state forms exhibit different MIR spectra represents rather a problem for such purposes. Thus, in the case that the spectmm of a compound, whose identity needs to be determined or confirmed, is different than that of the reference compound, pharmacopoeias suggest either to record the spectra in solution or to recrystallize both the substance and the reference using the same method before recording their solid-state spectra. 

Theophylline differs from caffeine solely by the lack of a methyl group in position 7 of the xanthine ring. Owing to the presence of a hydrogen-bond 

These sanples demonstrate that MIR spectroscopy provides important structural information about solid-state forms, particularly where different hydrogen-bond associations are present as well as for hydrates and solvates. However, from our experience, MIR spectra of conformational polymorphs [29] with aliphatic chains are often barely distinguishable owing to rather weak interactions between the molecules. In order to identify or quantify such forms by spectrometry, Raman or solid-state NMR spectroscopy stands a better chance of success. 

Developments in MIR spectrometers, particularly Fourier-transform (FT) techniques, have enabled the use of a variety of solid sampling techniques which overcome the disadvantages of classic IR-sampling techniques. Classic sampling techniques [30], such as alkali halide pellet preparation (with KBr or KCl) or mineral oil mull preparations, require a mechanical treatment of the sample and may thus induce solid-solid transformations or desolvations. 

Phenolic antioxidants in rubber extracts were determined indirectly photometrically after reaction with Fe(III) salts which form a red Fe(II)-dipyridyl compound. The method was applicable to Vulkanox BKF and Vulkanox KB [52]. Similarly, aromatic amines (Vulkanox PBN, 4020, DDA, 4010 NA) were determined photometrically after coupling with Echtrotsalz GG (4-nitrobenzdiazonium fluoroborate). For qualitative analysis of vulcanisation accelerators in extracts of rubbers and elastomers colour reactions with dithio-carbamates (for Vulkacit P, ZP, L, LDA, LDB, WL), thiuram derivatives (for Vulkacit I), zinc 2-mercaptobenzthiazol (for Vulkacit ZM, DM, F, AZ, CZ, MOZ, DZ) and hexamethylene tetramine (for Vulkacit H30), were mentioned as well as PC and TLC analyses (according to DIN 53622) followed by IR identification [52]. 8-Hydroquinoline extraction of interference ions and alizarin-La3+ complexation were utilised for the spectrophotometric determination of fluorine in silica used as an antistatic agent in PE [74], Also Polygard (trisnonylphenylphosphite) in styrene-butadienes has been determined by colorimetric methods [75,76], Most procedures are fairly dated for more detailed descriptions see references [25,42,44], 

UV/VIS spectrophotometry can also be used for extraction monitoring on the basis of the chromophore of functional classes and to follow up polymer impregnation with additives in scC02 [81]. 

Principles and Characteristics Vibrational spectroscopic techniques such as IR and Raman are exquisitely sensitive to molecular structure. These techniques yield incisive results in studies of pure compounds or for rather simple mixtures but are less powerful in the analysis of complex systems. The IR spectrum of a material can be different depending on the state of the molecule (i.e. solid, liquid or gas). In relation to polymer/additive analysis it is convenient to separate discussions on the utility of FUR for indirect analysis of extracts from direct in situ analysis. 

The vibrational and rotational motions of the chemically bound constituents of matter have frequencies in the IR region. Industrial IR spectroscopy is concerned primarily with molecular vibrations, as transitions between individual rotational states can be measured only in IR spectra of small molecules in the gas phase. Rotational - vibrational transitions are analysed by quantum mechanics. To a first approximation, the vibrational frequency of a bond in the mid-IR can be treated as a simple harmonic oscillator by the following equation  

A nonlinear molecule of N atoms with 3N degrees of freedom possesses 3N — 6 normal vibrational modes, which not all are active. The prediction of the number of (absorption or emission) bands to be observed in the IR spectrum of a molecule on the basis of its molecular structure, and hence symmetry, is the domain of group theory [82]. Polymer molecules contain a very high number of atoms, yet their IR spectra are relatively simple. This can be explained by the fact that the polymer consists of identical monomeric units (except for the end-groups). 

Around 500 K, the catalyst consumes H2, as shown by the sharp peak, while simultaneously H2S and some additional H2O are produced, which indicates that the catalyst has taken up too much sulfur at lower temperatures, which is now released in the form of H2S. At higher temperatures, the catalyst continues to exchange oxygen for sulfur until all the molybdenum is present as M0S2. TPS has proven very useful in studying the sulfidation of M0O3 as well as Co and Ni promoted catalysts. 

