Low energy barrier proton transfer in protonated benzene-water complex

Protonation of the rather weak piH-bonded benzene-water complex is studied at the MP2/6-31+G(d,p) computational level. It is shown that, contrary to t he fact that benzene is more basic than water by 13.5 kcal/mol, the excess proton favors to reside on water forming the benzene-oxonium complex. This complex is formed via the coupled electron-proton transfer from the benzeni um-water complex occurred through the low-energy barrier. The latter is ass ociated with a transition state resembling a pi -complex and resulting from an avoided crossing of the potential energy surfaces describing the dissoc iation channels of both benzenium-water and benzene-oxonium complexes, resp ectively.