Modified transition state theory and negative apparent activation energies of simple metathesis reactions: Application to the reaction CH 3 + HBr → CH 4 + Br

Document Type

Article

Publication Date

3-9-2006

Abstract

A modified transition state theory (MTST) has been developed for gas-phase reactions with "negative barriers". The theory was applied to the reactions CH 3 + HBr(DBr) → CH 4(CH 3D) + Br (1a, 1b), which exhibit negative temperature dependences. Accurate ab initio calculations performed with coupled cluster theory extrapolated to the complete basis set limit revealed a transition state located at -2.3 kJ mol -1 relative to the ground state of the reactants (in reaction la), as well as a shallow bound complex. The negative temperature dependence, the absolute values of the rate constant, and the isotope substitution effect are reproduced with good accuracy (10%), without any adjustment or fitting parameters. Analytical expressions are presented for MTST including angular momentum conservation, centrifugal barriers and tunneling. This analysis uses information about the possibly loose entrance barrier and the transition state but does not invoke a statistical intermediate complex. © 2006 American Chemical Society.

Identifier

33645666197 (Scopus)

Publication Title

Journal of Physical Chemistry A

External Full Text Location

https://doi.org/10.1021/jp054435q

ISSN

10895639

First Page

3110

Last Page

3120

Issue

9

Volume

110

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