Overtone methyl groups

Vibrational overtone spectroscopy has been applied to methyl-group conformational analysis in aromatic molecules (Henry, 1987). In addition to conformational data, very accurate information on the C—H bond lengths are obtained, showing that the methyl C—H bond eclipsing the ring plane is slightly shorter than the other methyl C—H bonds, in excellent agreement with ah initio calculations. 

A band at about 1370 cm is indicative of a methyl group. At what frequencies would the first and second overtones be ... 

In addition to the two methyl peaks near 5790 and 5735 cm, the methyl group in aromatic compounds has a band near 5660 cm, which has been found useful for quantitative analysis. Luty and Rohleder assigned this peak to be due to -i- 26. They also suggest that a peak at 4080 cm" is due to 38s, second overtone of a symmetric bending vibration of the CHj group. This peak is well isolated in compounds having multiple methyl groups such as penta and hexa-methyl benzene. 

The second overtone region (1150-1210 nm or 8264-8696 cm ) has also been used for quantitative measurements, in particular to measure methyl, methylene, methine, and aromatic contributions. The methyl groups of long-chain paraffinic hydrocarbons appear between 8365 and 8375 cm (1194-1195nm). In pentane and hexane, the methyl group absorbs at 8396 cm (1191 nm), in heptane it absorbs at 8388 cm (1192 nm), and in decane it is at 8378 cm (1194 nm). See Figure 2.4. 

The third overtone vibration of the methyl group appears at 10,953 in hexane. The methyl third overtone in some additional molecules is listed in Table 2.2. Wheeler assigns the fourth overtone s position to be at about 13,400 cm (746 nm). Fang and Swofford list the fifth overtone for a linear alkane methyl group C-H stretch to be at 15,690 cm (637 nm), and the sixth at about 17,890 cm" (560 nm). 

The 5800-cm peak is usually the strongest peak in the first overtone region of a series of linear hydrocarbons. Tosi and Pinto provide a formula for locating this peak for a series of linear hydrocarbons 5856-85 x weight-fraction of CH2, or about 5800 cm for hexane. They also mention that this peak splits into two closely spaced peaks possibly due to the influence of adjacent methyl groups. The absorptivity of this combined peak does not regularly increase with chain length, probably because it has contributions from two sources. 

In the second overtones, which are also shown in Figure 2.6, only one methyl and one methylene peak are normally observed at 8389 cm (1192 nm) and 8264 cm (1210 nm), although weaker peaks can be seen at 8673 cm (1153 nm) and 8503 cm (1176 nm) with higher resolution. In higher alkanes above dodecane, the methyl group becomes a shoulder in the methylene peak. ... 

The first overtone of the C-H stretch next to a double bond occurs at a higher wavenumber (lower wavelength) than saturated C-H stretch absorptions. This peak is strong and distinct in some structures, particularly the methylene group of terminal double. In most cases, however, it is weak and difficult to locate especially in the presence of methyl groups. The band position is near 61(X)-6200 cm (1640-1612 nm). 

The third overtone of terminal methylenes C-H stretch is fonnd at 11,390 cm (878 nm) in 1-alkenes. Its absorptivity is 0.002 1/mol-cm. The alkyl acrylate donblet occnrs at 11,905 and 12,500 cm (800 and 840 nm). The peak is at 10,776-11,360 cm (880-928 nm) in alkyl vinyl ethers, and 10,788-10,929 cm" (915-927 nm) in vinyl ethers and vinyl esters. The fourth through seventh overtones of terminal methylene C-H stretch peaks have been studied using photoacoustic spectroscopy." These were given as approximately 11,450, 14,090, 16,630, and 18,800 cm", respectively (873, 710, 600, and 532 nm, respectively). In the spectrum of propylene, this peak is split into a doublet of two peaks that are cis and trans to the methyl group. 

In a study of methyl-substituted pyridines, the aryl regions of the overtones show a simplified structure having one peak progression for each nonequivalent C-H. The methyl regions of the methylpyridines show complex profiles. The band profile in 3- and 4-methylpyridine is similar to that of toluene because the methyl groups of these compounds are free rotors, and all have a low-energy barrier to rotation. However, the methyl band profiles of 2-methylpyridine are complex, and these patterns indicate that vibration-torsional coupling is an important contribntor to the complex structure." ... 

In Fig. 5.4 we can see an example of a methyl group on an aromatic ring. This type of CH3 is somewhat different than the CH3 in an alkane, in that it has two prominent bands near 2925 and 2865 cnT both assigned to the in-phase symmetric CH3 stretch, in Fermi resonance with die CH31460 cm deformation band overtone. The CH3 1375 cm deformation band overtone is clearly seen near 2740 cm , with an intensity somewhat stronger than in... 

Halohydrins may be used as intermediates in protection of olefins as epoxides and there are some instances of the use of halohydrin acetates to protect double bonds. Overton [49] protected an olefin against both oxidative and reductive conditions by use of the halohydrin acetate and Levine and Wall [50] noted that formation of a halohydrin acetate of the A -olefin in (7) caused the Cj i methyl group to be protected in some way against bromination. Bromhydrin acetates can be... 

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. 

Figure 23 A proposal for dephasing in ethanol by solvent-assisted intramolecular vibrational redistribution (IVR). The yym-methyl stretch is initially excited, but rapidly equilibrates with one or more modes within kT (the ayym-methyl stretch and/or CH bend overtones). Dephasing occurs with this rapid equilibration time Tivr- However, significant population remains in the sym-methyl stretch after equilibration. Relaxation from this group of state to lower states causes the final relaxation of the population to zero, which is measured as Tj in energy relaxation experiments. (Adapted from Ref. 7.)...

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