Abstract
Equilibrium thermochemical measurements using the ion mobility drift cell technique have been utilized to investigate the binding energies and entropy changes associated with the stepwise association of HCN and CH3CN molecules with the benzene radical cation in the C6H6 center dot+(HCN) and C6H6 center dot+(CH3CN)(n) clusters with n = 1-6 and 1-4, respectively. The binding energy of CH3CN to the benzene cation (14 kcal/mol) is stronger than that of HCN (9 kcal/mol) mostly due to a stronger ion dipole interaction because of the large dipole moment of acetonitrile (3.9 D). However, HCN can form hydrogen bonds with the hydrogen atoms of the benzene cation (CH delta+center dot center dot center dot NCH) and linear hydrogen bonding chains involving HCN center dot center dot center dot HCN interaction. HCN molecules tend to form externally solvated structures with the benzene cation where the ion is hydrogen bonded to the exterior of HCN chains. For the C6H6 center dot+(CH3CN)(n) clusters, internally solvated structures are formed where the acetonitrile molecules are directly interacting with the benzene cation through ion dipole and hydrogen bonding interactions. The lack of formation of higher clusters with n > 4, in contrast to HCN, suggests the formation of a solvent shell at n = 4, which is attributed to steric interactions among the acetonitrile molecules attached to the benzene cation and to the presence of the blocking CH3 groups, both effects make the addition of more than four acetonitrile molecules less favorable.