Three /3-CH modes corresponding to in-plane C—H deformations are also observed (Table 22) and are probably best depicted as in (27), (28) and (29), although those for pyrrole will be modified as a result of interaction with the in-plane N—H deformation. The skeletal ring breathing mode (30) observed at ca. 1137 cm for 2-substituted pyrroles and 

Vibration Approximate description C2V Pyrrole Furan Thiophene Selenophene Tellurophene 

Structure of Five-membered Rings with One Heteroatom 

The y-CH modes arising from out-of-plane C H deformations should be characteristic of the substitution pattern and the observed frequencies are summarized in Table 23. For 2-substituted compounds these may be assigned as (31), (32) and (33). Additional characteristic bands for 2-substituted thiophenes are observed at 870-840m and 740-690s cm (67RTC37).  

In most cases the frequencies of substituent groups attached to these heterocycles differ little from those observed for their benzenoid counterparts. The only notable exception is the spectral behaviour of carbonyl groups attached to position 2. These have attracted much attention as they frequently give rise to doublets, and occasionally multiplets. In the case of (34), (35) (76JCS(P2)l) and (36) (76JCS(P2)597) the doublets arise from the presence of two conformers (cf. Section 3.01.5.2), whereas for the aldehydes (37) the doublets are 

The respective angles are typically 45° and 33° or 65° and 55°. IR-transparent Cap2 or ZnSe windows are used. 

Infrared spectroscopy (IRS) is a very powerful technique for the study of adsorbates on electrode surfaces. The two major obstacles to applying in situ IRS are strong absorption of infrared (IR) light by the electrolyte and the difficulty of detecting 

In the early work of Bewick and Robinson (1975), a simple monochromator system was used. This is called a dispersive spectrometer. In the experiment the electrode potential was modulated between two potentials, one where the adsorbed species was present and the other where it was absent. Because of the thin electrolyte layer, the modulation frequency is limited to a few hertz. This technique is referred to as electrochemically modulated infrared reflectance spectroscopy (EMIRS). The main problem with this technique is that data acquisition time is long. So it is possible for changes to occur on the electrode surface. 

A significant advance was the application of the Fourier transform technique to enhance the signal. The optical arrangement of a Fourier transform infrared (FUR) spectrometer is shown in Fig. 27.37 (Habib and Bockris, 1984). A beam of light from an IR source is directed to a beamsplitter, where part of the beam is transmitted to a 

FIGURE 27.37 Optical arrangement of an FTIR spectrophotometer. (From Habib and Bockris, 1984, with permission from Elsevier.) 

In the ATR FTIR study of the synthesis of cyclopentyl silsesquioxane 7F3, in situ ATR FTIR spectra of the reaction mixture were collected every 2 min during the reaction. The spectra obtained were plotted as a function of reaction time (Fig. 9.11). Pure component spectra and relative concentration profiles were subsequently recovered using a multivariate curve resolution (MCR) [59] technique based on a modified target factor analysis algorithm [60]. 

Principal component analysis (PCA) [61] was first used to determine the number of independently varying chemical species present and to provide initial estimates of the spectral shapes resulting from these species and of their concentration profiles. Reference ATR FTIR spectra for several components (the solvent acetonitrile and water, the reagent cyclopentyltrichlorosilane and the product o7h3) were measured to assist in the deconvolution of the data. Frequency windows were selected that allowed the best discrimination between the reference compounds (725-775 cm-1 for acetonitrile, 850-900 cm-1 for water and the silsesquioxane). Finally, the MCR technique was applied to the data in the selected frequency windows to find the component spectra and relative concentration profiles that best fit the observed spectra. 

228 I 9 Parallel Approaches to the Synthesis and Testing of Catalysts for Liquid-phase Reactions 

From Fig. 9.12 it can be seen that the pure component spectra obtained by the MCR technique closely match the reference spectra for acetonitrile (with a correlation coefficient R=0.987), water (R=0.993) and silsesquioxane (R=0.953). A poor match (R=0.214) was obtained for the cyclopentyltrichlorosilane reference spectra, indicating that cyclopentyltrichlorosilane cannot be detected at any stage during the reaction. This supports the MS results that also suggest that trichlorosilane is immediately hydrolyzed once water is added to the solution. 

Relative concentration profiles for the identified species are given in Fig. 9.13. Acetonitrile and water concentrations were reasonably constant during the reaction, reflecting the fact that both liquids were present in large quantities, and that any change in concentration is effectively negligible. 

Cosmic and gamma rays are very high energy and do not interact much with matter because no energy states with AE this large are available. The energy of X-rays corresponds to AE between electron shells and can excite and eject inner-shell electrons. 

Higher frequency The ener8/ of infrared light matches Shorter wavelength the AE between the vibrational energy levels of covalent bonds. When a molecule absorbs light, its bonds vibrate more rapidly. IR spectroscopy is discussed in this chapter. 

Radar employs microwave radiation. Microwave ovens use this radiation to excite rotational energy states of water and other molecules in food. 

