Simulations of ICl-(CO2)n photodissociation: effects of structure, excited state charge flow, and solvent dynamics

Experiments on the photodissociation of ICl(-) within size-selected CO(2) clusters [Nadal, M. E.; Kleiber, P. D.; Lineberger, W. C. J. Chem. Phys. 1996, 105, 504.] differ from analogous experiments on homonuclear systems in two major ways: an early onset of caged, ICl(-)-based products with cluster size peaking at n = 5 and a rapid decline of caging after n = 5 in favor of dissociated, Cl(-)-based fragments. Using an effective Hamiltonian model for the solute electronic structure and nonadiabatic MD trajectories, we have simulated the photodissociation of these clusters and found good agreement between experimental and simulated product distributions. The strong preference of the solvent for chlorine in the initial cluster structure, along with favorable kinematics, leads to the efficient caging at small n. At all cluster sizes, the Cl(-)-based fragments are formed by an adiabatic process and are accompanied by spin-orbit excited iodine. Caging of ICl(-) also occurs on the excited state potential surface, and electronic relaxation is not observed during the 100 ps trajectories. For n > 5, the initially caged products appear to be metastable, decaying to solvent-separated Cl(-)...I pairs that subsequently dissociate, leading to the falloff in the caging fraction in the larger clusters.