Torsional anharmonicity in transition state theory calculations.

We present a new application for the Torsional Path Integral Monte Carlo (TPIMC) method in which the TPI partition functions are introduced into the calculation of Transition State Theory (TST) rate constants. In this way, an explicit treatment of torsional anharmonicity is included in the TST calculations and the magnitude of these effects can be assessed. The new method is tested on the C(2)H(6) + H hydrogen abstraction reaction and concerted hydrogen transfer in the carbonic acid dimer, for which we have developed torsional potential energy surfaces. For the C(2)H(6) + H reaction the rate constants are halved at room temperature on including a treatment of torsional anharmonicity, while the effects are found to be much smaller for the hydrogen transfer reaction in the carbonic acid dimer.