Lower energy Lower frequency Longer wavelength 

Radio and TV broadcasts use radio frequency radiation. In addition, nuclear magnetic resonance spectroscopy, which causes transitions between nuclear spin states, uses radiation from this region. One typical NMR spectrometer operates at 2 X 108 Hz or 200 MHz (1.9 X 10 5 kcal/mol or 8 X 10 s kJ/mol). NMR spectroscopy is discussed in Chapter 14 

Pyrrole belongs to the 2 symmetry point group and exhibits 24 normal modes of vibration. Selected fundamental vibrations of pyrrole, 1-deuteropyrrole, and pentadeuteropyrrole are reported in Table 26, and have been reassigned based on high-level quantum-chemical calculations 2000JMT(507)75 . 

Because pyrrole has four C-H bonds, four C-H stretches are expected in two pairs, one pair of oscillators adjacent to the nitrogen and the other pair on the 3- and 4-positions. The origin of the fundamental C-H stretch vibrations in the gas phase has been intensively investigated 1995CPH(190)407 . The fundamental N-H stretch band has its origin at 3530.811343(82) cm and has been rotationally analyzed 1997CPH(220)311 . 

In low-temperature, solid inert matrixes (argon, xenon 7 = 9 K) pyrrole forms hydrogen-bonded aggregates that are predicted to be mainly cyclic trimers and tetramers, based on DFT analysis, with a significant cooperativity effect 2004PCA6953 . 

FTIR measurements and ab initio calculations show that iV-methylpyrrole interacts with hexafluoroisopropanol, trifluoroethanol, 2-chloroethanol, and 1-butanol to form 1 1 stoichiometric hydrogen-bonded complexes in which the OH group acts as H-donor and the aromatic 7t-system as acceptor 2003CPH(290)69 . 

Experimental and theoretical vibrational spectra for a series of pyrrole derivatives are listed in Table 27. 

the useful IR region is from 4000 to 400 cm corresponding tc energies of 48.0 kj/mol to 4.80kJ/moI (11.5-1.15 kcal/inol). 

1 Ultraviolet i I Visible Near Infrared 1 Infrared I Far infrared 1 Microwaves j 1 1 

Frequency and wavelength are inversely proportional (v = c/X), however, so energy and wavelength are inversely proportional  

When electromagnetic radiation strikes a molecule, some wavelengths—but not all—are absorbed. Only some wavelengths are absorbed because molecules have discrete energy levels. The energies of their electronic, vibrational, and nuclear spin states are quantized, not continuous. 

For absorption to occur, the energy of the incident electromagnetic radiation must match A . 

The larger the energy difference between two states, the higher the energy of radiation needed for absorption, the higher the frequency, and the shorter the wavelength. 

Problem 13.8 The difference in energy between two electronic states is -100 kcal/mol, whereas the difference in the energy between two vibrational states is -5 kcal/mol. Which transition requires the higher v of radiation  

SpectrpscoijyF ny technique involving - the pfbiluotion arid subsequent recc g of a speetrum af electromagnetic -radiatioT usyally in terms of wave enjth or energy.  

In addition to ultraviolet-visible (UV-vis) spectroscopy (p. 164), there are three other essential techniques that you will encounter during your laboratory course. They are  

Infrared (IR) spectroscopy, this is concerned with the energy changes involved in the stretching and bending of covalent bonds in molecules. 

Nuclear magnetic resonance (NMR) spectroscopy this involves the absorption of energy by specific atomic nuclei in magnetic fields and is probably the most powerful tool available for the structural determination of molecules (Chapter 29). 

Mass spectrometry (MS) this is based on the fragmentation of compounds into smaller units. The resulting positive ions are then separated according to their mass-to-charge ratio (mjz) (Chapter 30). 

How precisely the above methods will separate ferrihydrite from better crystalline oxides depends on the form and crystallinity of the latter. A positive relationship was found between Fco/Fct on the one hand and the surface area and XRD line width on the other for 14 synthetic goethites (range of FCo/Fct = 0.003-0.05) and 15 synthetic lepidocrocites (range of Foo/Fet = 0.06-077) (Schwertmaim, 1973). This shows that whereas goethites (and hematites) are essentially insoluble in oxalate irrespective of their crystal size, the method can only be used for well crystalline lepidocrocites. The same applies to the use of dilute strong acids. 

Infrared spectra arise as a result of interactions of solids with electromagnetic radiation (photons) in the wavelength range 1-500 pm (i.e. wave numbers of 10,000-20 cm ). These interactions involve excitation of vi- 

For Fe oxides, FTIR spectroscopy provides a rapid means of identification. It can detect traces (1-2%) of goethite in a sample of hematite. In addition, low levels of impurities arising from insufficient washing, for example nitrate in ferrihydrite (band at 1384 cm ) and carbonate in goethite (ca. 1300 and 1500 cm ) can be detected. Broadening of the absorption bands reflects a decrease in crystal perfection (Cambier, 1986). Structural substitution of Fe by A1 (Schulze Schwertmann, 1984), Mn (Stiers Schwertmann 1985), and Cr (Schwertmann et al. 1989) causes a shift in the positions of the bands. 

