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Science 22 March 2002:
Vol. 295. no. 5563, pp. 2245 - 2247
DOI: 10.1126/science.1068849
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Reports

Steric Effects and Solvent Effects in Ionic Reactions

Colleen K. Regan,1 Stephen L. Craig,2 John I. Brauman3*

Rates of SN2 reactions of chloride ion with methyl- and tert-butyl-substituted chloroacetonitrile were measured by using Fourier transform-ion cyclotron resonance spectrometry to follow the isotopic exchange reaction. Barrier heights for these reactions indicate that steric effects in the gas phase are diminished relative to apparent steric effects in solution. We attribute the increased barrier in solution to a solvation effect. Monte Carlo simulations done using statistical perturbation theory confirm that steric hindrance to solvation contributes to SN2 barriers in solution.

1 Department of Chemistry, Bryn Mawr College, Bryn Mawr, PA 19010, USA.
2 Department of Chemistry, Duke University, Durham, NC 27708, USA.
3 Department of Chemistry, Stanford University, Stanford, CA 94305-5080, USA.
*   To whom correspondence should be addressed. E-mail: brauman{at}stanford.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)