Collision-induced electronic transitions in complexes between benzene and molecular oxygen

The collision-induced electric dipole transition moments for the O₂ + C₆H₆ complex have been investigated by the multi-configurational self-consistent field linear response method. Cooperative simultaneous transitions, (¹Δ_g,³B_1u) ← (³Σ^-_g,¹A_1g) and (¹Δ_g,³E_1u) ← (³Σ^-_g,¹A_1g), as well as singlet-triplet and singlet-singlet transitions in each molecule have been studied in gas phase and in solvent phase as modelled by a self-consistent reaction field method. Strong intensity enhancement, respectively, reduction is predicted for the ³B_1u ← ¹A_1g and ¹E_1u ← ¹A_1g transitions in benzene colliding with oxygen in gas or solvent environments. We find that transitions involving singlet molecular oxygen are greatly enhanced at intermolecular distances easily reached at normal temperature conditions; the b¹Σ⁺_g - a¹Δ_g, a¹Δ_g - X³Σ^-_g and b¹Σ⁺_g-X³Σ ^-_g transition probabilities are predicted to become enhanced approximately by 10⁵, 10³ and 10⁰ times in O₂ + C₆H⁶ collisions. The enhancement is of such a magnitude that singlet molecular oxygen should be easy to detect by absorption and emission experiments tuned to the particular frequencies. It is argued that the benzene molecule is the most efficient choice among a set of organic colliders for promoting intensity of the Herzberg III band in molecular oxygen.