Abstract
Effects on the rate constant k, and on the temperature independent and temperature dependent factors of kh/kD, of variations in the eigenvalue (λ1) and eigenvector (L1) associated with the reaction coordinate have been studied by computation for the reaction H+Cl2 → HCl+Cl. A slightly bent transition state was assumed and its diagonal force field maintained invariant at partial bond levels. The properties of the transition state were varied by systematic manipulation of the three off-diagonal force constants. Values of the several temperature dependent quantities calculated were controlled largely by the interaction force constant between the H···Cl stretch and the H···Cl···Cl bend. Curving the potential barrier affects drastically the range of reaction coordinate ``compositions'' which correspond to acceptable transition states, but neither k nor kH/kD is sufficiently sensitive to barrier curvature to allow an estimate of a ``best value'' for λ1 by comparison of calculated and experimental values. Eigenvector variations have similarly modest effects on k, but the sensitivity of kH/kD seems enough greater that it could be used for model refinement if experimental results of moderate precision were available. Both these sensitivities would be enhanced if the transition state diagonal force field were treated as a parameter in concert with L1. The hypothesized general inverse correlation between isotope effects and reaction rates is shown to be limited by temperature.
Published Version
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