Rather little attention is normally paid to the intensities of i.r. absorption bands of steroids, but two papers now detail interesting variations in v(C=0) for primary, secondary, and tertiary acetoxy-groups,85 and in v(C=0) and v(C=C) for variously substituted progesterone derivatives.85-86 I.r. absorption intensities of progesterone derivatives have been studied with a Fourier-transform spectrometer.87 

The organic molecule is irradiated continuously with IR light (of changing wavelength), and the absorption of energy is recorded by an 

IR spectrometer (which has the same design as a UV spectrometer see Section 10.3). The absorption corresponds to the energy required to vibrate bonds within a molecule. 

The absorption of energy, which gives rise to bands in the IR spectrum, is reported as frequencies, and these are expressed in wavenumbers (in cm-1). The most useful region of radiation is between 4000 and 400 cnT1. 

The frequency of vibration between two atoms depends on the strength of the bond between them and on their atomic weights (Hooke s law). A bond can only stretch, bend or vibrate at specific frequencies corresponding to specific energy levels. If the frequencies of the IR light and the bond vibration are the same, then the vibrating bond will absorb energy. 

The IR spectrum can therefore be viewed as a unique fingerprint of an organic compound, and the region below 1500 cm-1 is known as the fingerprint region. Fortunately, the vibrational bands of functional groups in different compounds do not change much, and they appear at 

Identifying compounds - the combination of techniques described in this and the following chapters can often provide sufficient information to identify a compound with a low probability of error. 

Spectroscopy - any technique involving the production and subsequent recording of a spectrum of electromagnetic radiation, usually in terms of wavelength or energy. 

Spectrometry - any technique involving the measurement of a spectrum, e.g. of electromagnetic radiation, molecular masses, etc. 

The discussion that follows is necessarily selective and is pitched at a simplistic level. Although in this section we derive the number of vibrational modes for some simple molecules, for more complicated species it is necessary to use character tables. The reading list at the end of the chapter gives sources of detailed discussions of the relationship between group theory and normal modes of vibration. 

Infrared (IR) and Raman (see Box 3.1) spectroscopies are branches of vibrational spectroscopy and the former technique is the more widely available of the two in student teaching laboratories. 

One of the key advantages of supercritical solvents over conventional liquids is their unique combination of gas-like and liquid-like properties. Thus, reactant gases such as H2 or N2 can, under the correct conditions of temperature and pressure, be totally miscible with SCF solutions. At Nottingham, this unique property has been exploited to synthesize a range of novel organometallic complexes with metals bonded directly to dinitrogen [17] and dihydrogen [7,18], as shown in eq (3.1.1). These experiments are outlined in more detail in chapter 4.2. 

Initially, photochemical studies were performed in a miniature spectroscopic cell which was designed with a low volume (ca. 1 mL), to minimize stored potential energy and to permit safe operation on the open bench. This modular 

FIGURE 10.5 The NMR line width Ah (the unit is the tesla, T) as a function of temperature for poly(tetrafluoroethylene). The mechanical damping curve (.) is included for compar- 

The infrared spectra of highly stereoregular polymers are distinguishable from those of their less regular counterparts, but many of the differences can be attributed to crystallinity rather than tacticity as such. The application of infrared to stereostructure detenniuation in polymers is less reliable than NMR, but has achieved moderate success for PMMA and polypropylene. In PMMA, a methyl deformation at 7.25 pm is unaffected by microstructure, and comparison of this with a band at 9.40 pm, which is presort only in atactic or syndiotactic polymers, allows an estimate of the syndiotaeticity to be made from the ratio A(9.40 pm)/A(7.25 pm). Similarly, A(6.75 pm)/A(7.25 pm) provides a measure of the isotactic content. An alternative method is to ealeulate the quantity 7 as an average of the two equations 

For spectral analysis below 2000cm , FTIR transmission spectra were obtained. A small amount of sample (approximately 10-20 mg) was placed between two NaCl plates, which were then rotated to disperse the particles. Spectra were obtained at 4 cm nominal resolution with a DTGS detector by co-addition of 16 scans. 

The FTIR spectra of certain oxide samples were recorded over the 4000-400cm range by means of a FTIR 1725x (Perkin-Elmer) or a Specord M-80 (Carl Zeiss) spectrophotometer. Before FTIR measurements, samples were dried at 200°C for 2h. They (0.33 wt%) were stirred with KBr (Merck, spectroscopy grade) and then pressed to form appropriate tablets. 

The perturbation degree 1 = 1 - (///q), where Iq and I are intensities of a band of free silanols at 3750 cm of initial silica and silica-polymer samples, respectively. I was used as a measure of interaction of polymers with the silica surface, because free silanols are the main adsorption sites for polar compounds adsorbed on the silica surface. 

