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 (ΔHn-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.

Gas-Phase External Solvation of Protonated Benzonitrile by Eight Methanol Molecules.

The gas-phase sequential association of methanol onto protonated benzonitrile (C6H5CNH+) and the proton-bound dimer (C6H5CN)2H+ have been examined experimentally by equilibrium thermochemical measurements and computationally by density functional theory (DFT). The bonding enthalpy (ΔH°) for the association of methanol with protonated benzonitrile (25.2 kcal mol-1) reflects the strong electrostatic interaction provided by the formation of an ionic hydrogen bond in the C6H5CNH+OHCH3 cluster in excellent agreement with a DFT-calculated binding energy of 24.9 kcal mol-1. The sequential bonding enthalpy within the (C6H5CN)H+(OHCH3)n clusters decreases from 25.2 to 10.6 kcal mol-1 for the eighth solvation step (n = 8), which remains more than 25% above the enthalpy of vaporization of liquid methanol (8.4 kcal mol-1). The nonbulk convergence of ΔH°n-1,n with eight solvent molecules is attributed to the external solvation of a benzonitrile molecule by an extended hydrogen bonding network of protonated methanol clusters H+(CH3OH)n. In the external solvation of protonated benzonitrile by methanol, the proton resides on the methanol subcluster and the neutral benzonitrile molecule remains outside and bonded to the surface of the protonated methanol cluster. The bonding enthalpy of methanol to the proton-bound benzonitrile dimer (C6H5CN)H+(NCC6H5) is measured to be 18.0 kcal mol-1, in good agreement with a DFT-calculated value of 17.1 kcal mol-1, which reflects the association of the proton with the lower proton affinity methanol molecule, thus forming a highly stable structure of protonated methanol terminated by two ionic hydrogen bonds to the two benzonitrile molecules. The external solvation of benzonitrile by methanol ices in space allows benzonitrile to remain on the ice grain surface rather than being isolated inside the ice. This could provide accessibility for reactions with incoming ions and molecules or for photochemical processes by UV irradiation, leading to the formation of complex organics on the surface of ice grains.