Tunneling Assists the 1,2‐Hydrogen Shift in N‐Heterocyclic Carbenes |
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| Authors: | Sharmistha Karmakar Dr Ayan Datta |
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| Affiliation: | Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur ‐ 700032, West Bengal (India) |
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| 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 105 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. |
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| Keywords: | density functional calculations dimerization kinetics N‐heterocyclic carbenes quantum chemistry |
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