Intermolecular Coupling of Vibrations in Molecular Crystals. III. Intermolecular Potential and Crystal Structure of Ethylene

Intermolecular force constants and vibrational frequency splittings are calculated for ethylene and deuteroethylene on the assumption of certain types of intermolecular potentials. The results of a calculation based on intermolecular hydrogen repulsions as the mechanism of vibrational coupling are in reasonable agreement with the observed band splittings. To a limited extent electrostatic (dipole) interactions are of importance. In the calculations detailed consideration of the as yet uncertain crystal structure were necessary, and the results lend support to the P21/n space group for ethylene at 65°K. The hydrogen repulsion parameter obtained from analysis of the data was found to be consistent with a theoretically obtained result and with values obtained from independent experimental studies. A certain amount of information about the variation of repulsive potential with distance is now available from crystal studies.