Effect of temperature and pressure on proton transfer rate from a photoacid to ethanol solution

The proton dissociation of photoacids is studied as a function of temperature and pressure in liquid ethanol. For this purpose we used a strong photoacid, 5,8-dicyano-2-naphthol (DCN2) (pK_a* ∼ - 4.5 in water), capable of transferring a proton to alcohols. At high temperatures, the proton transfer rate is almost temperature independent, while at low temperatures the rate constant has strong temperature dependence. At relatively low pressures, the proton transfer rate increases with pressure while, at high pressures, the rate constant decreases as the pressure increases. The unusual temperature dependence is explained by a two coordinate stepwise mechanism. The two coordinates are the generalized solvent coordinate and the actual proton coordinate between two oxygen atoms. The pressure dependence is explained using the same model used for the temperature dependence. The decrease of the proton transfer rate at high-pressures denotes the solvent-control limited, while the increase in rate at low-pressures denotes the nonadiabatic limit.