Ikble 4-3. Techniques that have been used to characterize encaged clusters. 

Extraction metal carbonyl clusters internal or external location of metal carbonyl cluster in zeolite Carbonyl cluster encaged in the zeolite cages cannot diffuse through the zeolite aperture and cannot be extracted out effective for anionic dusters but not effective for some neutral carbonyl clusters that are difficult to dissolve in solvent. 

Metal-catalyzed reduction of methylviologen metal clusters internal or external location of metal cluster in zeolite Characteristic blue color of the reduced viologen radicals makes it ea to locate dusters. 

FIG U RE 7.21 Schematic representation of inductively coupled plasma (ICP) torches (a) for 

ICP-AES, NAZ is the normal analytical zone and IRZ is the initial radiation zone (b) for ICP-MS, P -p5 are zones of decreasing pressure, p, = 760 Torr, P2 1 Torr, and pj ss 10 -10 Torr s, sampling cone and Sji skimmer cone. 

FIGURE 7.22 Fourier transform infrared (FTIR) spectra of brown humic acid (BHA) obtained and purified from a commercial humic substance (HS) (Acros Organics) (a) BHA alone, (b) loaded with 80 mmol-kg Co, (c) loaded with 1280 mmol-kg Co, (d) and (e) the same with NF , and (f) and (g) the same with Cu. (Reprinted with permission from Alvarez-Puebla, R.A. et al., 2004, Langmuir, 20, no. 9,3657-3664. Copyright 2004 American Chemical Society.) 

H-bonded -COOH. The higher variations are consistently found in samples doped with a greater amount of metal there is an increase in the bands at about 3420,1620, and 1380 cm and a decrease in intensity in those near 1710 and 1240 cm , which indicates that the metal is bound by surface complexation (specific adsorption). The samples doped with 80 mmol kg show lower variations, suggesting that electrostatic retention plays a role in ion binding. 

In addition, there are many types of modifiers applied to calcium carbonate surface modification. We need to 

As seen from Table 5.3, the products modified by tita-nate NDZ-101, sodium stearate, stearic acid, and Zinc stearate acid zinc exert the highest activation index nevertheless, the modification effects of other modifiers are relatively poor. The results of contact angle also support this point. Among these four modifiers, sodium stearate modified products hold the maximum contact angle and have lower costs.  

The advantages of the treatment of calcium carbonate whiskers by wet processing include even dispersion and good effect. The disadvantages are that wet processing is generally done in solution, and therefore a dispersion medium is needed. It also 

Surface Modification Agent Activation Index (%) Contact Angle n 

If the matrix resin contains processing molding additives, such as a plasticizer, stabilizer, lubricant, and so forth, the different compatibilities among these additives, between additives and resin, and between the filler and additives, make the interfacial area of the filler and the matrix and the interfacial distribution complicated, and the performance of the composite material will be influenced by many factors. 

By plotting 1 /p or (1/po — ) versus T/r], we can obtain information about V if r is known or about x if V is known from other sources. 

Infrared (IR) spectroscopy has long been used to determine molecular structure and to identify unknown compounds. IR data have been used to obtain information about the chemical composition, configuration, and crystalienity of polymeric materials. Recently, IR spectroscopy has encountered competition from other techniques, such as NMR and x-ray diffraction. Nevertheless, IR spectroscopy s importance as an experimental technique continues largely because of the rapid development of Fourier transform infrared (FTIR) spectroscopy, which is sensitive to the detailed structure of a molecule. 

Using the wavenumber scaie resuits in iR frequencies in a numericai range that is easier to report than the corresponding frequencies given in hertz (4000-400 cm compared to 1.2 x 10. 2 x 10 Hz). 

Organic chemists use infrared (IR) spectroscopy to identify the functional groups in a compound. 

When a molecule is irradiated with electromagnetic radiation, energy is absorbed if the frequency of the radiation matches the frequency of the vibration. The result of this energy absorption is an increased amplitude for the vibration in other words, the spring connecting the two atoms stretches and compresses 

The length reported for a bond between two atoms is an average length, because in reality a bond behaves as if it were a vibrating spring. A bond vibrates with both stretching and bending motions. 

A stretch is a vibration occurring along the line of the bond a stretching vibration changes the bond length. 

A diatomic molecule such as H—Cl can undergo only a stretching vibration because it has no bond angles. 

