Determination of concerted or stepwise mechanism of hydrogen tunneling from isotope effects: Departure between experiment and theory

Abstract

Isotope substitution is an important experimental technique that offers deep insight into reaction mechanisms, as the measured kinetic isotope effects (KIEs) can be directly compared with theory. For multiple proton transfer processes, there are two types of mechanisms: stepwise transfer and concerted transfer. The Bell–Limbach model provides a simple theory to determine whether the proton transfer mechanism is stepwise or concerted from KIEs. Recent scanning tunneling microscopy (STM) experiments have studied the proton switching process in water tetramers on NaCl(001). Theoretical studies predict that this process occurs via a concerted mechanism; however, the experimental KIEs resemble the Bell–Limbach model for stepwise tunneling, raising questions on the underlying mechanism or the validity of the model. We study this system using ab initio instanton theory, and in addition to thermal rates, we also considered microcanonical rates, as well as tunneling splittings. The instanton theory predicts a concerted mechanism, and the KIEs for tunneling rates (both thermal and microcanonical) upon deuteration are consistent with the Bell–Limbach model for concerted tunneling but could not explain the experiments. For tunneling splittings, partial and full deuteration change the size of it in a similar fashion to how they change the rates. We further examined the Bell–Limbach model in another system, porphycene, which has both stepwise and concerted tunneling pathways. The KIEs predicted by instanton theory are again consistent with the Bell–Limbach model. This study highlights differences between KIEs in stepwise and concerted tunneling and the discrepancy between theory and recent STM experiments. New theory/experiments are desired to settle this problem.