The effect of intermolecular interactions on the electric field gradients in solid ammonia, tetrazole and imidazole

Abstract

The electric field gradients (efg) at the nitrogen and hydrogen nuclei in hydrogen bonded ammonia clusters, as well as the nitrogen efg's in H-bonded tetrazole and imidazole, calculated by ab initio self-consistent field (SCF) techniques, are reported. The monomer to cluster shifts in the efg's that were calculated for ammonia clusters and a tetrazole dimer were decomposed into electrostatic, polarization, exchange and charge transfer contributions according to the Morokuma method. The results of electrostatic model calculations on ammonia, tetrazole and imidazole are also presented, where a single molecule, treated quantum mechanically, is surrounded by a point charge distribution that represents the molecules in the crystal lattice. The results suggest that while next nearest neighbour interactions in the crystal lattice are relatively unimportant, the intermolecular exchange, charge transfer and polarization that are associated with the direct H-bond interaction are important in understanding the gas phase to solid state nitrogen efg shifts that are observed for these compounds. In contrast with the nitrogen efg, the crucial factor responsible for the large observed shift in the ²H efg in ammonia is shown to be the increase in the monomeric N-H distance as ammonia forms a solid.