A THEORETICAL-STUDY OF THE PROTON-BOUND AMMONIA DIMER

A detailed investigation into the structure, energetics, and charge di stribution of the cation H3N-H...NH3+ is reported. We have employed nb initio molecular orbital calculations to investigate the optimal geom etry and electronic structure of this and related systems. Using the H F/6-311++G* level of theory, N3H7+ is found to be asymmetrically prot onated, in line with previous findings. At this level, the hydrogen bo nd strength is predicted to be 95.8 kJ.mol(-1), and the protonation en ergy of a model ''proton sponge'' is predicted to be 1051.7 kJ.mol(-1) . Decomposition of the properties of these systems into atomic contrib utions; using subsystem quantum mechanics, allows us to identify the u nderlying reasons for the high stability of N2H7+ when compared to its constituents. We show that charge transfer, as well as the proximity of the nitrogens, contributes to the hydrogen bond strength, which ult imately comes from stabilization of the NH4+ fragment. We also demonst rate that the high proton affinity of a proton sponge comes solely fro m the [N-H...N](+) moiety, with important factors being loss of repuls ion between nitrogens and increased electronegativity of nitrogens.