Molecular Rotation in Crystals of N2 and CO

Abstract

Molecular rotation in low‐temperature crystals of nitrogen and carbon monoxide is investigated. The following contributions to the crystal field hindering free rotation are taken into account: (a) the interaction of the molecular quadrupoles; (b) the effect of the anisotropic molecular polarizabilities; and (c) the short‐range repulsive forces. The results of a theoretical calculation of the minimum‐energy configuration at 0°K support the generally accepted structure; it is shown that the crystal field in this structure is axially symmetric at each lattice site. The Schrödinger equation for a rigid rotor in the crystal field is solved for the lowest rotational states of N₂ and CO. It is found that a classical treatment of the rotation is not justified for these crystals due to the large separations of the lowest eigenvalues. The zero‐point energy of rotation is found to be about half as large as the zero‐point vibrational energy for these crystals.