CONFORMATIONAL-ANALYSIS OF FLAVONE - VIBRATIONAL AND QUANTUM-MECHANICAL STUDIES

Combined molecular mechanics (MMX), semi-empirical (AM1) and ab initio calculations were applied to an investigation of the conformational and electronic properties of the isolated flavone molecule in the ground state. The calculated molecular structure shows that the angle between the benzopyrone moiety and the phenyl ring is about 30-degrees. The barrier to rotation in this molecule is about 1.8 kcal mol-1. The frequency shifts observed in the vibrational spectra of flavone in solution and in crystalline form are tentatively connected with a conformational change of the molecule with the physical state. The energy gap between a planar and a twisted configuration is very low and the intermolecular interactions in the crystal, which are expected to be larger than the modest intramolecular potential, must strongly reduce the dihedral angle. As a flat molecule should give more favorable packing, it is reasonable that flavone should exist in this form in the crystal. Electronic energy levels, charge distributions and bond orders were calculated using semi-empirical methods. Electronic interactions of the rings are discussed.