Hydrolysis kinetics of photoexcited 6-methoxyquinoline in aqueous acetonitrile solutions

6-Methoxyquinoline undergoes pseudo-first-order hydrolysis and its conjugate acid, second-order proton abstraction by hydroxide ion, in the lowest excited singlet state. The proton transfer kinetics in water containing acetonitrile up to a mole fraction of about 0.1 have been evaluated as a function of acetonitrile concentration. At mole fractions above 0.13 of acetonitrile, proton transfer does not occur. At mole fractions below 0.1 steady-state and pulsed-source fluorimetries show the rate constant for hydrolysis to decrease exponentially with the mole fraction of acetonitrile. This is believed to be due to penetration of the aqueous solvent cage of the 6-methoxyquinoline by acetonitrile rather than to specific solvation by the organic cosolvent. The rate of neutralization of the conjugate acid by hydroxide ion is found to vary only slightly and depends on the bulk dielectric properties of the solvent.