Tunnelling corrections in hydrogen abstractions by excited‐state ketones

Abstract

Hydrogen abstraction from 1‐phenylethanol by triplet acetophenone occurs from both CH and OH bonds. The reaction path of the Interacting‐State Model (ISM) is used with the Transition‐State Theory (TST) and the semiclassical correction for tunnelling (ISM/scTST) to help rationalizing the experimental kinetic results and elucidate the mechanisms of these reactions. The weak exothermicity of the abstraction from the strong OH bond is compensated by electronic effects, hydrogen bonding and tunnelling, and is competitive with the more exothermic abstraction from the α‐CH bond of 1‐phenylethanol. The alkoxy radical formed upon abstraction from OH reacts within the solvent cage and the primary product of this reaction is 1‐phenylethenol. The corresponding kinetic isotope effect is ca. 3 and is entirely consistent with a tunnelling correction ca. 9 for H abstraction. We therefore demonstrate that the tunnelling correction is the major contributor to the kinetic isotope effect. Copyright © 2010 John Wiley & Sons, Ltd.

A pictorial representation of zero‐point energy and tunnelling contributions to primary hydrogen isotope effects

“…The measurements have raised the question of whether enzymes might facilitate tunnelling. With some notable exceptions45–47 it seems fair to say that this question has been addressed more actively by computational or theoretical chemists26, 33, 48 than by organic or inorganic chemists undertaking measurements of isotope effects for non‐enzymatic reactions. It would be a useful upshot of the symposium therefore if more experimentalists were encouraged to take an interest in the field.…”

Analytical solution for time-resolved photoacoustic calorimetry data and applications to two typical photoreactions

“…The enthalpy of the hydrogen abstraction step was recently estimated using thermochemical cycles, and is DH H = -32.6 kcal mol -1 . 26 The enthalpy of the nonreactive quenching directly back to the acetophenone ground state is just its triplet energy in nonpolar solvents, DH Q1 = E T = 73.5 kcal mol -1 . 27a The enthalpy of the nonreactive quenching to 1-phenylethenol is The distinction between these mechanisms can be made on the basis of their transient heat decays but requires accurate PAC data analysis.…”

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

“…[38]; *: enzyme catalyzed H-atom transfers from ref. [40]. The line refers to the ideal correlation.…”