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The thermally and photochemically induced dissociation of 9-cyanoanthracene-dimer molecules (9-CNAD) into two monomer molecules is examined using optical spectroscopy and ESR. Optical spectroscopy gives an activation energy of 1.67(5) eV for the thermally induced formation of monomer pairs within the dimer crystal. The photoinduced reactions are examined by ESR spectroscopy of the monomer pairs. The presence of only one Δms=±1 ESR triplet line pair indicates the existence of only one photochemically active species. This results from the nondegeneracy of the ground state energies of the four dimer molecules within the unit cell. The fine structure constants D and E and the orientation of the anthracene monomer molecules have been calculated from the angular dependence of the ESR spectra. The orientation of the crystallographic unit cell axes in the crystal has been determined by Laue diffraction.
Sixl et al. (1987) studied this question.