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 (KIE_app) were observed to increase with extent of reaction. The KIE_app–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  K_eq = k_f/k_b(ii) ArCH₃⁺˙/B → ArCH₂˙ + BH⁺  k_p(iii) ArCH₂˙ + ArCH₃⁺˙ + B → Products  fast

Resolution of the kinetics into the relevant microscopic rate constants resulted in real deuterium kinetic isotope effects (KIE_real) which are much larger than KIE_app and were observed to increase markedly with decreasing temperature. Values of KIE_real 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.