Quantum Suppression of Intramolecular Deuterium Kinetic Isotope Effects in a Pericyclic Hydrogen Transfer Reaction

Abstract

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 °C 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.

Large Anharmonic Effects on Tunneling and Kinetics: Reaction of Propane with Muonium

“…Kinetic isotope effects (KIEs) can be used to elucidate mechanisms and, because they are especially sensitive to tunneling and zero-point energy (ZPE), they provide quantitative tests of the reliability of simulations. − Inclusion of quantal effects in simulations of complex processes such as enzyme catalysis, proton-coupled electron transfer, and hydrogen-atom transfer in atmospheric reactions is critical for calculating accurate rate constants and KIEs. ,,,− Here, we demonstrate the importance of reaction-coordinate-dependent anharmonicity for calculating KIEs.…”