Faster Dissociation:  Measured Rates and Computed Effects on Barriers in Aryl Halide Radical Anions

Norihiko Takeda, Pavel V. Poliakov, Andrew R. Cook, and John R. Miller*
Contribution from the Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000
J. Am. Chem. Soc., 2004, 126 (13), pp 4301–4309
DOI: 10.1021/ja0389671
Publication Date (Web): March 13, 2004
Copyright © 2004 American Chemical Society

Abstract

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Carbon−halogen bond dissociation rates for a series of aryl halide radical anions (ArX-:  X = Cl, Br) in NMP were measured at room temperature by pulse radiolysis with 10-11 s time resolution. To obtain accurate dissociation rates, care was taken to measure and correct for competing decay channels. The observed rates correlated well with activation energies computed in the gas phase by density functional (DFT) calculations. The rates did not correlate well with electron affinities or dissociation energies obtained by the same computational methods, although such correlations are reported in the literature and are expected on the basis of simple models. The calculations also found that the transition state structures had bent carbon−halogen bonds. Bending enables large reductions of the activation energies by an electronic effect involving mixing of π* and σ* states. This bending-induced mixing is computed to increase the dissociation rates by a few orders of magnitude and is thus essential to understanding these reactions.

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History

  • Published In Issue April 07, 2004
  • Received October 9, 2003

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