A quantum chemistry study of benzene dimer

We have performed a detailed quantum chemistry study of the gas-phase benzene dimer. Large atomic orbital basis sets with multiple polarization functions were used. The effects of basis set size, electron correlation, and basis set superposition error were investigated for the low-energy planar sandwich (D6h and C6v), parallel displaced (C2h), and T-shaped (C2v) dimer structures. Our studies indicate that the C2h-symmetry parallel displaced geometry is the lowest-energy structure for the benzene dimer. The T-shaped structure was found to be a low-energy saddle point for interconversion between parallel displaced structures, while the planar sandwich structure was found to be a saddle point on a higher-energy interconversion path between parallel displaced structures. Detailed analysis of the low-energy (T-shaped saddle point) path revealed the presence of a shallow minimum corresponding to a tilt angle between phenyl ring planes of about 45°. Much of the behavior of the benzene dimer observed through molecular jet spectroscopy can be explained by considering a population of the low-energy parallel displaced structure and structures along the low-energy interconversion path, including the tilt- and T-shaped structures.