Kinetics and energetics of solvent-mediated proton transfer, electron transfer, and bond dissociation.

摘要

This work has two parts, with the common theme of solvent mediation of reaction rates. Using time-resolved transient absorption spectroscopy, the rates of chemical reactions of photo-excited organic molecules are measured. First, proton transfer reactions within the contact radical ion pairs of photoexcited benzophenones/N,N-disubstituted anilines and photoexcited benzophenones/triethylamine are studied. Comparison with theory indicates that the proton transfer occurs nonadiabatically, such that the proton tunnels from reactant to product state. This tunneling process occurs when the energies of the reactant and product states are equilibrated by reorganization of the solvent. In the second part of this work, the kinetics of photoexcited diphenylmethyl acetates and diphenylmethyl chlorides are studied. Following photoexcitation, diphenylmethyl acetates and chlorides undergo homolytic or heterolytic bond dissociation to form either geminate radical pairs or contact ion pairs. The contact ion pairs collapse to re-form the starting material or diffuse apart. Comparison with predictions made by transition state theory shows that the collapse of the contact ion pair takes place in the polarization caging regime; the solvent must reorganize before the bond can be re-formed. The higher energy species, the geminate radical pair, either diffuses to form free radicals or moves to the lower energy contact ion pair surface. The kinetics of geminate radical pair diffusion reveal an energy of interaction between the two radicals while the conversion of radical pairs to ion pairs can be modeled by nonadiabatic electron transfer theory, indicating that this process is also solvent-mediated. In addition, the coupling between the two diabatic energy surfaces (one covalent, one ionic) is calculated.