Peaks infrared

However, full structural analysis of a lipid will often necessitate further analysis of the collected column effluent for a single GLC peak. Infrared and NMR spectroscopy and mass spectrometry are all useful techniques which will give information for identification purposes, including the position and configuration of any double bonds. 

As shown in Fig. 4.1, similar nitro compounds with substituted carbon atoms at different positions have different infrared absorption peaks. Infrared absorption peaks of different nitro compounds are shown in Table 4.1. 

The infrared spectrum, also known as molecule vibration and rotation spectrum, is used in fundamental research of molecular structure and analysis of chemical components, and the latter is the widest application of the infrared spectrum. The structure of an unknown sample can be deduced according to the position and shape of absorption peaks in the spectrum and contents of the components of the mixture can be detected by the strength of the characteristic peaks. Infrared spectrometry has become the most widely used analysis and test instrument because of its analytical characteristics of high efficiency, high sensitivity, lower sample quantity, and good sample applicability. 

Pulsed difference frequency generation from the outputs of a ruby laser and a dye laser mixed in LiNbO has achieved 6 KW of infrared power tunable between 3.1 and 4.5 ym [7.89], Spectral narrowing of the dye laser output reduces the bandwidth to less than 1 cm , and peak infrared powers of several hundred watts with repetition rates up to 30 s have been generated with a long-term frequency stability of better than 1 GHz [7.90]. 

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation. 

Figure Bl.5.15 SFG spectrum for the water/air interface at 40 °C using the ssp polarization combination (s-, s- and p-polarized sum-frequency signal, visible input and infrared input beams, respectively). The peaks correspond to OH stretching modes. (After [ ].)...

Polyatomic molecules vibrate in a very complicated way, but, expressed in temis of their normal coordinates, atoms or groups of atoms vibrate sinusoidally in phase, with the same frequency. Each mode of motion functions as an independent hamionic oscillator and, provided certain selection rules are satisfied, contributes a band to the vibrational spectr um. There will be at least as many bands as there are degrees of freedom, but the frequencies of the normal coordinates will dominate the vibrational spectrum for simple molecules. An example is water, which has a pair of infrared absorption maxima centered at about 3780 cm and a single peak at about 1580 cm (nist webbook). 

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. 

Alkylidenehydrazinothiazoles (297) can be prepared either from 2-hydrazinothiazoles (549) or by direct heterocyclization (527). Their characteristic infrared bands have been reported (550). The main mass spectrometric peaks of (4-coumarinyl-2-thiazolyl)hydrazone (302) (Scheme 179) (134, 551) are situated at mle = 361. 244, 243, 118, 216, 202, 174, 117 the proposed interpretation of the fragmentation pattern should, however, be reconsidered. Scheme l80 summarizes some representative reactions of this class of compounds. 

A particular vibration will give an absorption peak in the IR spectrum only if the dipole moment of the molecule changes dunng the vibration Which vibration of carbon dioxide the sym metric stretch or the antisymmetric stretch is infrared active 2... 

Section 20 21 Acyl chlorides anhydrides esters and amides all show a strong band for C=0 stretching m the infrared The range extends from about 1820 cm (acyl chlorides) to 1690 cm (amides) Their NMR spectra are characterized by a peak near 8 180 for the carbonyl carbon H NMR spectroscopy is useful for distinguishing between the groups R and R m esters (RCO2R ) The protons on the carbon bonded to O m R appear at lower field (less shielded) than those on the carbon bonded to C=0... 

A certain compound has a molecular weight of 83 and contains nitrogen Its infrared spec trum contains a moderately strong peak at 2270 cm Its H and NMR spectra are shown in Figure 20 10 What is the structure of this compound" ... 

Infrared The absorptions of interest m the IR spectra of amines are those associated with N—H vibrations Primary alkyl and arylammes exhibit two peaks m the range 3000-3500 cm which are due to symmetric and antisymmetric N—H stretching modes... 

Infrared The IR spectra of phenols combine features of those of alcohols and aro matic compounds Hydroxyl absorbances resulting from O—H stretching are found m the 3600 cm region and the peak due to C—O stretching appears around 1200-1250 cm These features can be seen m the IR spectrum of p cresol shown m Figure 24 3... 