When the irreducible representations for the vibrations of a molecule have been identified, the collective motions of the atoms that constitute each mode can be thought of as a simple oscillator to be described with quantum mechanics. As the molecule vibrates in a given mode it moves through a potential energy surface that is set by the chemical bonds of the system. For small displacements, which are all that occur at room temperature, we 

Each mode has a set of energy levels that form a regular ladder of states, with energies En given by 

n is a quantum number taking values 0, 1, 2. ... etc., the vibrational frequency, v, is in s units of the mode and h is the Planck constant (6.626 x 10 J s). The lowest energy state (n = 0) and the first excited state (n = 1) differ in energy and in the amplitude of the oscillation. In the higher energy state the atoms can move further from the minimum point before bond strain forces cause them to return. 

In Appendix 6 it is shown that the simple form of Equation 6.1 is a direct consequence of the harmonic approximation, and anharmonic corrections are required for the most accurate spectroscopic analysis. However, the harmonic model is perfectly adequate for most cases in which spectroscopy is used to identify polyatomic molecules, and so we will continue to use it here. 

In Equation (6.4) the volume of an infinitesimal region required for the integration is written dr. 

1 Determination of Hydroxy Groups in Dinitropropyl Acrylate Prepolymer 

Kim and co-workers [18] studied variables such as temperature, concentration, bulk dielectric properties, and the structure of the alcohols to determine their effects on the characteristics of the THF-associated OH absorption peak. These studies show that the infrared method has a general applicability. The hydroxy equivalent weight values by this method compare well with expected values on a variety of dinitroacrylate polymers [18]. This method is subject to interferences (see Table 3.5). 

Prepolymers Vendor s eqivalent weight IR method Chemical methods  

Reprinted with permission from C.S.Y. Kim, A.L. Dodge, S.R Lau and A. Kowasaki, Analytical Chemistry, 1982, 54, 2, 232. 1982, American Chemical Society [18]  

Infrared spectroscopy has also been used to determine hydroxyl groups in polyethers [20], polyethylene ether carbonate [21], and carboxy terminated polybutadiene [22, 23] and hydroxybutadienes [24]. 

Which is higher in energy, FM radio waves with a frequency of 1.015 X 10 Hz (101.5 MHz) or visible green light with a frequency of 5 X 10 Hz  

Remember the equations e = hv and e = hc/, which say that energy increases as frequency increases and as wavelength decreases. 

Since visible light has a higher frequency than radio waves, it is higher in energy. 

It s useful to develop a feeling for the amounts of energy that correspond to different parts of the electromagnetic spectrum. Calculate the energies in kj/mol of each of the following kinds of radiation  

The most fundamental problem in evaluating the structure of polymer gels is the structural analysis on the molecular level, in particular, the determination of molecular conformation and quantitative analysis. 

Polyurethane EO copolymer N-H—ether hydrogen bonding noted 298 

Poly(l-vinyl Imidazole) Hydroxyl con-taining polymers Frequency shifts in OH (3550 cm ) noted 299 

Perflorinated sulfonic acid copolymer EA-4VP copolymer Vinyl pyridine in-plane aromatic ring vibration at 1416 cm decreases, protonated pyridine N band appears at 1642 cm sulfonic acid copolymer = Nafion 300 

Sulfonated PS EA-4VP copolymer S-0 stretching at 900 cm and 0=S=0 stretching bands at 1176 and 1350 cm disappear in blends, pyridinium band emerges at 1638 cm , Additional peaks observed 301 

PAA PVPh Miscible when cast from DMF self-association of PAA and also PVPh disrupted and replaced with PAA/PVPh interactions as shown by FTIR 302 

Consider a weight suspended from a spring (or rubber band) that is dropped and allowed to boimce. Before seeing a demonstration, predict with your group which case below will have the fastest frequency (number of bounces per unit time) and which will have the slowest frequency. 

Circle each of the following statements that fits with what you observed in the demonstration. 

Cross out each one that does not fit with what you observed. 

Frequency can be a measure of any repeating phenomenon. What other phenomena around us are measured in frequency  

Many phenomena aroimd us can be measured in frequency (Hz), including ocean waves and light waves. 

NMR has become the dominate technique for the characterization of the structure of polymers. The development of new 2D-NMR pulse sequences is continuously increasing the capabilities offered. Compared to most of the earlier known conducting polymers, the solubility of P3ATs, thus should allow a much more detailed structural analysis. Surprisingly, there are, however, only a few reports presenting NMR results on P3ATs [31-34]. 

FIGURE 4. NMR spectrum of P30T, synthesized by Grignard coupling [34]. 

MMPT was developed with this situation in mind. On one hand, the most important degrees of freedom along the PT motif are parameterized with anharmonic functions and fitted to high-level ab initio calculations (MP2/6-311++G(d,p)). On the other hand, the environment is described with a computationally efficient force field. 

