Perturbations of proton transfer potentials caused by polar molecules

Abstract

The influence of polar molecules on the proton transfer potential within a H-bonded system is studied by ab initio methods, An external HF molecule is placed in various locations in the vicinity of the (H{sub 2}OH {center dot} {center dot} OH{sub 2}){sup +} system. It was found that for most positions the effects on the proton transfer potential could be reproduced very accurately by replacing the full HF molecule by a simple point dipole of magnitude equal to the moment of HF. This result implies that the effects of HF are primarily electrostatic and, further, that attempts to model a large array of solvent molecules by a collection of point dipoles are likely to be successful. The only exceptions occur when H-bonding is possible between HF and (H{sub 2}OH {center dot} {center dot} OH{sub 2}){sup +}. In these cases, the transfer potential is altered to a significantly greater degree by the HF molecule than by the point dipole. It is only when the pertinent H-bonding atoms approach within about 3 {angstrom} that the effects of forces other than electrostatic become noticeable.