Doublet first overtone

Fig,, 3. Doublet separation for the fundamental and first overtone as a function of q... 

The principal saturated hydrocarbon functional groups of concern are methyl, methylene and methyne (—CH3, —CH2—, = CH—). The spectra of typical hydrocarbon mixtures (for example as in gas oil and gasoline) are dominated by two pairs of strong bands in the first overtone and combination regions (5900-5500 cm-1 and 4350-4250 cm-1). These are predominantly methylene (—CH2—). The methyl end groups typically show up as a weaker higher-frequency shoulder to these methylene doublets. 

The doublet that is observed in the range 2860-2700 cm for an aldehyde is a result of Fermi resonance (p. 17). The second band appears when the aldehyde C—H stretching vibration is coupled with the first overtone of the medium intensity aldehyde C—H bending vibration appearing in the range 1400-1350 cm . ... 

The absorption of the single methine proton in the presence of methyl and methylenes is generally too small to be observed and is little mentioned in the literature. In the mid-infrared, the fundamental peak is near 2900 cm , between the methyl and methylene doublets. This would suggest that its first overtone was also in the envelope of small peaks in the 5882-5555-cm- (1700-1800-nm) region. 

The first overtone doublet near 6000 cm (1670 nm) is actually composed primarily of IR-inactive C-H-stretching vibrations. As described in Table 4.1 and Figure 4.1, both of the bands near 6000 cm are composed of a Raman active vibration and a vibration that is neither IR nor Raman active. The benzene NIR spectrum is shown in Figure 4.2. 

The first overtone of a free hydroxyl group in dilute CCI4 solution or a low-density gas is at about 7090 cm (1410 nm). This peak is at different positions for primary, secondary, and tertiary alcohols, as seen in Figure 5.1. Primary and secondary butanols can be split into doublets by rotational isomerization. The splits are better seen in Figure 5.2, in the second derivation spectra of the same spectral region. Maeda et al. observed an additional peak in the first overtone region when they subtracted the spectrum at a lower temperature from that at a higher one. They felt that temperature effects further separated species that were weakly bonded to the carbon tetrachloride solvent and a terminal free OH of a self-associated species. 

As shown in Figure 8.1, primary and secondary amines are distinctly different in the first overtone region near 6600 cm. Primary amines have a doublet, and secondary a single peak. Tertiary amines have no peak as they have no N-H functionality and are not shown in the figure. The asymmetric and symmetric NH-stretching peaks occur at 6553 and 6730 cm (1625 and 1486 nm), respectively, in butyl amine in carbon tetrachloride. The first overtone of secondary amines has only one band near 6530 cm (1530 nm). 

The NH-stretching bands of aromatie amines such as aniline show a doublet in the first overtone region, but it is shifted to lower wavelengths relative to aliphatic amines. Figure 8.2 compares the near-infrared spectrum of aniline in CCI4 with that of n-butyl amine. The asymmetric vibration occurs... 

The assignment of secondary amides has also been studied extensively. A single band between 6711 and 6803 cm (1470 to 1490 nm) for A-methylacetamide and other simple amides was attributed to the first overtone NH stretch. Dilute solutions of A-methylacetamide in either carbon tetrachloride or water have a band at 6711 cm (1470 nm), whereas more concentrated solutions have a doublet in the 6666-6880-cm (1500-1700-nm) range. In CCI4, the bonded doublet appears at concentrations above 1 M, whereas in water the unassociated band is present until about 8 M. 

The first overtone of the S-H group can be seen in Figure 9.1 as the weak peak near 5050 cm k The phosphorus-thiol group (P-SH) shows a shift from the normal thiol overtone and gives rise to a weak doublet at 5076-5002 cm (1970 and 1999 nm)." ... 

From a spectroscopic viewpoint, although the two fundamental absorption bands of CO2 do not present a major problem for flow-cell-based SFC/FT-IR measurements, a broad doublet in the region near 1400 cm does. These bands are assigned to the first overtone of the bend and the (infrared forbidden) symmetric stretch, which exhibit second-order coupling, or Fermi resonance (see Section 1.2). In the supercritical state, this doublet becomes weakly allowed (see Figure 23.8). 

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