Transition path sampling: Rearrangement of cage water hexamer

The hydrogen bond exchange reaction of cage water hexamer has been investigated. With an aid of transition path sampling method, a transition state for this reaction has been found. The transition state ensemble has been investigated as a function of energy, and the distribution of the structures of sampled transition states is consistent with that obtained by other method. From the ab initio calculations, it has been confirmed to be a transition state with one imaginary frequency along the reaction coordinate. The most important structural parameters for the transition are retardation (increase of r _a and O⋯O distance) and C₂ rotation of a water molecule. I obtained rate constants at temperatures ranging from 0 to 300 K considering quantum mechanical tunneling effect. At temperatures below 100 K, the tunneling effect is very important, while at high temperatures, the classical transition state theory approach gives reasonable data. Transition path sampling method is very useful to find transition states for many complicated chemical reactions in which we do not know about transition states or it is hard to find a transition state by quantum mechanical approaches.