Semiempirical treatment of the benzophenone molecule as a function of twist angle

The benzophenone molecule has been treated in MNDO, MNDOC, and MINDO/3 approximation as a function of the twist angle α of the phenyl rings. Calculated were (i) the energies of S0 and T1 state, (ii) delocalization of the n and π* orbitals involved in the 3nπ* state, (iii) net atomic charges,(iv) the force constants of the C=O vibration in the S0 and T1 state, respectively, (v) vertical transition energies for T1←S0 and S1←S0, and (vi) the position of upper triplet states. The results are discussed in relation to the spectroscopic properties of the T1 state of a vitreous benzophenone phase. It is concluded that diagonal disorder is due to a significant fluctuation of α causing both a spread of the site energies of the glass-forming molecules and a variation of spin-orbit coupling.