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.

Hydride-exchange reactions between NADH and NAD+ model compounds under non-steady-state conditions. Apparent and real kinetic isotope effects

“…(7) and (8). [21][22][23][24][25] The point of importance here is that KIE real depends only on k f and the two experimental steadystate apparent rate constants, (k app H ) s.s. and (k app D ) s.s. . Since small experimental errors are expected in the latter and relatively small errors accompany the determination of k f in the fitting procedure, we expect the error in KIE real values to be of the order of ±10% or less.…”

Novel studies on the electrochemical oxidation of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) in acetonitrile at platinum electrodes

“…(15) should be expressed by a more complex one such as that indicated in Eq. (2) [45]. The fact that an increase in the DPMB concentration accelerated the decay reaction of DPMB Åþ is unequivocal proof of a dimerisation step after the radical cation generation.…”

“…Several studies have been carried out on the reactivity of electrogenerated aromatic cation radicals in aprotic solvents [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Both the radical cation-nucleophile combination reaction [1,[14][15][16][17][18][19] and also the radical cation proton transfer reaction [14,15,[20][21][22][23] have been studied in depth for some time.…”

6 Reaction mechanisms : Part (iii) Radical and radical ion reactions

“…Lu et al investigated the proton transfer reaction between methylphenylanthracene radical cation and 2,6-lutidine by measuring the kinetics of the process using derivative cyclic voltammetry (DCV). 52 The reaction could occur by two possible mechanisms, the first being a second-order single-step proton transfer. The other mechanism involves two steps, the formation of a complex between the radical cation and the base followed by intramolecular proton transfer.…”