Reaction path Hamiltonian analysis of dynamical solvent effects for a Claisen rearrangement and a Diels-Alder reaction

The solvent effects for a Claisen rearrangement and a Diels-Alder reaction are investigated. Electronic structure methods are used to generate the fre quencies, couplings, and curvatures along the minimum energy paths for thes e reactions in the gas phase and in the presence of two water molecules. Th e geometries and charge distributions along the minimum energy paths are an alyzed to determine the structural and electrostatic roles of the water mol ecules. Reactive flux molecular dynamics methods based on a reaction path H amiltonian are used to calculate the dynamical transmission coefficients, w hich account for recrossings of the transition state. The transmission coef ficients for the Claisen rearrangement are nearly unity both in the gas pha se and in the presence of two water molecules. The transmission coefficient s for the Diels-Alder reaction are 0.95 and 0.89 in the gas phase and in th e presence of two water molecules, respectively. These differences in the t ransmission coefficients are explained in terms of the locations and magnit udes of the curvature peaks along the reaction path, as well as the shape o f the potential energy along the reaction coordinate near the transition st ate. Analysis of the dynamical trajectories provides insight into the dynam ical role of the water molecules and elucidates possible reaction mechanism s.