Paramagnetic resonance in phosphorescent aromatic hydrocarbons

Electron resonance spectra of the phosphorescent state of C6H6 solute molecules in a C6D6 host crystal have been recorded at 1·95°k. The fine structure is described by the zero-field energies: if g is assumed isotropic and equal to the free electron value. The experiments indicate that the latter assumption must be correct to within 0·1 per cent. The z principal axis is perpendicular to the molecular plane and the x axis coincident with the C-C diagonal that is almost parallel to the (001) crystallographic plane. Hyperfine structure has been recorded for H‖z and H‖x (approximately). The results are consistent with a spin density distribution A simple theoretical discussion is presented on the basis of the Born-Oppenheimer model. A comparison of the fine structure with theoretical calculations leads to an asymmetry in the spin distribution equal to the upper limit of that allowed by the hyperfine structure.