Energy Deposition in SN2 Reaction Products and Kinetic Energy Effects on Reactivity

Document Type

Article

Publication Title

Journal of Physical Chemistry A

Department

Chemistry

ISSN

1089-5639

Volume

106

Issue

13

DOI

10.1021/jp0141070

First Page

3804

Last Page

3813

Publication Date

3-20-2002

Abstract

Translational energy dependence of reaction kinetics and internal energy depositions in reaction products were examined for gas-phase SN2 reactions of Cl- and F- with NCCO2CH3 and CF3CO2CH3 in a Fourier transform ion cyclotron resonance mass spectrometer (FTICR). The energy dependence of the kinetics was examined by measuring the SN2 rate constants as a function of reactant ion translational energies and by comparing the experimental results with those calculated using RRKM theory. Negative kinetic energy dependence was observed, and the changes of rate constants agree with RRKM theory predictions. The internal energy deposition in the SN2 products was examined by measuring the fraction of secondary fragments resulting from dissociation of the ionic SN2 product. A statistical model was used to calculate the statistical fraction of internally excited SN2 product ion under thermal conditions. For the least exothermic reaction (<20 kcal/mol), the measured fraction of secondary ion is comparable to the statistical fraction of internally excited ionic product. For a highly exothermic reaction (>40 kcal/mol), the internal energy deposition in the ionic SN2 product is significantly lower than the statistical value, indicating that most of the excess energy is trapped in the neutral product. Additional translational energy has little effect on the observed fractions of secondary ions.

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