The Molecular Structure and Thermodynamics of Propane The Vibration Frequencies, Barrier to Internal Rotation, Entropy, and Heat Capacity

A substantially complete picture is proposed for the vibrational and torsional motions of the propane molecule. The vibrational assignment of Wu and Barker is extended and slightly revised so that definite values are assigned to all normal modes. It is believed that, with possibly one exception, all are correct within 2 percent. Comparison with the experimental entropy and heat capacity shows excellent agreement. The free energy, heat content, and heat capacity functions are then calculated over the range 180 to 1500°K. By obtaining the fundamental frequency of one torsional mode from a combination band in the infra-red spectrum, it was possible to evaluate the other from the entropy. The force constants calculable therefrom indicate that the apparent increase of potential barrier from ethane to propane arises from interaction of methyl group hydrogen atoms. For that mode in propane where these distances remain larger the torsional force constant is the same as that for ethane.