Femtosecond real-time probing of reactions. XXI. Direct observation of transition-state dynamics and structure in charge-transfer reactions

Abstract

This paper in the series gives our full account of the preliminary results reported in a communication [Cheng, Zhong, and Zewail, J. Chem. Phys. 103, 5153 (1995)] on real-time femtosecond (fs) studies of the transition state of charge-transfer (CT) reactions, generally described as harpooning reactions. Here, in a series of experimental studies in a molecular beam, and with the help of molecular dynamics, we elucidate the microscopic elementary dynamics and the structure of the transition states for the isolated, bimolecular reaction of benzenes (electron donor) with iodine (electron acceptor). The transition state is directly reached by fs excitation into the CT state of the complex Bz⋅I2, and the dynamics is followed by monitoring the product build up or the initial transition-state decay. We further employed the fs resolution in combination with the kinetic-energy resolved time-of-flight and recoil anisotropy techniques to separate different reaction pathways and to determine the impact geometry. Specifically, we have studied: (1) the temporal evolution of the transition state (τ‡) and of the final products (τ); (2) the product translational-energy distributions; (3) the recoil anisotropy (β) in each channel; (4) the reaction time dependence on the total energy; (5) the dynamical and structural changes with varying CT energy (ionization potential–electron affinity–Coulomb energy). Such a change is made by replacing the electron donor from benzene to toluene, and to xylenes and trimethylbenzenes of different symmetries. We have also studied deutrobenzene as a donor. The reaction mechanism involves two exit channels. The first one (ionic) follows the ionic potential of the CT state. Following the harpooning (Bz+⋅I−2), the transition state [Bz+⋅⋅I−⋅⋅I]*‡ evolves on the adiabatic potential to produce Bz+⋅I− and I products. The second channel (neutral) is due to the coupling of the transition state to neutral, locally excited, iodine repulsive states and, in this case, the products are Bz⋅I+I. The latter process is an intermolecular electron transfer and occurs on an ultrafast time scale of 250 fs, resulting in a greater yield for the neutral channel. Molecular dynamics simulations support this dynamical picture and provide the time scales for trajectories in the transition-state region and in the product valley. The geometry of the transition state is determined from the anisotropy measurements and we found a nearly axial geometry with the iodine axis of recoil tilted 30°–35° away from the transition moment. These angular dependencies are related to the molecular structure and the electronic structure with highest occupied molecular orbit–lowest occupied molecular orbit descriptions. By increasing the level of solvation from the 1:1 complex structure to clusters, we address the dynamics of caging in small and large solvent structures. We also report studies in the liquid phase and compare our results with those from other laboratories in an attempt to unify the nature of the dynamics and structure in going from the isolated gas phase complex to the liquid.