Y. Georgievskii et Aa. Stuchebrukhov, Concerted electron and proton transfer: Transition from nonadiabatic to adiabatic proton tunneling, J CHEM PHYS, 113(23), 2000, pp. 10438-10450
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].