Photoinduced electron and proton transfer in phenol and its clusters with water and ammonia

Ab initio (RHF, MP2, CASSCF, and CASPT2) calculations have been performed for the electronic ground and lowest excited singlet states of phenol, the complexes of phenol with water and ammonia, and the corresponding cations. In agreement with recent experiments it is found that proton transfer is a barrierless process in the phenol-(H₂O)₃ and phenol-NH₃ cations, whereas no proton transfer occurs in the phenol-H₂O cation. Novel aspects of the reaction dynamics in the excited-state manifold of the neutral clusters are revealed by the calculations. Predissociation of the S₁(ππ*) state by a low-lying ¹πσ* state leads to a concerted electron and proton-transfer reaction from the chromophore to the solvent. The excited-state reaction is endothermic in phenol-H₂O and phenol-(H₂O)₃ clusters but exothermic (though activated) in the phenol-NH₃ complex. These results substantiate recent reinterpretations of spectroscopic and kinetic data on hydrogen-transfer reactions in phenol-ammonia clusters. The close relationship of the concerted electron-proton-transfer process in phenol-water complexes with the formation of hydrated electrons in the photolysis of phenol and tyrosine in liquid water is pointed out.