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

Abstract

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.