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 (H2OH ·· OH2)+ 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 (H2OH ·· OH2)+. 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 Å that the effects of forces other than electrostatic become noticeable.
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