Saturation effect in the temperature dependence of a proton recombination with a photobase

The temperature dependence of the reaction of a proton recombination with a photobase is studied in glycerol-water mixtures. For this purpose, we used a strong photobase, 7-hydroxy-4-methyl coumarin (4CUOH). The experimental data are analyzed using the bimolecular irreversible Smoluchowski theory. At high temperatures, the proton recombination rate is almost temperature independent, whereas at low temperatures, the rate constant has strong temperature dependence. The unusual temperature dependence is explained using proton-tunneling theory, based on the Landau-Zener curve crossing formulation. The high-temperature behavior of the rate constant denotes the nonadiabatic limit, whereas the low-temperature behavior denotes the adiabatic limit. We used an approximate expression for the proton-transfer rate, which bridges the nonadiabatic and the solvent controlled adiabatic limit to fit the temperature dependence curve of the experimental proton-transfer rate constant.