title: Tunneling assists the 1,2-hydrogen shift in N-heterocyclic carbenes.
abstract: At room temperature, 1,2-hydrogen-transfer reactions of N-heterocyclic carbenes, like the imidazol-2-ylidene to give imidazole is shown to occurr almost entirely (>90 %) by quantum mechanical tunneling (QMT). At 60 K in an Ar matrix, for the 2, 3-dihydrothiazol-2-ylidene→thiazole transformation, QMT is shown to increase the rate about 10(5)  times. Calculations including small-curvature tunneling show that the barrier for intermolecular 1,2-hydrogen-transfer reaction is small, and QMT leads to a reduced rate of the forward reaction because of nonclassical reflections even at room temperature. A small barrier also leads to smaller kinetic isotope effects because of efficient QMT by both H and D. QMT does not always lead to faster reactions or larger KIE values, particularly when the barrier is small.
authors.affiliation: Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur - 700032, West Bengal (India).
authors.forename: Sharmistha
authors.initials: S
authors.lastname: Karmakar
authors.forename: Ayan
authors.initials: A
authors.lastname: Datta
keywords: N-heterocyclic carbenes; density functional calculations; dimerization; kinetics; quantum chemistry