Stability and structure of benzene dimer cation (C6H6)+2 in the gas phase

The gas-phase equilibria of the clustering reaction (C6H6)+n−1+C6H6 =(C6H6)+n were studied with a pulsed electron-beam high-pressure mass spectrometer. The enthalpy changes −ΔH0n−1,n for the reaction with n=2–4 were determined to be 20.6±1.0, 7.8±0.5, and ∼7 kcal/mol, respectively. The structure of dimer cation (C6H6)+2 was determined by ab initio spin-restricted open-shell Hartree–Fock molecular orbital (ROHF MO) calculations. Two isomers, ‘‘axial C2 ’’ and ‘‘parallel C2v, ’’ are calculated to be located at the potential-energy minima. The axial C2 is 3.0 kcal/mol more stable than the parallel C2v by ROHF/6-31G**//ROHF/3-21G.