It is also possible to determine frequencies from quantum simulations using a range of approximations. This is illustrated for MA as an example [73-76]. Fairly accurate fuU-dimensional quantum dynamics investigations on truncated PESs 

The dynamics of a shared proton in symmetrical systems, including protonated water [3, 20, 78-87] and ammonia dimer [5, 24, 66, 88, 89], formic acid dimer [90-92], MA [26, 73-76], or protonated diglyme [20] has a long history. Experimentally, systems such as NH - - - NHj have been characterized through spectroscopic techniques [93, 94] or thermodynamic measurements [95]. Even more important, the protonated water dimer has provided much fundamental insight into the shared proton [3, 20, 70, 71,85,96-98], culminating inafully dimensional quantum study of its IR spectrum [65]. 

Another important experimental observable is the rate of a chemical reaction. With a validated PES at hand, such rates can be determined in a variety of ways, ranging from transition state theory (TST) to computationally demanding fully QM rates based on flux correlation functions [99]. As MMPT is a dissociable force field, it can be used in all such formalisms. Furthermore, because the environmental modes are described with a conventional force field and therefore computationally inexpensive to evaluate, and because only the motion along the PT motif is more highly parameterized, MMPT is a particularly efficient empirical representation of the intermolecular interactions. 

In situations where tunneling plays a role, PT should rather be analyzed in terms of a sphtting than in terms of a rate constant. To estimate the tunneling splitting, a suitable Hamiltonian for the quantum dynamics calculation is required. One such Hamiltonian is the HBA Hamiltonian [26, 101, 102] 

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To ensure disposal water quality is in line with regulatory requirements (usually 40 ppm), the oil content in water is monitored by solvent extraction and infrared spectroscopy. The specification of 40 ppm refers to an oil in water content typically averaged over a one month period. 

The polymer concentration profile has been measured by small-angle neutron scattering from polymers adsorbed onto colloidal particles [70,71] or porous media [72] and from flat surfaces with neutron reflectivity [73] and optical reflectometry [74]. The fraction of segments bound to the solid surface is nicely revealed in NMR studies [75], infrared spectroscopy [76], and electron spin resonance [77]. An example of the concentration profile obtained by inverting neutron scattering measurements appears in Fig. XI-7, showing a typical surface volume fraction of 0.25 and layer thickness of 10-15 nm. The profile decays rapidly and monotonically but does not exhibit power-law scaling [70]. 

M. L. Hair, Infrared Spectroscopy in Surface Chemistry, Marcel Dekker, New York,... 

L. H. Little, Infrared Spectra of Adsorbed Molecules, Academic, New York, 1966. 68a. M. L. Hair, Infrared Spectroscopy in Surface Chemistry, Marcel Dekker, New... 

Infrared Spectroscopy. The infrared spectroscopy of adsorbates has been studied for many years, especially for chemisorbed species (see Section XVIII-2C). In the case of physisorption, where the molecule remains intact, one is interested in how the molecular symmetry is altered on adsorption. Perhaps the conceptually simplest case is that of H2 on NaCl(lOO). Being homo-polar, Ha by itself has no allowed vibrational absorption (except for some weak collision-induced transitions) but when adsorbed, the reduced symmetry allows a vibrational spectrum to be observed. Fig. XVII-16 shows the infrared spectrum at 30 K for various degrees of monolayer coverage [96] (the adsorption is Langmuirian with half-coverage at about 10 atm). The bands labeled sf are for transitions of H2 on a smooth face and are from the 7 = 0 and J = 1 rotational states Q /fR) is assigned as a combination band. The bands labeled... 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

Iwasita T and Mart F C 1997 In situ infrared spectroscopy at electrochemical interfaces Prog. Surf. Sc/. 55 271... 

Fehrensen B, Luckhaus D and Quack M 1999 Inversion tunneling in aniline from high resolution infrared spectroscopy and an adiabatic reaction path Hamiltonian approach Z. Phys. Chem., NF 209 1-19... 

Infrared and Raman spectroscopy each probe vibrational motion, but respond to a different manifestation of it. Infrared spectroscopy is sensitive to a change in the dipole moment as a function of the vibrational motion, whereas Raman spectroscopy probes the change in polarizability as the molecule undergoes vibrations. Resonance Raman spectroscopy also couples to excited electronic states, and can yield fiirtlier infomiation regarding the identity of the vibration. Raman and IR spectroscopy are often complementary, both in the type of systems tliat can be studied, as well as the infomiation obtained. 

Figure Bl.2.6. Schematic representation of a Michelson interferometer. From Griffiths P R and de Flaseth J A 1986 Fourier transfonn infrared spectroscopy Chemical Analysis ed P J Hiving and J D Winefordner (New York Wiley). Reprinted by pemiission of Jolm Wiley and Sons Inc.

Elving P J and Winefordner J D (eds) 1986 Fourier Transform Infrared Spectroscopy (New York Wiley)... 

In addition to covering Raman microscopy, this book has a wealth of information on Raman instrumentation in general. Elving P J and Winefordner J D (eds) 1986 Fourier Transform Infrared Spectroscopy (New York Wiley)... 

