Intermolecular Forces and Vibrational Spectra of Crystalline Benzene and Naphthalene

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Abstract
Theoretical calculations are presented for the splittings, caused by intermolecular forces, of vibrational spectroscopic transitions in crystalline benzene and naphthalene. Semi-empirical potential functions for the interactions between pairs of molecules are assumed. The interaction between two molecules is further assumed to be composed of central forces acting between pairs of atoms, one on each molecule. Several semi-empirical potentials for hydrogen-hydrogen, hydrogen-carbon and carbon-carbon interactions are used in the computations. The infrared spectra of single crystals of naphthalene have been remeasured, and the observed splittings are compared with the theoretically calcualated values; comparisons are also made for benzene. It is concluded that short-range potentials similar to the ones assumed are primarily responsible for the observed splittings, that both hydrogen-hydrogen and hydrogen-carbon interactions are important, and that carbon-carbon interactions are only of marginal importance. Although one specific set of potential parameters gave the best results, it was not possible to refine the parameters significantly.