Computational study of structures and proton transfer in hydrogen-bonded ammonia complexes using semiempirical valence-bond approach

In this study, the seGVB method was implemented for the N-H bonding system, specifically for hydrogen-bonded ammonia complexes, and the model well reproduces the MP2 geometries and energetics. A comparison between the ammonia dimer and water dimer is given from the viewpoint of valance-bond structures in terms of the calculated bond energies and pair-pair interactions. The linear hydrogen bond is found to be stronger than the bent bonds in both cases, with the difference in energy between the linear and cyclic structures being comparable in both cases although the N ... H bonds are generally weaker. The energy decomposition clearly demonstrates that the changes in electronic energy are quite different in the two cases due to the presence of an additional lone pair on the water molecule, and it is this effect which leads to the net stabilization of the cyclic structure for the ammonia dimer. Proton-transfer profiles for hydrogen-bonded ammonia complexes [NH2-H-NH2](-) and [NH3-H-NH3](+) were calculated The barrier for proton transfer in [NH3-H-NH3](+) is larger than that in [NH2-H-NH2](-), but smaller than that in the protonated water dimer. The different bonding structures substantially affect the barrier to proton transfer, even though they are isoelectronic systems. (C) 1999 John Wiley & Sons, Inc. Int I Quant Chem 73: 357-367, 1999.