Water-Assisted Proton Transfer in the Sequential Hydration of Benzonitrile Radical Cation CHCN(HO): Transition to Hydrated Distonic Cation CHCNH(HO) with ≥ 4.

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