C-H stretching modes and the structure of n-alkyl chains. 2. Long, all-trans chains
The C-H and C-D stretching bands in the infrared spectra of some crystalline n-alkanes at low temperature (<10 K) have been measured and analyzed. Most of the n-alkanes studied contain 16-22 carbons, and all are in the all-trans conformation. Assignments are proposed for the C-H stretching bands associated with the polymethylene chain and for the C-H bands of deuterium isolated CHD groups remaining in a perdeuterated sample of n-nonadecane. In addition, assignments are proposed for the C-H and C-D stretching bands of CH3, CH2D, CHD2, and CD3 methyl groups. There are complexities in the symmetric C-H stretching band of the polymethylene chain that are due to Fermi resonance interaction with binary combinations of HCH bending modes. As in the Raman case, these complexities can be accounted for with the use of a simple resonance model. The methyl C-H stretching bands show the effects of nonequivalent in- and out-of-plane force constants; the spectra of hydrogen impurity bands in a sample of n-C19D40 exhibit these differences conspicuously. The C-D stretching spectra of n-alkanes with selectively deuterated methyl groups are generally much more complex than the corresponding C-H spectra. This is attributed to the fact that DCD bending fundamentals, whose combinations and overtones are involved in resonances, are themselves numerous and complex because of interaction with other kinds of motion. Specific resonances involving the symmetric and asymmetric stretches of the CH3 and CD3 groups are discussed in detail. © 1984 American Chemical Society.
