A dynamic 15N NMR study of kinetic hydrogen/deuterium isotope and tunnel effects on the triple proton transfer in crystalline 3,5‐dimethylpyrazole

Abstract

AbstractUsing dynamic solid state 15N CPMAS NMR spectroscopy (CP≡cross polarization, MAS≡magic angle spinning) the kinetics of the degenerate intermolecular triple proton and deuteron transfer in the cyclic trimers of 15N‐labeled polycrystalline 3,5‐dimethylpyrazole (DMP) have been studied in a wide temperature range. At high temperatures, rate constants of the various isotopic HHH, HHD, HDD, and DDD transfer reactions are obtained in the millisecond timescale by lineshape analysis of partially deuterated doubly 15N‐labeled DMP. At low temperatures, the kinetics were followed by magnetization transfer methods in the laboratory frame. In order to suppress artifacts arising from 15N‐spin diffusion these experiments were performed on singly 15N labeled DMP, partially diluted in the non‐labeled material. As multiple kinetic hydrogen/deuterium isotope effects on triple proton transfer reactions have not yet been studied these effects are modeled theoretically for the single barrier case involving a concerted proton motion and for the triple barrier case where the three protons are transferred stepwise. The experimental kinetic isotope effects obey the rule of the geometric mean, i.e. kHHH/kHHD ≈︁ kHHD/kHDD ≈︁ kHDD/kDDD ≈︁ 3.6 i.e. ≈︁ kHHH/kDDD ≈︁ 47 at 300 K, which is indicative of a single barrier where all hydrons loose zero‐point energy in the transition state. At low temperatures, strong deviations from an Arrhenius behavior are observed for all isotopic reactions, indicating incoherent triple hydron tunneling processes, which are described in terms of a modified Bell tunneling model.