Treatment of 2 4 6 tn tert butylphenol with bromine in cold acetic acid gives the compound CigH29BrO in quantitative yield The infrared spectrum of this compound contains absorptions at 1630 and 1655 cm Its H NMR spectrum shows only three peaks (all singlets) at 8 1 2 13 and 6 9 in the ratio 9 18 2 What is a reasonable structure for the compound" ... 

Benzyl alcohol Infrared spectrum has peaks for O—H and sp C—H lacks peak for C=0... 

The so-called peak power delivered by a pulsed laser is often far greater than that for a continuous one. Whereas many substances absorb radiation in the ultraviolet and infrared regions of the electromagnetic spectrum, relatively few substances are colored. Therefore, a laser that emits only visible light will not be as generally useful as one that emits in the ultraviolet or infrared ends of the spectrum. Further, witli a visible-band laser, colored substances absorb more or less energy depending on the color. Thus two identical polymer samples, one dyed red and one blue, would desorb and ionize with very different efficiencies. 

An obvious difficulty here is deciding the location of the broken line portions of the peaks in the region of overlap. Some features of the infrared absorption spectrum may also be analyzed by the same procedure to yield values for 6. 

Infrared spectra of fats and oils are similar regardless of their composition. The principal absorption seen is the carbonyl stretching peak which is virtually identical for all triglyceride oils. The most common appHcation of infrared spectroscopy is the determination of trans fatty acids occurring in a partially hydrogenated fat (58,59). Absorption at 965 - 975 cm is unique to the trans functionaHty. Near infrared spectroscopy has been utilized for simultaneous quantitation of fat, protein, and moisture in grain samples (60). The technique has also been reported to be useful for instmmental determination of iodine value (61). 

A computer file of about 19,000 peak wavenumbers and intensities, along with search software, is distributed by the Infrared Data Committee of Japan (IRDC). Donated spectra, which are evaluated by the Coblentz Society in coUaboration with the Joint Committee on Atomic and Molecular Physical Data (JCAMP), are digitized and made avaUable (64). Almost 25,000 ir spectra are avaUable on the SDBS system developed by the NCLl as described. A project was initiated at the University of California, Riverside, in 1986 for the constmction of a database of digitized ftir spectra. The team involved also developed algorithms for spectra evaluation (75). Other sources of spectral Hbraries include Sprouse Scientific, Aston Scientific, and the American Society for Testing and Materials (ASTM). 

CS4JSI/SND. The Canadian Scientific Numeric Database Service (CAN/ SND) is provided by the Canada Institute for Scientific and Technical Information (ClSTl), a division of the National Research Council of Canada. It contains 140,000 ir spectra of 96,000 compounds. Entries consist of peak locations and some intensities. This system is searchable on-line using the SPIR (Search Program for Infrared Spectra) (85). Table 9 summarizes the available databases in the area of spectra. 

Infrared Data Com-mittee of Japan ir 19,000 PC-based peak wavenumbers and intensities... 

Q are the absorbance and wavenumber, respectively, at the peak (center) of the band, p is the wavenumber, and y is the half width of the band at half height. Liquid band positions ate usually shifted slightly downward from vapor positions. Both band positions and widths of solute spectra are affected by solute—solvent interactions. Spectra of soHd-phase samples are similar to those of Hquids, but intermolecular interactions in soHds can be nonisotropic. In spectra of crystalline samples, vibrational bands tend to be sharper and may spHt in two, and new bands may also appear. If polarized infrared radiation is used, both crystalline samples and stressed amorphous samples (such as a stretched polymer film) show directional effects (28,29). 

Spectroscopy. Infrared spectroscopy (48) permits stmctural definition, eg, it resolves the 2,2 - from the 2,4 -methylene units in novolak resins. However, the broad bands and severely overlapping peaks present problems. For uncured resins, nmr rather than ir spectroscopy has become the technique of choice for microstmctural information. However, Fourier transform infrared (ftir) gives useful information on curing phenoHcs (49). Nevertheless, ir spectroscopy continues to be used as one of the detectors in the analysis of phenoHcs by gpc. 

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