STRUCTURES, STABILITIES, AND INTERMOLECULAR VIBRATIONAL FREQUENCIES OF SMALL AMMONIA COMPLEXES BY MOLECULAR MECHANICS FOR CLUSTERS ANALYSIS

The electrically based molecular mechanics for clusters (MMC) approach has been applied to complexes of ammonia, including the ammonia dimer, trimer, and tetramer as well as mixed, binary complexes with hydrogen, water, hydrogen fluoride, and hydrogen cyanide. The MMC approach is designed for the generation of potential energy surfaces for weakly interacting assemblies, and equilibrium structures, stabilities, dipole moments, and harmonic frequencies for the intermolecular vibrational modes are obtained directly. This information is reported for the ammonia clusters that were studied and serves to characterize the nature of ammonia's weak interactions. Generally, the good comparison with available spectroscopic data indicates that the weak bonding of ammonia is principally a consequence of its electrical properties and its shape. The trimer and tetramer are found to be symmetric, cyclic structures and the relative orientations of a pair of ammonias progress smoothly from the dimer to the trimer to the tetramer. © 1990 American Institute of Physics.