Rotational transition states: relative equilibrium points in inelastic molecular collisions

Transition states (TSs) are a key ingredient in the understanding of many chemical reactions. We present here a generalization of TS theory towards rotational excitation in molecular collisions, in a multi-dimensional classical Hamiltonian framework. The treatment is based on relative equilibrium, where the two colliding molecules behave as a single rotating solid object. We illustrate the theory with the help of a simple, yet meaningful, model potential for collisions between H2O and H-2, which is of great importance in the astrophysical context. We show that it is the occurrence of a rotational TS that opens up the possibility of significant angular momentum transfer.