Molecular Rotation in Crystals of N2 and CO

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.