PICOSECOND STUDIES OF PROTON-TRANSFER IN CLUSTERS .2. DYNAMICS AND ENERGETICS OF SOLVATED PHENOL CATION

The dissociative proton transfer (DPT) of the phenol cation was studied in molecular clusters of NH3, CH3OH, H2O, and other hydrogen-bonding solvents. These studies confirm the existence of a double well reaction coordinate and lead to estimates of the activation energy, enthalpy, and shape of the potential curves as a function of solvent cluster size. By using two-color picosecond excitation and delayed ionization from an excited neutral proton transfer state, cluster ions were produced nearly exclusively in either the strongly bound inner well or the shallow and extended outer well. We have measured very different reaction efficiencies from these two potential wells. Potential energy surfaces are constructed for stepwise solvation that predict the existence of a double well reaction coordinate and explain the origin of the activation barrier. Upper limits to the barrier height for (NH3)n solvation range from about 1.5 (n = 1) to 1.0 eV (n = 4). The gas-phase bimolecular energy barrier relative to the separated reactant molecules is about 0.6 eV.