Infrared study of the amide group isolated in an amorphous hydrocarbon matrix at low temperature: Aggregation and rearrangement upon annealing

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.