A transition-state based rotational sudden (TSRS) approximation for polyatomic reactive scattering.

A transition-state based rotational sudden (TSRS) approximation for the calculation of differential and integral cross sections is introduced. The TSRS approach only requires data obtained from reactive scattering calculations for the vanishing total angular momentum (J = 0). It is derived within the quantum transition state framework and can be viewed as a generalization and improvement of existing J-shifting schemes. The TSRS approach assumes a sudden decay of the activated complex and separability of the overall rotation and motion in the internal coordinates. Depending on the choice of the body fixed frame, different variants of the TSRS can be derived. The TSRS approach is applied to the calculation of integral cross sections of various isotopomers of the H2O+H→H2+OH reaction, the reverse reaction H2+OH→H2O+H, and the H2O+Cl→HCl+OH reaction. Comparison with accurate close-coupling calculations and established approximate schemes shows that a scattering frame based TSRS approximation yields more accurate results than the centrifugal sudden approximation and standard J-shifting for the H2O+H→H2+OH reaction and all isotopomers studied. For the H2+OH→H2O+H and the H2O+Cl→HCl+OH reactions, the TSRS results as well as the results of the other approximate schemes agree well with the exact ones. The findings are rationalized by an analysis of the different contributions to the moment of inertia matrix at the transition state geometry.