CH combination bands

The second overtone of the nonbonded OH stretch occurs at about 10,400 cm" (960 nm), and the third at about 13,500 cm (740 nm) for simple alcohols. The second overtone has also been used for a number of hydrogen-bonding studies. Variations in the structure of the alcohol result in splitting of the band and systematic shifts. Second overtones of the OH stretch appear to have less interference from CH combination bands than first overtones, and can therefore be more useful for thermodynamic studies. Additional overtones of the nonbonded hydroxyl stretch of alcohols, using gaseous ethanol as a model, are the fourth at 16,700 cm" (600 nm) and the fifth at 19,500 cm" (510 nm). Additional bonded hydroxyl bands include the OH-stretch second overtone at 9550 cm (1047 nm), a combination of the first overtone of the OH stretch and twice the methyl CH deformation at 9386 cm" (1065 nm), and a combination of the OH-stretch first overtone plus three times the CO stretch at 9720 cm" (1029 nm). i... 

A similar frequency shift is observed for their overtones or combination bands (204). It was also established that the proton-donating ability of the thiazole CH groups decreases in the order, 2>5>4 (204). 

The spectra of substituted thiazole derivatives, especially the methyl derivatives, can be used to determine harmonic and combination bands of the T(ch) modes 27(c.,h> T(c.hi + T[Pg.351]

In addition to the VCD from the methine C H stretching vibration, which alone gives rise to a strong positive bias in the CH stretching region, the CH stretching VCD of amino acids contains contributions from two other sources. Minor features can be attributed to combination bands of the very intense antisymmetric carboxylate stretch at 1610 cm with the symmetric carboxylate... 

Overtone/combination band involving fundamentals in the 1810-1050 cm"1 (5.52-9.52 pm) range, aromatic CH stretch (high u), aliphatic CH stretch, ... 

Naturally, the bands in this region may well represent a blend of the (v = 1) —(v = 2) and (n = 2) — (n = 3) aromatic CH stretching transitions with overtones and combinations involving aromatic CC stretches as well as aliphatic CH stretches. Many PAHs which do not have aliphatic side groups show weak absorptions near these frequencies. For example, Fig. 6 shows that chrysene, pyrene and coronene all show substructure on a broad component. Chrysene and coronene show a peak at about 2910 and 2845 cm-1 while pyrene has a broad (weak) plateau from 2950-2880 cm-1, which is similar to the emission plateau observed from the astronomical object BD + 30°3639 [44]. In the laboratory spectra these are due to overtone and combination bands which have been perturbed sufficiently by solid state effects to absorb weakly [35, 36, 37, 38, 39]. The perturbations within the PAH clusters that are suspended in salt pellets induce IR activity and broaden the individual bands causing them to overlap. In free vibrationally excited PAHs, perhaps Fermi resonances between the overtones and combinations of C-C stretching vibrations with the highly excited C-H modes can sufficiently enhance the intensities of these presumably weak bands to produce the observed intensites. 

Figure 8.5 NIR image of Tempo lotioned nonwoven tissue generated from the integrated area under the band at 4323 cm-1. Red areas represent puddles of petrolatum (lotion) with integrated area under the CH bend-stretch combination band at 4323 cm-1 > 0.18. Black areas represent thin areas or holes in the paper. Blue areas represent thick areas of the paper.

In addition to providing data on the compositions of coexisting phases, infrared data is also of potential value in providing structural information about phases. The mid-IR spectral region, particularly the CH-stretching and -bending regions, has been extensively used for this purpose (17-18). The near-IR bands ve observe between 4500 and 4000 cur"1" are the CH bend-stretch combination bands. It seemed likely that their band positions and intensities could also be sensitive to phase structure. 

Figure 6.27 shows the 1(,5(, infrared combination band of acetylene, where v, is the symmetric CH stretching vibration and v5 the cis bending vibration, as an example of a I7 — Zg band of a linear molecule. Note that the P branch starts with P(2), rather than P( ) as it would in a Z-Z type of transition, and that there is an intensity alternation of 1 3 for J"... 

For clarity, the wavenumber scale of the CH-stretching region is magnified besides the A type band at 2989.5 cm (t n), there is a B type band at higher wavenumbers whose rotation-vibration lines go through a marked intensity minimum (3105.5 cm ( 9). Further two A type bands of medium intensity (1443.5 cm 1 12 and a combination band at 1889.6 cm, assigned to r v -f i s) can be observed. 

F%, 6, Changes in the relative intensities of the free and associated NH stretching + NH in plane bending combination band of self-assodated N-ethylacetamide. The solutions contained equimolar amounts of one of the following anesthetics E, QH F, CFCI3, G, CH Clj H. CHCI3. From G. Trudeau, K. C. Cole, R, Massuda and C. Sandorfy, Can. J. Chon. 56, 1681 (1978). Reproduced with permission from the National Research Coimdl of Canada... 

The NIR region of the spectrum contains overtones and combination bands that are primarily attributed to hydrogen vibrations (OH, CH, NH). These overtones and combination bands are much weaker than the fundamental vibrations, thus, the molar absorptivities are much smaller than those of the corresponding infrared bands. Smaller molar absorptivities allow the use of undiluted samples and penetration of solid samples with good results. 

Another possibility is the investigation of combination bands of CH stretch and deformation, both with torsion. Confirmation of the assignment is also a severe problem in this case. By using the spectroscopic combination principle, the distance between torsional levels may be determined. Both of these infrared methods finally have to use the Hamiltonian Hj, with the consequence that Ia influences the results for V3 in a way discussed under the microwave intensity method. The infrared results on CH3CH2CN are compared in Table 1. 

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