Low Energy Barrier Proton Transfer in Protonated Benzene−Water Complex

Protonation of the rather weak πH-bonded benzene−water complex is studied at the MP2/6-31+G(d,p) computational level. It is shown that, contrary to the 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 benzenium−water complex occurred through the low-energy barrier. The latter is associated with a transition state resembling a π-complex and resulting from an avoided crossing of the potential energy surfaces describing the dissociation channels of both benzenium−water and benzene−oxonium complexes, respectively.