SOLVATION ULTRAFAST DYNAMICS OF REACTIONS - 14 - MOLECULAR-DYNAMICS AND AB-INITIO STUDIES OF CHARGE-TRANSFER REACTIONS OF IODINE IN BENZENECLUSTERS

Previous experiments have been carried out in this laboratory to inves tigate the dynamics of iodine-benzene charge-transfer reactions. Both 1:1 solute-solvent complexes and 1:n clusters were studied on the femt osecond time scale with kinetic energy time-of-flight mass spectrometr y (Cheng, P. Y.; Zhong, D.; Zewail, A. H. J. Chern. Phys. 1996, 105, 6 216). Here, we report theoretical studies of the structure and dynamic s of iodine-benzene clusters with direct comparison to experimental fi ndings. In particular, ab initio calculations confirm that iodine bind s 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 reactio n and with the angular and kinetic energy distributions of product fra gments. Experimental observations have shown that the dynamics of iodi ne dissociation under cluster solvation is described by two caging tim e scales. Using Monte Carlo and molecular dynamics simulations, we fin d that this effect arises from a fundamental asymmetry in the structur e of 1:n iodine-benzene clusters. The benzenes tend to form mouth-shap ed 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 struc ture, 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.