Combination bands group frequencies

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). 

Figure 8.9 Diffuse reflectance infrared spectrum of a silica support, showing silica vibrations at frequencies below 1300 cm1, overtones and combination bands between 1700 and 2050 cm-1, and various hydroxyl groups at frequencies above 3000 cm 1. The sharp peak at 3740 cm"1 is due to isolated OH groups, the band around 3550 cm 1 to paired, H-bonded OH groups, and the band around 3660 cm 1 to hydroxyls inside the silica (courtesy of R.M. van Hardeveld, Eindhoven).

The functional groups almost exclusively involved in NIRS are those involving the hydrogen atom C-H, N-H, O-H (see Figure 5.1). These groups are the overtones and combinations of their fundamental frequencies in the mid-infrared and produce absorption bands of useful intensity in the NIR. Because the absorptivi-ties of vibrational overtone and combination bands are so much weaker, in NIRS the spectra of condensed phase, physically thick samples, can be measured without sample dilution or the need to resort to difficult short-path length sampling techniques. Thus conventional sample preparation is redundant, and fortunately so, because most PAT applications require direct measurement of the sample " either in situ, or after extraction of the sample from the process in a fast loop or bypass. 

However, in polyatomic molecules, transitions to excited states involving two vibrational modes at once (combination bands) are also weakly allowed, and are also affected by the anharmonicity of the potential. The role of combination bands in the NIR can be significant. As has been noted, the only functional groups likely to contribute to the NIR spectrum directly as overtone absorptions are those containing C-H, N-H, O-H or similar functionalities. However, in combination with these hydride bond overtone vibrations, contributions from other, lower frequency fundamental bands such as C=0 and C=C can be involved as overtone-combination bands. The effect may not be dramatic in the rather broad and overcrowded NIR absorption spectrum, but it can still be evident and useful in quantitative analysis. 

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. 

As seen earlier, some fundamental vibrations are relatively weak. Furthermore, some overtone and combination bands become unusually strong when Fermi resonance (accidental degeneracy) occurs. A typical example is given by C02, where the frequency of the first overtone of the v2(667 cm-1) is very close to that of the vj fundamental (1,337 cm-1). Since vj and 2v2 belong to the same symmetry species ( +), they interact with each other to give rise to two strong Raman bands at 1,388 and 1,286 cm-1. Finally, it should be noted that the point group symmetry in the crystalline state is not necessarily the same as that in the isolated state. Thus, this method must be applied with caution. 

A further development in the characterization resulted from a study of the near infrared region, by Washburn and Mahoney who showed that the first overtone of the cyclopropyl C-H stretching frequency absorbs at 6097 cm with a combination band also present at ca. 4465 cm . The overtone band is sharp though weak and separated from the first overtone of the saturated aliphatics. Any possible ambiguity with terminal methylene groups can be removed in infrared spectroscopic terms by analysis of the fundamental of the methylene. 

The molecular structure is the nonplanar configuration from the electron diffraction study of Akishin et al. (8). A planar model (also point group < 2 ) was assumed by Hisatsune et al. (9) in their approximate normal coordinate analysis of the infrared and Raman spectra. The frequency assignments of these authors are listed above in the order for the planar model, although the vibrations for the nonplanar form will separate differently into the species 5A, 3A2, 3B and 4B2. Hisatsune et al. (9) estimated the N-O -N deformation frequency (8 cm ) from combination bands in the solid and gas phase spectra. The JANAF thermodynamic functions were obtained using these frequencies and assuming the two N0 groups to be hindered internal rotators. 

---