Carbon isotope effects on proton transfers from carbon, and the question of hydrogen tunneling

Abstract

We wish to report evidence that the tunnel effect makes an important contribution to hydrogen isotope effects in proton transfers but does not control the form of the dependence of the isotope effect on the strength of the attacking base.

Kinetic Isotope Effects in Enzymology

“…Recently measured carbon and hydrogen isotope effects in the elimination reactions of 2-phenylethyl-dimethylsulfonium and 2-phenylethyl-trimethylammonium ions vary in distinctly different ways, consistent with calculations based on force constant changes. Although the carbon isotope effect was observed to be normal at the region where kH/kD is maximal (instead of slightly inverse as predicted from models), indi- 38 194 cating a tunnel correction to the isotope effect of approximately 1.015-1.025 for carbon and 1.5-2.0 for hydrogen, these data are consistent with vibrational frequency changes being the primary determinant of the observed variations in the isotope effect (32). The foregoing considerations suggest a small isotope effect on the tunnel correction, QH/QD = 1.5 when AG = 0; this Correction is consistent with isotope effects of 10-1 1, rather than 7, for hydrogen abstraction from carbon.…”

Reactions that involve tunneling by carbon and the role that calculations have played in their study

“…H/D KIEs have proven to be invaluable in studying tunneling by hydrogen in a very wide variety of reactions. 7 -13 As early as 1975, Saunders used 12 C/ 13 C KIEs to show that motion by carbon contributes to the reaction coordinate for tunneling in an E2 reaction; 60 and in 1980 he used 12 C/ 14 C for the same purpose. 61 This section describes several different reactions for which 12 C/ 13 C KIES have been calculated and/or measured more recently.…”

Tunnelling control of chemical reactions – the organic chemist's perspective

“…21 The elimination reactions of 39 and 41 showed that the solvent composition (DMSO and H 2 O) affects the KIE, most likely by influencing the force constants and thus the barrier width. 60,61 Experiments with the respective β-13 C-39 compound allowed breaking down the overall isotope effect into a semiclassic and a QMT part. The authors showed that heavy atom movement is coupled to the reaction coordinate and lowers the k H /k D values; 61 the whole system tunnels rather than just the directly involved proton.…”