Infrared and Raman

The pathlength for IR samples is in the range 2-3 mm. Gases are easiest to sample in a longer pathlength cell, typically 10 cm in length. Fibre optic cables can be used where the sample is remote from the spectrometer. 

For Raman analysis, sample preparation is much easier than with IR. In fact, the source light is simply focussed onto the solid or liquid sample directly. If a cuvette is used, quartz or glass windows can be used. If a slide or surface is used, a background spectrum should be taken to remove the possibility of any interfering peaks. Glass tubes are often used and since water is a weak Raman scatterer, aqueous samples can be easily analysed. Reflectance measurements, as distinct from transmissive measurements above, can also be made and are useful for studying Aims on metal surfaces or samples on diamond surfaces. Measurements should also ideally take place in the dark to remove ambient light interferences. 

The most common detectors in IR are thermal, i.e. thermocouples, thermistors and bolometers. A thermocouple is based on the use of two different conductors connected by a junction. When a temperature difference is experienced at the junction, a potential difference can be measured. A series of thermocouples together is called a thermopile. Thermistors and bolometers are based on a change in resistance with temperature. They have a faster response time than thermocouples. With a Fourier Transform IR (FTIR), where rapid response and improved sensitivity is key, lead sulflde and InGaAs detectors are used as for NIR. Some arrays are also used. 

The most common detectors in Raman instruments are PDAs and CCDs but for FT-Raman, single channel detectors are used, e.g. InGaAs. An extra requirement for the FT-Raman instrument is a notch or edge filter it is included to reject scattered laser light at the strong Rayleigh line, which could otherwise obscure the FT-Raman spectrum. 

The real strength of IR is its ability to identify the strnctnral groups in a molecule, e.g. olehn, carbonyl, and so IR absorption spectroscopy is a powerful identification technique. In particnlar, the fingerprint region below 1500 cm is very dependent on the molecule s environment and it may be possible to identify a molecnle by comparing its transmission bands in this region with spectra from an IR library. Mid IR can also determine the quality or consistency of a sample and the amonnt of components in a mixture. Examples of an FTIR and an FT-Raman instrnment are shown in Fignres 2.16 and 2.17, respectively. 

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Herzberg G 1945 Molecular Spectra and Molecular Structure II Infrared and Raman Spectra of Polyatomic Molecules (New York Van Nostrand-Reinhold)... 

Wilson E B Jr, Decius J C and Cross P C 1955 Molecular Vibrations The Theory of Infrared and Raman Vibrational Spectra (New York McGraw-Hill)... 

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. 

Both infrared and Raman spectroscopy provide infonnation on the vibrational motion of molecules. The teclmiques employed differ, but the underlying molecular motion is the same. A qualitative description of IR and Raman spectroscopies is first presented. Then a slightly more rigorous development will be described. For both IR and Raman spectroscopy, the fiindamental interaction is between a dipole moment and an electromagnetic field. Ultimately, the two... 

Advances in Infrared and Raman Spectroscopy [36] provides review articles, both fiindamental and applied, in the fields... 

Lee D and Albrecht A C 1985 A unified view of Raman, resonance Raman, and fluorescence spectroscopy (and their analogues in two-photon absorption) Advances in Infrared and Raman Spectroscopy vo 12, ed R J H Clark and R E Hester (New York Wiley) pp 179-213... 

Nakamoto K 1978 Infrared and Raman Spectra of Inorganic and Coordination Compounds 3rd edn (New York Wiley-Interscience)... 

Bewiok A and Pons S 1985 Infrared speotrosoopy of the eleotrode-eleetrolyte solution interfaoe Advances in Infrared and Raman Spectroscopy ed R J FI Clark and R E Flester (New York Wiley Fleyden) 12 1-63... 

Wliile infrared and Raman speetroseopy are limited to vibrations in whieh a dipole moment or the moleeular polarizability ehanges, EELS deteets all vibrations. Two exeitation meehanisms play a role in EELS dipole... 

This book, originally published in 1950, is the first of a classic tliree-volume set on molecular spectroscopy. A rather complete discussion of diatomic electronic spectroscopy is presented. Volumes 11 (1945) and 111 (1967) discuss infrared and Raman spectroscopy and polyatomic electronic spectroscopy, respectively. 

D. Lin-Vien, N.B. Colthup, W.G. Fately, J. G. Grasselli, Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, New York, 1991. 