Comprehensive coverage of all fundamental aspects of Fourier transform infrared spectroscopy. 

Since the vibrational eigenstates of the ground electronic state constitute an orthonomial basis set, tire off-diagonal matrix elements in equation (B 1.3.14) will vanish unless the ground state electronic polarizability depends on nuclear coordinates. (This is the Raman analogue of the requirement in infrared spectroscopy that, to observe a transition, the electronic dipole moment in the ground electronic state must properly vary with nuclear displacements from... 

Laser Raman diagnostic teclmiques offer remote, nonintnisive, nonperturbing measurements with high spatial and temporal resolution [158], This is particularly advantageous in the area of combustion chemistry. Physical probes for temperature and concentration measurements can be debatable in many combustion systems, such as furnaces, internal combustors etc., since they may disturb the medium or, even worse, not withstand the hostile enviromnents [159]. Laser Raman techniques are employed since two of the dominant molecules associated with air-fed combustion are O2 and N2. Flomonuclear diatomic molecules unable to have a nuclear coordinate-dependent dipole moment caimot be diagnosed by infrared spectroscopy. Other combustion species include CFl, CO2, FI2O and FI2 [160]. These molecules are probed by Raman spectroscopy to detenuine the temperature profile and species concentration m various combustion processes. 

Vreugdenhil A J and Butler I S 1998 Investigation of MMT adsorption on soils by diffuse reflectance infrared spectroscopy DRIFTS and headspace analysis gas-phase infrared spectroscopy HAGIS Appl. Organomet. Chem. 

Korzeniewski C 1997 Infrared spectroscopy in electrochemistry new methods and connections to UhV surface science Crit. Rev. Anai. Chem. 27 81... 

Hamm P, Urn M and Hochstrasser R M 1998 Structure of the amide I band of peptides measured by femtosecond nonlinear-infrared spectroscopy J. Phys. Chem. B 102 6123-38... 

Wynne K, Haran G, Reid G D, Moser 0 0, Dutton P L and Hochstrasser R M 1996 Femtosecond infrared spectroscopy of low-lying excited states in reaction centers of Rhodobacter sphaeroides J. Rhys. Chem. 100 5140-8... 

As described above, classical infrared spectroscopy using grating spectrometers and gas cells provided some valuable infonnation in the early days of cluster spectroscopy, but is of limited scope. However, tire advent of tunable infrared lasers in tire 1980s opened up tire field and made rotationally resolved infrared spectra accessible for a wide range of species. As for microwave spectroscopy, tunable infrared laser spectroscopy has been applied botli in gas cells and in molecular beams. In a gas cell, tire increased sensitivity of laser spectroscopy makes it possible to work at much lower pressures, so tliat strong monomer absorjDtions are less troublesome. 

Infrared spectroscopy can also be carried out in molecular beams. The primary advantages of beam spectroscopy are tliat it dispenses almost entirely witli monomer absorjitions tliat overlap regions of interest, and tliat tlie complexes are... 

Most infrared spectroscopy of complexes is carried out in tire mid-infrared, which is tire region in which tire monomers usually absorb infrared radiation. Van der Waals complexes can absorb mid-infrared radiation eitlier witli or without simultaneous excitation of intennolecular bending and stretching vibrations. The mid-infrared bands tliat contain tire most infonnation about intennolecular forces are combination bands, in which tire intennolecular vibrations are excited. Such spectra map out tire vibrational and rotational energy levels associated witli monomers in excited vibrational states and, tluis, provide infonnation on interaction potentials involving excited monomers, which may be slightly different from Arose for ground-state molecules. 

Far-infrared and mid-infrared spectroscopy usually provide the most detailed picture of the vibration-rotation energy levels in the ground electronic state. However, they are not always possible and other spectroscopic methods are also important. 

Anderson D T, Schwartz R L and Todd M W and Lester M I 1998 Infrared spectroscopy and time-resolved dynamics of the ortho-Hj-OH entrance channel complex J. Chem. Phys. 109 3461-73... 

Schneider J, Erdelen C, Ringsdorf H and Rabolt J F 1989 Structural studies of polymers with hydrophilic spacer groups. 2. Infrared-spectroscopy of Langmuir-Blodgett multilayers of polymers with fluorocarbon side-chains at ambient and elevated temperatures Macromolecules 22 3475-80... 

Porter M D, Bright T B, Allara D L and Chidsey C E D 1987 Spontaneously organized molecular assemblies. 4. Structural characterization of normal-alkyl thiol monolayers on gold by optical ellipsometry, infrared-spectroscopy, and electrochemistry J. Am. Chem. Soc. 109 3559-68... 

Eden G J, Gao X and Weaver M J 1994 The adsorption of suiphate on goid(111) in acidic acqueous media Adiayer structurai interferences from infrared spectroscopy and scanning tunneiing microscopy J. Electroanal. Chem. 375 357-66... 

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