Simplification of the transition state concept in reactive island theory: Application to the HCN<=>CNH isomerization.

The reactive island (RI) theory of chemical reaction rates is modified so that a precise knowledge of the transition state is not required. This revised form of the theory is applied to a realistic two (J=0) and three degree of freedom model (J≠0) of the isomerization HCN⇄CNH. The detailed reaction dynamics is examined and understood in terms of the cylindrical manifolds embedded in the molecular phase space. Of special interest we find a T-shaped conformer in addition to the HCN and CNH moieties at J=0. Rates of unimolecular decay are examined by numerical simulation and theory. Good agreement is found between theory and simulation except for a case where a substantial amount of phase space is consumed by regular motion. In all cases studied, the theory is a significant improvement over the Rice–Ramsperger–Kassel–Marcus (RRKM) theory, which is up to three orders of magnitude too large. [ABSTRACT FROM AUTHOR]