Combination bands region, overtone

DRS has been applied both to the analysis of vibrational spectra of surface species in the fundamental, overtone, and combination band regions,and to the determination of time correlation motion of adsorbed molecules by Fourier inversion of the spectra onto... 

NIR spectrophotometry in the region from 8000 to 4000 cm-1 was used to measure the kinetics of copolymerization of an aromatic bismaleimide (72) derived from an aromatic diamine (e.g. 5a), taking place at 160 to 180 °C. The following NIR spectral ranges were useful for this study primary amine first overtones (vn h) at 7000 to 6400 cm-1, double bond first overtone (vc=c-h) at 6100 cm-1, aromatic first overtones (vc-h) at 6000 to 5750 cm-1, aliphatic first overtones (vc-h) at 5750 to 5350 cm-1 and primary aromatic amine combination bands first overtones (vn h + <5nh2) at 5150 to 4800 cm-1. The process consisted mainly of a second-order Michael addition, as depicted in equation 14, and not the plausible imide opening to yield a maleic dianilide (119), as shown in equation 15. A Michael addition between maleimide moieties and secondary amine moieties present in the products (118) also takes place, however at a rate of about one fourth of that of the primary amine moieties. To improve the SNR of the measurements, usually the results of... 

As already mentioned, absorption bands in this region are overtones or combination bands of fundamental stretching bands that occur in the 3000-1700 cm1 region (Figure 12). The bonds involved are usually C—H, N—H, and —H. Because the bands are overtones or combinations, their molar absorptivities are low and detection limits are around 0.1%. 

An example of Fermi resonance in an organic structure is the doublet appearance of the C=0 stretch of cyclopentanone under sufficient resolution conditions. Figure 3.2 shows the appearance of the spectrum of cyclopentanone under the usual conditions. With adequate resolution (Fig. 3.3), Fermi resonance with an overtone or combination band of an a-methylene group shows two absorptions in the carbonyl stretch region. 

Weak combination and overtone bands appear in the 2000-1650 cm-1 region. The pattern of the overtone bands is not a reliable guide to the substitution pattern of the ring. Because they are weak, the overtone and combination bands are most readily observed in spectra obtained from thick samples. The spectrum of Figure 3.13 is that of a typical aromatic (benzenoid) compound. 

Unfortunately, liquid ammonia possesses a very intense absorption band at 15,320 A. (2 i and/or 2vz) so that this region is inaccessible for quantitative measurements even with a 1 mm. cell. If ND8 is used as the solvent instead of NH8, no intense overtone or combination bands of the solvent occur in the spectral region of interest (Figure 1). The spectra of dilute solutions of lithium, sodium, potassium, calcium, and barium in liquid ND8, and the perturbations arising from concentration changes, temperature changes, and the presence of inert salts have been investigated. 

Figure 7. IR spectra in the combination/overtone region showing the photochemical reaction of Cr(CO)6 in scC2H4 at ambient temperature. The first spectrum shown in the Figure was recorded just before the UV lamp was switched on, and subsequent spectra were taken at ca. 10 min intervals. The bands labelled are those of Cr(CO)6 and those labelled A show the growth of the final photoproduct Cr(CO)4(C2H4)2. The steady state concentration of the intermediate species Cr(CO)5(C2H4) is relatively low, hence the overtone/combination bands of this species are not observed in this experiment.

Near-infrared reflectance analysis is particularly well suited to chemical sensing because it operates on "as is" samples and yet has chemical specificity. The absorptions observed originate from vibrations of a relatively few chemical groups whose overtones and combination bands appear in the near infrared region. 

There are a number of weak bands in this region. Unresolved shoulders straddle the major band at about 3085 and 2995 cm-1 (see Fig. la, 2a and 3a). These fall in the range generally assigned to overtones and combination bands involving CC stretching fundamentals in the 1810 to 1050 cm-1 region and are responsible for most of the weak absorptions in Fig. 3c, e, and g [35, 36, 37,38, 39], These features should always... 

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. 

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