Unusual inverse secondary isotope effects: A statistical study of CD2HNC and CH3NC isomerization

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Abstract
Two formulations of microcanonical transition‐state theory are used to compute isomerization rate constants k(E,J=0) for CD2HNC and CH3NC. The first of these uses a Monte Carlo method to approximate the classical flux integral expression for k(E,J=0). This method allows a direct comparison to be made with classical trajectory results. The second approach uses a quantum, harmonic state counting method to evaluate k(E,J=0) within the familiar RRKM formalism. The results of the transition‐state theory calculations differ at low energies, but at high energies an unusual inverse secondary isotope effect is observed in both. This is analyzed in terms of the RRKM formalism and it is concluded that the presence of a methyl free‐rotor mode at the transition state is responsible for the onset of the inverse isotope effect. A similar, but somewhat larger inverse isotope effect has been observed in high‐energy classical trajectory calculations.