Use of Quantum Methods with Transition State Theory: Application to H-Atom Metathesis Reactions

The use of quantum chemical methods to determine rate constants for some H-atom metathesis reactions using transition state theory and tunneling corrections is explored. Comparisons are made among several methods (DFT, MP2, QCISD), all of which yield similar structures and frequencies for the transition states, but quite different barrier heights. Tunneling corrections are made using either the well-known Eckart method or one based on the WKB approach. We find that we can fit the extant data by varying the barrier heights using either tunneling approach, although the WKB method is both more accurate and more labor intensive. Values of the barrier heights obtained this way are not in good agreement with those obtained from any of the quantum methods.