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

The kinetics of the proton transfer reactions between the 9-methyl-10-pheny lanthracene radical cation (MPA(+.)) with 2,6-lutidine were studied in acet onitrile-Bu4NBF4 (0.1 M) using derivative cyclic voltammetry. Comparisons o f extent of reaction-time profiles with theoretical data for both the simpl e 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 expe rimental to theoretical data, in the pre-steady-state period, for the compl ex mechanism were observed, In this time period, the apparent deuterium kin etic isotope effects (KIEapp) 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 comple x mechanism resulted in a real kinetic isotope effect (KIEreal) equal to 82 at 291 K. Arrhenius activation parameters (252-312 K) for the reactions of MPA(+.) with 2,6-lutidine in acetonitrile-Bu4NBF4 (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 temper ature range, KIEreal varied from 46 at the highest temperature to 134 at th e lowest. The large KIEreal, along with the Arrhenius parameters. are indic ative of extensive tunneling for the proton transfer steps.