Control of proton-transfer reactions with external fields

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Abstract
The possibility of controlling the tunneling of a proton in a condensed phase with the use of static or time varying external fields, which couple to the transition dipole moment of the tunneling proton, is investigated. Starting from a Hamiltonian, an equation of motion describing the tunnel dynamics of the proton as a stochastically modulated, externally driven, two-level system is derived under suitable restrictions. For external fields that satisfy a precise connection between frequency and amplitude, whereby the resulting Floquet eigenvalues (quasienergies) are degenerate, tunneling can be suppressed in the absence of the medium. With the medium present, we examine the consequences to this tunnel suppression. Static fields, if sufficiently strong, can also suppress tunneling. Expressions are derived for the effect of a static external field on the medium-influenced, tunnel-rate constant. The rate constant can be enhanced or decreased, depending on the sizes of the medium-reorganization energy and external field and the latter’s direction relative to the tunnel system. It is demonstrated that proton tunneling in dicarboxylic acids would be a good candidate to exhibit a proton-transfer rate dependent on the relative orientation of the external field and proton tunnel system.