The amide group in a number of methyl N-alkyl and C-alkyl amides has been isolated at low temperature in a hydrocarbon environment. The infrared spectrum of the free amide group has been measured and its characteristic bands assigned. The amide bands for some of the hydrogen bonded n-mers have also been characterized. The isolation was achieved by the condensation of alkyl amide vapor onto the surface of a window at 8 K. Dilution with n-alkane vapor during the deposition was employed to increase the degree of isolation. The as-deposited films at 8 K were completely amorphous. A series of major structural changes occurred while the as-deposited films were annealed. These were monitored by infrared spectroscopy. With increasing temperature, the relative concentrations of the various n-mer species change to favor the higher n-mers. In addition, the hydrocarbon chains undergo conformational ordering, accompanied by a transformation of the amorphous solid to a crystalline solid. The triclinic subcell of the crystal changes to orthorhombic perpendicular with increasing temperature. The onset temperature and the chain-length dependence of the onset temperature for the various solid-solid transformations were compared to the corresponding values reported previously for similarly prepared amorphous films of the n-alkanes.

Bands associated with delocalized vibrational modes were identified in the isotropic Raman spectra of a series of polyglycine oligomers in aqueous solution as zwitterions and as cations. The dependence of these bands on conformational disorder and chain length was determined. The observed dependence is closely mimicked in spectra calculated for a series of corresponding model polypeptides. The simulated spectra were calculated in a skeletal approximation for ensembles of conformationally disordered chains. As the chain length of the conformationally disordered polypeptides increases, the observed isotropic spectra rapidly approach the spectrum of the infinitely long disordered chain. Convergence is nearly complete at the tripeptide for both the zwitterion and the cation. The stimulated spectra behave in essentially the same way. Convergence to the spectrum of the infinitely long chain is much more rapid for the conformationally disordered polyglycines than for the ordered polyglycines because of the mode localization that results from disorder. In the low-frequency region the bands in the calculated spectra have frequencies that are systematically dependent on chain length. These bands are related to the longitudinal acoustic modes of the ordered chain.