Correlations between Characteristic Frequencies of the Paraffins and the Skeletal Frequencies of Some Structurally Analogous Polar Molecules

Some close analogies are found in the region 1500 to 600 cm—1 between the frequencies of a number of paraffins and their more polar analogs in which CH3 groups are replaced by F, OH, or NH2 groups. The great intensities of the infrared bands of the polar molecules have led to their assignment to skeletal stretching modes; a similar assignment for the frequencies of the paraffins may therefore be deduced. On this criterion the 1053 cm—1 band of propane, the 1170 cm—1 band of isobutane, and the 1249 cm—1 band of neopentane are to be attributed principally to skeletal stretching vibrations. Some apparently contradictory conclusions reached earlier from studies of deuterium substituted propanes and neopentanes are discussed.