Concerted electron and proton transfer: Transition from nonadiabatic to adiabatic proton tunneling

A concerted electron-proton transfer reaction is discussed, in which proton tunneling occurs simultaneously with electronic transition. It is assumed that the potential in which the proton moves is formed by two electronic st ates, which in the absence of their interaction would cross in the region b etween the two minima of the proton adiabatic potential. The proton tunneli ng between the two wells is, therefore, coupled to a switch between the two electronic states. The later occurs only when the proton is in the tunneli ng region under the barrier. A simple analytical expression for the tunneli ng matrix element T-DA is derived, which is uniformly correct for small and large values of the electronic coupling. For small electronic coupling our expression coincides with that obtained in the nonadiabatic theory of prot on-coupled electron transfer reactions. For large electronic coupling the e xpression is reduced to that obtained in the Born-Oppenheimer approximation . The transition from nonadiabatic to adiabatic tunneling is governed by th e magnitude of the Landau-Zener parameter defined for the tunneling process . The obtained result is discussed in the context of the proton tunneling t ime. (C) 2000 American Institute of Physics. [S0021-9606(00)70147-4].