The vibrational states of a molecule are observed experimentally via infrared and Raman spectroscopy. These techniques can help to determine molecular structure and environment. In order to gain such useful information, it is necessary to determine what vibrational motion corresponds to each peak in the spectrum. This assignment can be quite difficult due to the large number of closely spaced peaks possible even in fairly simple molecules. In order to aid in this assignment, many workers use computer simulations to calculate the vibrational frequencies of molecules. This chapter presents a brief description of the various computational techniques available. 

E. B. Wilson Jr., J. C. Decius, P. C. Cross, Molecular Vihrations The Theory of Infrared and Raman Vihrational Spectra Dover, New York (1980). 

Infrared and Raman spectra of A-4-thiazoline-2-thione and of isotopi-cally labeled derivatives (56. 59) were interpretated completely. (Table VII-41. 

Until 1962 the infrared and Raman spectra of thiazole in the liquid state were described by some authors (173, pp. 194-200) with only fragmentary assignments. At that date Chouteau et al. (201) published the first tentative interpretation of the whole infrared spectrum between 4000 and 650 cm for thiazole and some alkyl and haloderivatlves. They proposed a complete assignment of the normal modes of vibration of the molecule. 

The infrared and Raman studies of thiazole derivatives are numerous (111,173,197-226) though often only fragmentary. The only studies leading to a complete assignment of the observed bands are those of Chouteau and Davidovics et al. (201,203.204,227,228). 

The infrared and Raman spectra of many alkyl and arylthiazoles have been recorded. Band assignment and more fundamental work has been undertaken on a small number of derivatives. Several papers have been dedicated to the interpretation of infrared spectra (128-134, 860), but they are not always in agreement with each other. However, the work of Chouteau (99, 135) is noteworthy. The infrared spectrum of thiazole consists of 18 normal vibrations as well as harmonic and combination bands. 

The section on Spectroscopy has been retained but with some revisions and expansion. The section includes ultraviolet-visible spectroscopy, fluorescence, infrared and Raman spectroscopy, and X-ray spectrometry. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon induction coupled plasma, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-19, and phosphoms-31. 

Herzberg, G. (1990) Infrared and Raman Spectra, Krieger, Florida. 

Equations (6.5) and (6.12) contain terms in x to the second and higher powers. If the expressions for the dipole moment /i and the polarizability a were linear in x, then /i and ot would be said to vary harmonically with x. The effect of higher terms is known as anharmonicity and, because this particular kind of anharmonicity is concerned with electrical properties of a molecule, it is referred to as electrical anharmonicity. One effect of it is to cause the vibrational selection mle Au = 1 in infrared and Raman spectroscopy to be modified to Au = 1, 2, 3,. However, since electrical anharmonicity is usually small, the effect is to make only a very small contribution to the intensities of Av = 2, 3,. .. transitions, which are known as vibrational overtones. 

One effect of mechanical anharmonicity is to modify the Au = t infrared and Raman selection rule to Au = 1, 2, 3,. .., but the overtone transitions with Au = 2, 3,... are usually weak compared with those with Au = t. Since electrical anharmonicity also has this effect both types of anharmonicity may contribute to overtone intensities. 

As for a diatomic molecule, the general harmonic oscillator selection mle for infrared and Raman vibrational transitions is... 

Table 6.4 Fundamental vibration wavenumbers of crotonaldehyde obtained from the infrared and Raman spectra...

In addition to bands in the infrared and Raman spectra due to Au = 1 transitions, combination and overtone bands may occur with appreciable intensity, particularly in the infrared. Care must be taken not to confuse such bands with weakly active fundamentals. Occasionally combinations and, more often, overtones may be used to aid identification of group vibrations. 

What group vibrations would you hope to identify in the infrared and Raman spectra of... 

Schrader, B. (1995) Infrared and Raman Spectroscopy, Wiley-VCH, Weinheim. 

It is important to realize that electronic spectroscopy provides the fifth method, for heteronuclear diatomic molecules, of obtaining the intemuclear distance in the ground electronic state. The other four arise through the techniques of rotational spectroscopy (microwave, millimetre wave or far-infrared, and Raman) and vibration-rotation spectroscopy (infrared and Raman). In homonuclear diatomics, only the Raman techniques may be used. However, if the molecule is short-lived, as is the case, for example, with CuH and C2, electronic spectroscopy, because of its high sensitivity, is often the only means of determining the ground state intemuclear distance. 

A small but artistically interesting use of fluorspar is ia the productioa of vases, cups, and other ornamental objects popularly known as Blue John, after the Blue John Mine, Derbyshire, U.K. Optical quaUty fluorite, sometimes from natural crystals, but more often artificially grown, is important ia use as iafrared transmission wiadows and leases (70) and optical components of high energy laser systems (see Infrared and RAMAN spectroscopy Lasers) (71). 

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