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Anomalous kinetic hydrogen isotope effect in the oxidation of formate by bromine: Tunneling via an intermediate
Author(s) -
Brusa M. A.,
Colussi A. J.
Publication year - 1982
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550141105
Subject(s) - chemistry , kinetic isotope effect , quantum tunnelling , kinetic energy , arrhenius equation , formate , bromine , transition state theory , aqueous solution , decomposition , hydrogen , deuterium , kinetics , reaction rate constant , activation energy , atomic physics , catalysis , physics , organic chemistry , quantum mechanics
Rates of oxidation of XCOO − (X = H, D) by Br 2 in acid aqueous media were measured between 274 and 332 K. The derived Arrhenius parameters for both reactions\documentclass{article}\pagestyle{empty}\begin{document}$$\log k_{\rm H} (M^{- 1} {\rm s}^{{\rm - 1}}) = (11.18 \pm 0.10) - (14.33 \pm 0.13)/\theta $$\end{document}\documentclass{article}\pagestyle{empty}\begin{document}$$\log k_{\rm D} (M^{- 1} {\rm s}^{{\rm - 1}}) = (13.77 \pm 0.13) - (17.62 \pm 0.04)/\theta $$\end{document}where θ = 4.575 T × 10 −3 kcal/mol, with ( k H / k D ) 298K = 2.85, reveal a primary isotope effect, but the difference ( E D ‐ E H ) = 3.29 kcal/mol and the ratio A D / A H = 91 fall beyond the limits imposed by semiclassical transition‐state theory, suggesting tunneling or a multiple‐stage mechanism. However, it can be shown that either tunneling in a single step or a three‐step, internal return mechanism can be ruled out as alternative models, since both require unreasonable kinetic parameters to fit the data. The simplest scheme accounting for the present observations involves tunneling in the decomposition of a charge transfer complex in equilibrium with the reactants.