For the past 60 years, the standard model for the interpretation of the mechanism for proton transfer has been based upon transition-state theory, which posits that the transition state is found in the proton transfer coordinate involving the breaking and making of bonds. However, the observed dynamics of proton transfer within the triplet contact radical ion pair, derived from a variety of substituted benzophenones complexed with N,N-dimethylaniline, cannot be accounted for within the standard model for proton transfer. Instead, the kinetic behavior is in accord with nonadiabatic proton transfer theory that has the transition state in the solvent coordinate. Evidence for the importance of the solvent coordinate comes from the existence of an inverted region; as the driving force for reaction increases, the rate of proton transfer decreases. This kinetic behavior is not found in the standard model. The present paper employs density function theory to examine the question as to whether the inverted region can be attributed to the transition state being in the solvent coordinate or whether the inverted region is an artifact produced by changes in the structure of the triplet contact radical ion pair with the placement of substituents upon the p,p′ positions of benzophenone. It is concluded that the inverted region is not an artifact of substituent effects upon structure. These results support the conclusion that the transition state for proton transfer resides in the solvent coordinate and challenges the validity of the standard model for interpreting the mechanism of proton transfer. Copyright © 2014 John Wiley & Sons, Ltd.
The dynamics of proton transfer within a variety of substituted benzophenones–N-methylacridan contact radical ion pairs in benzene were examined. The correlation of the rate constants for proton transfer with the thermodynamic driving force reveals both normal and inverted regions for proton transfer in benzene. Employing the isotopically labeled compounds N-methyl-d3-acridan and N-methylacridan-9,9-d2, the kinetic deuterium isotope effects were examined. The isotope dependence for the transfer process was examined within the context of the Lee–Hynes model for non-adiabatic proton transfer. The theoretical analysis of the experimental data suggests that the reaction path for proton–deuteron transfer involves tunneling. Conventional transition-state theory with the inclusion of the Bell correction for tunneling in the region of the transition state cannot account for the observed kinetic behavior. Copyright © 2004 John Wiley & Sons, Ltd.
