Non-steady-state kinetic studies of the real kinetic isotope effects and Arrhenius activation parameters for the proton transfer reactions of 9,10-dimethylanthracene radical cation with pyridine bases†

Abstract

The kinetics of the reactions between 9,10-dimethylanthracene radical cation and 2,6-diethylpyridine (DEP) in dichloromethane–Bu4NPF6 (0.2 M) as well as that with 2,6-dimethylpyridine (LUT) in acetonitrile–Bu4NPF6 (0.1 M) were studied at temperatures ranging from 252 to 312 K. In the time period before steady-state was reached for both reaction systems at all temperatures, the apparent deuterium kinetic isotope effects (KIEapp) were observed to increase with extent of reaction. The KIEapp–extent of reaction profiles provide strong evidence for a two-step mechanism [eqns. (i),(ii)] consisting of reversible complex formation prior to rate determining proton transfer.(i) ArCH3+˙ + B ⇌ ArCH3+˙/B Keq = kf/kb(ii) ArCH3+˙/B → ArCH2˙ + BH+ kp(iii) ArCH2˙ + ArCH3+˙ + B → Products fast Resolution of the kinetics into the relevant microscopic rate constants resulted in real deuterium kinetic isotope effects (KIEreal) which are much larger than KIEapp and were observed to increase markedly with decreasing temperature. Values of KIEreal ranged from 62 to 247. It is concluded that a significant degree of quantum mechanical tunneling is involved for both reaction systems. Activation parameters for apparent and microscopic rate constants are discussed with reference to the proton tunneling effect.