Water-Assisted Proton Transfer in the Sequential Hydration of Benzonitrile Radical Cation C 6 H 5 CN •+ (H 2 O) n : Transition to Hydrated Distonic Cation • C 6 H 4 CNH + (H 2 O) n with n ≥ 4.

The stepwise hydration of the benzonitrile •+ radical cation with one-seven H 2 O molecules was investigated experimentally and computationally with density functional theory in C 6 H 5 CN •+ (H 2 O) n clusters. The stepwise binding energies (Δ H n -1, n ° ) were determined by equilibrium measurements for C 6 H 5 CN •+ (H 2 O) and for • C 6 H 4 CNH + (H 2 O) 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 C 6 H 5 CN •+ (H 2 O) n clusters were observed only in trace abundance due to fast depletion processes leading to the formation of the hydrated distonic cations • C 6 H 4 CNH + (H 2 O) n with n = 4-7. The observed transition occurs between conventional radical cations hydrated on the ring in C 6 H 5 CN •+ (H 2 O) n clusters with n = 1-3 and the protonated radical • C 6 H 4 CNH + (distonic ion) formed by a proton transfer to the CN nitrogen and ionic hydrogen bonding to water molecules in • C 6 H 4 CNH + (H 2 O) n clusters with n = 4-7. The measured binding energy of the hydrated ion C 6 H 5 CN •+ (H 2 O) (8.8 kcal/mol) is similar to that of the hydrated benzene radical cation (8.5 kcal/mol) that involves a relatively weak CH δ+ ···O hydrogen bonding interaction. Also, the measured binding energies of the • C 6 H 4 CNH + (H 2 O) n clusters with n = 5-7 are similar to those of the protonated benzonitrile (methanol) n clusters [C 6 H 5 CNH + (CH 3 OH) 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 C 6 H 5 CN •+ (H 2 O) n clusters to form the distonic • C 6 H 4 CN···H + (OH 2 ) n clusters. The distonic clusters • C 6 H 4 CN···H + (OH 2 ) n constitute a new class of structures in radical ion/solvent clusters.