The dissociative proton transfer (DPT) of the phenol cation was studied in molecular clusters of NH3, CH30H, H20, 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 (NH,), solvation range from about 1.5 ( n = 1) to 1 .O eV (n = 4). The gas-phase bimolecular energy barrier relative to the separated reactant molecules is about 0.6 eV.
The rate of proton transfer from the acidic S1 state of phenol to the basis solvent (NH3)n was measured as a function of solvent cluster size n. A distinct reaction threshold was observed for solvent size n=5 for 266 nm picosecond excitation. The proton transfer rate was measured to be ka=(60±10 ps)−1 for n=5–7. A competitive recombination rate of k−a =(350±100 ps)−1 occurs for n=5. Additional solvation stabilizes the product side causing the reaction enthalpy and consequently k−a to decrease. No evidence of proton transfer was observed when phenol was seeded in the less basic solvent clusters (CH3OH)n and (H2O)n.
Recently, we reported the direct detection of two distinct and stable reactive complexes confirming a double-minima barrier process for the reaction PhOH+ + NH3 -PhO + NH4+ (Steadman, J.; Syage, J. A. J. Am. Chem. Soc. 1991,113,6786).A model explaining the origin of the barrier and its generality to proton-transfer reactions involving delocalized charge was also presented. However, in that study, the barrier height could not be experimentally estimated because the ion-complex energy distribution was not known. In this report, we present photoelectron spectra to measure the energy distribution of the complex form PhOH+-(NH3), produced by resonance two-photon ionization of the corresponding neutral clusters. The collective photoelectron and mass spectra results indicate that the barrier to proton transfer from the reactive complex is 1.0-1.5 eV.
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