Electrical properties of ammonia and the structure of the ammonia dimer

Abstract

Electrical multipole moments, polarizabilities, and hyperpolarizabilities of ammonia have been calculated with large basis sets through application of derivative Hartree–Fock (DHF) theory. With these properties, the orientational dependence of the electrical interaction between two ammonia molecules has been examined. The electrical interaction model indicates that the orientational angles of the two ammonias with respect to the a-inertial axis are strongly coupled. Internal rotation about the ammonia C3 axes is coupled as well. There are 18 equivalent minimum energy structures for the ammonia dimer and the lowest barrier for inversion is found to be about 250 cm−1. The predicted equilibrium structure differs noticeably from the recently determined experimental structure. Certain of the assumptions used in obtaining the experimental structure, we feel are not justified, in light of the coupling of internal motions apparent in the potential surface.