Proton transfer reactions of methylanthracene radical cations with pyridine bases under non-steady-state conditions. Real kinetic isotope effect evidence for extensive tunneling.

Abstract

The kinetics of the proton transfer reactions between the 9-methyl-10-phenylanthracene radical cation (MPA(+)(.)) with 2,6-lutidine were studied in acetonitrile-Bu(4)NBF(4) (0.1 M) using derivative cyclic voltammetry. Comparisons of extent of reaction-time profiles with theoretical data for both the simple single-step proton transfer and a mechanism involving the formation of a donor-acceptor complex prior to unimolecular proton transfer were made. The experimental extent of reaction-time profiles deviated significantly from those simulated for the single-step mechanism, while excellent fits of experimental to theoretical data, in the pre-steady-state period, for the complex mechanism were observed. In this time period, the apparent deuterium kinetic isotope effects (KIE(app)) were observed to vary significantly with the extent of reaction as predicted by the complex mechanism. Resolution of the apparent rate constants into the microscopic rate constants for the complex mechanism resulted in a real kinetic isotope effect (KIE(real)) equal to 82 at 291 K. Arrhenius activation parameters (252-312 K) for the reactions of MPA(+)(*) with 2,6-lutidine in acetonitrile-Bu(4)NBF(4) (0.1 M) revealed E(a)(D) - E(a)(H) equal to 2.89 kcal/mol and A(D)/A(H) equal to 2.09. In this temperature range, KIE(real) varied from 46 at the highest temperature to 134 at the lowest. The large KIE(real), along with the Arrhenius parameters, are indicative of extensive tunneling for the proton transfer steps.