Ion-molecule recombination and other activationless processes in solution: Foundations of a capture model

A three dimensional model Hamiltonian is used to mimic and interpret the results of full molecular dynamics simulations of an ion-molecule activationless recombination process in a solvent of structureless atoms. By making an adiabatic separation of variables it is shown that the gas phase capture model, suitably modified to incorporate the dynamical role of the solvent motion, can be used also in solution. Specifically, a motion along one uncoupled coordinate describes the capture process. The angular momentum for this coordinate is constant during the approach motion and thereby it provides a suitable criterion for capture. The motion of the approaching reactants is shown to be in the strong coupling adiabatic limit. In this limit there is a combination of two effects: A weak ion-molecule attractive interaction at large separations and a substantial solvation of the ion by the liquid. Thus the solvent is able to follow the motion along the reaction coordinate and to take part in the crossing of the centrifugal barrier. A second implication of the model is the efficient deactivation of the ion pair as a result of nonadiabatic V-T transitions. These transitions are confined to the ion-pair polarization well region, i.e., to the left of the adiabatic region of the centrifugal barrier. If a "solvent-separated" ion pair is formed the recombination process is delayed and the reorganization of the solvent is required to facilitate a successful capture. To model this effect a nonlinear, space dependent, coupling term is used in the model Hamiltonian. Comparison is made throughout between the results of full molecular dynamics simulations, computational results for the model Hamiltonian, and the predications of the adiabatic separation. The role of strong solvation in activationless recombination reactions is discussed in terms of the adiabatic separation and its breakdown. The conclusions are compared, and contrasted, with the case of activated bimolecular reactions.