A THEORETICAL-STUDY OF THE PROTON-BOUND AMMONIA DIMER

A detailed investigation into the structure, energetics, and charge distribution of the cation H3N-H...NH3+ is reported. We have employed nb initio molecular orbital calculations to investigate the optimal geometry and electronic structure of this and related systems. Using the HF/6-311++G** level of theory, N3H7+ is found to be asymmetrically protonated, in line with previous findings. At this level, the hydrogen bond strength is predicted to be 95.8 kJ.mol(-1), and the protonation energy of a model ''proton sponge'' is predicted to be 1051.7 kJ.mol(-1). Decomposition of the properties of these systems into atomic contributions; using subsystem quantum mechanics, allows us to identify the underlying 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 ultimately comes from stabilization of the NH4+ fragment. We also demonstrate that the high proton affinity of a proton sponge comes solely from the [N-H...N](+) moiety, with important factors being loss of repulsion between nitrogens and increased electronegativity of nitrogens.