Proton transfer in the [phenol-NH3]+ system: An experimental and ab initio study

Abstract

Mass-analyzed threshold ionization (MATI) has been used to prepare phenol cations in selected vibrational states, including the ground state. Reactions of ground state ${\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{5}}{\mathrm{OH}}^{\mathrm{+}}$ with ${\mathrm{ND}}_{\mathrm{3}},$ studied in a guided ion-beam apparatus, are reported, along with related ab initio calculations. This paper focuses on the energetics and product branching in the proton transfer (PT) channel. Based on thermochemistry in the literature, combined with calculations of the intracomplex PT barrier, PT was expected to make up a large fraction of the total reactive scattering. Experimentally, it is found that PT has a small cross section with clear threshold behavior, and the conclusion is that the PT reaction is endoergic by $\text{4.5±1\hspace{0.05em}}\mathrm{kcal/mole}.$ Assuming that ${\mathrm{NH}}_{\mathrm{3}}$ has a proton affinity of 204.0 kcal/mole, this results in a proton affinity for phenoxy radical of 208.7 kcal/mole, and a neutral PhO–H bond energy of 91.1 kcal/mole. The results are used to reinterpret previous dissociative photoionization studies of phenol-ammonia complexes.