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Published May 29, 1997 | public
Journal Article

Theory of Rates of S_N2 Reactions and Relation to Those of Outer Sphere Bond Rupture Electron Transfers

Abstract

A model is considered for S_N2 reactions, based on two interacting states. Relevant bond energies, standard electrode potentials, solvent contributions (nonequilibrium polarization), and steric effects are included. A unified approach is introduced in which there can be a flux density for crossing the transition state, which is either bimodal, one part leading to S_N2 and the other to ET products, or unimodal with a less marked energy-dependent separation of the rates of formation of these products. In a unified description an expression is given for the reorganization energy, which reduces in the appropriate limits to the pure S_N2 and ET/bond rupture cases. Expressions are obtained for the S_N2 rate constant and for its relation to that of the concerted electron transfer/bond rupture reaction. Applications of the theory are made to the cross-relation between rate constants of cross and identity reactions, experimental entropies and energies of activation, the relative rates of S_N2 and ET reactions, and the possible expediting of an outer sphere ET reaction by an incipient S_N2-type interaction. Results on the photoelectron emission threshold energies of ions in solution provide some information on a solvation term, and another quantity can be estimated using data from gas phase S_N2 reactions or from quantum chemistry calculations. Also introduced for comparison is an adiabatic model that is an extension of a bond energy−bond order formulation for gas phase reactions.

Additional Information

© 1997 American Chemical Society. Received: November 7, 1996; In Final Form: February 26, 1997. It is a pleasure to acknowledge the support of this research by the National Science Foundation and the Office of Naval Research. The author has benefitted considerably from an extensive correspondence with Jean-Michel Savéant. I am also particularly indebted to John Brauman, Henning Lund, Sason Shaik, and Lennart Eberson for their helpful comments.

Additional details

Created:
August 19, 2023
Modified:
October 23, 2023