Quantum Suppression of Intramolecular Deuterium Kinetic Isotope Effects in a Pericyclic Hydrogen Transfer Reaction
mediaposted on 11.03.2019, 00:00 by Xiao Li, Darrin M. York, Matthew P. Meyer
It is generally accepted that hydrogen tunneling enhances both primary and secondary H/D kinetic isotope effects (KIEs) over what would be expected under the assumptions of classical barrier transition. Previous studies have exclusively shown that the effects of tunneling upon primary H/D KIEs have been much larger than those observed for secondary H/D KIEs. Here we present a series of experimental H/D KIE results associated with the Chugaev elimination of methyl xanthate derived from β-phenylethanol over the temperature range of 180 to 290 °C. Intramolecular H/D KIEs computed according to classical transition state theory (TST) are markedly overestimated relative to experimentally measured values. Experimental intermolecular H/D KIEs and direct dynamic calculations based on canonical variational transition state theory (CVT) with small-curvature tunneling correction (SCT) reveal that this result is largely the consequence of extraordinary tunneling enhancement of the secondary H/D KIE. This unexpected behavior is examined in the context of other similar hydrogen transfer reactions.
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canonical variational transition state theorybarrier transitionCVThydrogen transfer reactionstunneling enhancementisotope effectsTSTIntramolecular Deuterium Kinetic Isotope EffectsPrevious studiesPericyclic Hydrogen Transfer Reactionquantum SuppressionChugaev eliminationhydrogen tunnelingSCTtransition state theorymethyl xanthateβ- phenylethanoltemperature rangesmall-curvature tunneling correctionKIE