Femtosecond Real-Time Probing of Reactions. 24. Time, Velocity, and Orientation Mapping of the Dynamics of Dative Bonding in Bimolecular Electron Transfer Reactions

Abstract

In this paper, we give a full account of our studies of the dynamics of electron-transfer reactions. We examine bimolecular reactions of various donors and acceptors and focus on the reversible and dissociative elementary steps probed directly using femtosecond time, speed, and angular resolutions. In particular, we report studies of the bimolecular systems of the following electron donors:  diethyl sulfide, p-dioxane, acetone, and benzene. The electron acceptors are iodine and iodine monochloride. The general phenomena of reversible and dissociative electron transfer are found for all systems studied. The dynamics of the dative bonding, from the transition state (TS) to final products, involve two elementary processes with different reaction times, speed and angular distributions. For example, for the diethyl sulfide·iodine system, it is shown that after charge separation, the entire complex is trapped in the TS region and the reversible electron transfer occurs in less than 500 femtosecond (lifetime), followed by the rupture of the I−I bond with the release of the first exterior I-atom. However, the second process of the remaining and trapped (caged) interior I-atom takes 1.15 ps with its speed (500 m/s) being much smaller than the first one (1030 m/s). The initial structure is determined to be a nearly linear configuration of S−I−I (165°), consistent with the ab inito calculations and predictions of the HOMO−LUMO frontier orbitals. The observed time scales and bifurcation of the wave packet, with different speeds, are illustrated on the global potential energy surface with the help of molecular dynamics simulations. The findings on this and the other systems reported here elucidate the mechanism and address the concepts of nonconcertedness, caging, and restricted energy dissipation, which are important to the description of reaction mechanisms in the condensed phase, on surfaces, and in electrochemical studies.