| Converged quantum mechanical partition functions for H2O 2 and its isotopologs have been calculated, which allow for benchmark calculations of hindered rotor (or torsion) methods to be carried out on these molecules. Various moment-of-inertia schemes and partition function formulas for the torsional mode have been tested. The results of this study are applied to three difficult systems.;The first application is the study of the H2S + OH → SH + H2O reaction. Previous theoretical studies have consistently underestimated the rate constants compared to experiment. The use of torsional methods and new density functionals cause the theoretical calculations to agree with the experimental results for the first time.;The second application is the study of the H2O2 + H → OH + H2O and H2O2 + H → HOO + H2 reactions. These reactions play an important role during the combustion of hydrogen, which has been proposed as an alternative fuel source. The torsional methods have been used while calculating rate constants, leading to an agreement between theory and several of the experimental studies.;The final application is the calculation of the 12C/ 13C kinetic isotope effect (KIE) for the CH4 + OH → CH3 + H2O reaction. Only the use of appropriate torsional methods allows for the limited experimental KIE data to be reproduced and for the temperature dependence to be properly predicted. |
