Tunnelling in low-temperature hydrogen-atom and proton transfers

Abstract

The reaction path of the interacting-state model with the Lippincott-Schroeder potential for hydrogen bonds, is used in transition-state theory calculations with the semiclassical correction for tunnelling (LS-ISM/scTST) to estimate proton and hydrogen-atom transfer rates at low temperatures. Down to 100 K, the semiclassical correction leads to semi-empirical rates and isotope effects that are in good agreement with the thermal tautomerism of porphine, and the excited-state tautomerisms of salicylideneanilines and 2-(2 0 -hydroxyphenyl)benzoxazole. For lower temperatures, the tunnelling corrections become extremely high and unreliable. It is shown that the permeability of an Eckart barrier fitted to the curvature of the LS-ISM reaction path leads to good estimates of these reaction rates down to 2 K. q

Understanding Chemical Reactivity: The Case for Atom, Proton and Methyl Transfers

“…[36];~: intramolecular H-atom and proton transfers from ref. [39] and proton transfers from Table 3 of ref. [34] lozenges: methyl transfers from Figure 6 of ref.…”

A chemical understanding for the enhanced hydrogen tunnelling in hydroperoxidation of linoleic acid catalysed by soybean lipoxygenase‐1

“…It increases with the propensity of the 'ligands' A and C to participate in partial electron transfer with the transition state (low I P and/or high E A ), but otherwise leads to no stabilization (high I P and/or low E A ). The electronic saturation further stabilizes the transition state and was introduced in the ISM reaction coordinate through a modification of the reactant and product Morse curves [9,17,18,25,26] V…”

Tunnelling corrections in hydrogen abstractions by excited‐state ketones

“…We recently developed a semi‐empirical model that gives barrier shapes in very good agreement with those of ab initio calculations for small systems, but that can be employed to calculate the barriers of polyatomic systems without added difficulty. This model, named Intersecting/Interacting State Model (ISM), has been described in detail elsewhere 5–9, 17, 18, 20–22. Here we present only a brief description of ISM with emphasis on the factors that contribute to tunnelling corrections.…”

“…Over 30 years ago, one of us proposed that tunnelling should play a major role in the mechanism of photochemical H‐abstractions 3, 4. Here we present new data on such reactions and include them in the general approach recently developed for the calculation of H‐transfer rates in a very wide variety of systems, from gas phase to solution and to enzymes 5–9. It is not our intention to review the controversies in the literature on the adequacy of thermal activation versus nuclear tunnelling models in the description of photochemical H‐abstractions.…”