Solvation ultrafast dynamics of reactions. 14. Molecular dynamics and ab initio studies of charge-transfer reactions of iodine in benzene clusters

Previous experiments have been carried out in this laboratory to investigate the dynamics of iodine-benzene charge-transfer reactions. Both 1:1 solute-solvent complexes and 1:n clusters were studied on the femtosecond time scale with kinetic energy time-of-flight mass spectrometry (Cheng, P. Y.; Zhong, D.; Zewail, A. H. J. Chem. Phys. 1996, 105, 6216). Here, we report theoretical studies of the structure and dynamics of iodine-benzene clusters with direct comparison to experimental findings. In particular, ab initio calculations confirm that iodine binds to benzene near-axially with an energy of 3.5 kcal/mol (MP2/6-311G**), which is consistent with the experimental time scale of the reaction and with the angular and kinetic energy distributions of product fragments. Experimental observations have shown that the dynamics of iodine dissociation under cluster solvation is described by two caging time scales. Using Monte Carlo and molecular dynamics simulations, we find that this effect arises from a fundamental asymmetry in the structure of 1:n iodine-benzene clusters. The benzenes tend to form mouth-shaped structures around the iodine, causing one of the iodine atoms to be more strongly solvated than the other. The dynamics therefore reflect two types of solvation forces, free and bound, in a homogeneous structure, rather than a distribution of structures. The two distinct time scales, femtosecond and picosecond, are a result of caging dynamics in the solvent structure. In this way, caging dynamics may be used as a probe of structural features of solvation.