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Tunneling Assists the 1,2‐Hydrogen Shift in N‐Heterocyclic Carbenes
Author(s) -
Karmakar Sharmistha,
Datta Ayan
Publication year - 2014
Publication title -
angewandte chemie
Language(s) - English
Resource type - Journals
eISSN - 1521-3757
pISSN - 0044-8249
DOI - 10.1002/ange.201404368
Subject(s) - chemistry , quantum tunnelling , activation barrier , hydrogen , hydrogen atom , kinetic energy , kinetic isotope effect , intermolecular force , thiazole , chemical physics , computational chemistry , crystallography , stereochemistry , molecule , atomic physics , density functional theory , condensed matter physics , physics , organic chemistry , deuterium , alkyl , quantum mechanics
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.