REACTIVITY OF MOLECULAR CLUSTERS IN THE GAS-PHASE - PROTON-TRANSFER REACTION IN NEUTRAL PHENOL-(NH3)N AND PHENOL-(C2H5NH2)N

Photoionization of phenol(B)n (B = NH3, C2H5NH2) clusters formed in a supersonic expansion has been studied by the mass-selected two-color two-photon resonance-enhanced multiphoton ionization technique. This study clearly shows that a reaction of proton transfer takes place in the excited state (S1) of the neutral clusters, depending on the nature of the base molecule (B) and the size of the cluster. The clusters with n ≤ 3 for ammonia and n ≤ 2 for monoethylamine show an ionization cross section steeply increasing in the 7.5-7.8-eV range. This corresponds to the vertical ionization potential of the clusters lowered by about 1 eV with respect to free phenol, due to hydrogen bonding. For larger clusters the onset lies at about 6.8-7.0 eV and corresponds to the ionization of the proton-transferred species. These results are corroborated by fluorescence measurement: an emission similar to that of free phenol is observed for the small clusters while an emission similar to that of phenolate anion is observed for the larger ones. The cluster-size dependence of the proton-transfer reaction is mainly governed by the gas-phase basicity of Bn cluster (its proton affinity), i.e., by its proton solvation ability. © 1990 American Chemical Society.