Femtosecond dynamics of dative bonding: Concepts of reversible and dissociative electron transfer reactions

With fs time, speed, and angular resolution of the elementary steps in elec tron transfer reactions? we report direct observation of reversible and dis sociative processes for dative bonding involving covalent and ionic charact ers. For bimolecular reactions of various donors and accepters we find stro ng correlation between the structure and the dynamics. The dynamics from th e transition state to final products involve two elementary processes, with different reaction times, speed, and angular distributions. For example, f or the R2S . I-2 (R = C2H5) system, it is shown that after charge separatio n, the reversible electron transfer occurs in less than 150 fs (fastest tra jectory) and is followed by the rupture of the I-I bond with the release of the first I-atom in 510 Is. How-ever, the second process of the remaining and trapped I-atom takes 1.15 ps with its speed (500 m/s) being much smalle r than the first one (1,030 m/s). The S-I-I average angle is 130 degrees. T hese findings, on this and the other systems reported here, elucidate the m echanism and address some concepts of nonconcertedness, caging, and restric ted energy redistribution.