Variation of atomic charges during proton transfer in hydrogen bonds

The point atomic charges in a number of ionic H-bonded systems are studied by ab initio calculations as functions of the proton transfer coordinate. In the proton-bound complexes of water–water, ammonia–ammonia, formamide–water, formamide–ammonia, and dimethylether–ammonia, the net atomic charges were obtained using Mulliken population analysis and from the diagonal elements of the atomic polar tensors calculated at the HF/4–31G and MP2/6–31 + G** levels. The dependence of the atomic charges upon the coordinate of the transferring proton was found to be close (within an error of 0.02 e) to a linear function for intermolecular distances in the 2.5–2.8 Å range. The obtained charge and charge flux dependencies highlight the electron redistribution during the proton transfer process and provide insights into the source of the high infrared (IR) intensities of stretching modes of N? H and O? H bonds undergoing hydrogen bonding. © 1994 by John Wiley & Sons, Inc.