Water-Assisted Proton Transfer in the Sequential Hydration of Benzonitrile Radical Cation C6H5CN•+(H2O) n : Transition to Hydrated Distonic Cation •C6H4CNH+(H2O) n with n ≥ 4

The stepwise hydration of the benzonitrile•+ radical cation with one–seven H2O molecules was investigated experimentally and computationally with density functional theory in C6H5CN•+(H2O) n clusters. The stepwise binding energies (ΔH n–1,n °) were determined by equilibrium measurements for C6H5CN•+(H2O) and for •C6H4CNH+(H2O) n with n = 5, 6, and 7 to be 8.8 and 11.3, 11.0, and 10.0 kcal/mol, respectively. The populations of n = 2 and 3 of the C6H5CN•+(H2O) n clusters were observed only in trace abundance due to fast depletion processes leading to the formation of the hydrated distonic cations •C6H4CNH+(H2O) n with n = 4–7. The observed transition occurs between conventional radical cations hydrated on the ring in C6H5CN•+(H2O) n clusters with n = 1–3 and the protonated radical •C6H4CNH+ (distonic ion) formed by a proton transfer to the CN nitrogen and ionic hydrogen bonding to water molecules in •C6H4CNH+(H2O) n clusters with n = 4–7. The measured binding energy of the hydrated ion C6H5CN•+(H2O) (8.8 kcal/mol) is similar to that of the hydrated benzene radical cation (8.5 kcal/mol) that involves a relatively weak CHδ+···O hydrogen bonding interaction. Also, the measured binding energies of the •C6H4CNH+(H2O) n clusters with n = 5–7 are similar to those of the protonated benzonitrile (methanol) n clusters [C6H5CNH+(CH3OH) n , n = 5–7] that involve CNH+···O ionic hydrogen bonds. The proton shift from the para-•C ring carbon to the nitrogen of the benzonitrile radical cation is endothermic without solvent but thermoneutral for n = 1 and exothermic for n = 2–4 in C6H5CN•+(H2O) n clusters to form the distonic •C6H4CN···H+(OH2) n clusters. The distonic clusters •C6H4CN···H+(OH2) n constitute a new class of structures in radical ion/solvent clusters.