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

Abstract

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 CH^δ+···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.