Dynamic Quantum Isotope Effects on Multiple Proton-Transfer Reactions

Abstract

Abstract In order to investigate static quantum isotope effects on the stability of proton-transferred structures, we defined an effective quantal potential energy hypersurface (EQPES), which includes mass-dependent quantum effects. We demonstrated the difference between the ordinary potential energy surface and EQPES of the double well potential as a simple example. The minimum energy path on EQPES describes the zero-point energy-corrected structures and tunneling motion, which is characterized as a classically forbidden motion. We also performed the EQPES analysis of model proton-transfer reactions in DNA base pairs. It was found that the double proton-transferred structure of an adenine–thymine (AT) pair is quantum mechanically unstable and that of a guanine–cytosine (GC) pair is stable within a least uncertainty regime. In order to investigate dynamic quantum isotope effects on the stability of the proton-transferred structures, we performed quantal cumulant dynamics (QCD) simulations of the GC pair. Results show that the proton-transferred structure of the protonated isotopomer is dynamically unstable though the EQPES analysis predicts its stability. It is relevant to include dynamic effects in the investigation of the quantum isotope effects on geometric stability of systems with a small energy gap between global and metastable structures.