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 a nd electronic properties of the isolated flavone molecule in the groun d state. The calculated molecular structure shows that the angle betwe en 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 fre quency shifts observed in the vibrational spectra of flavone in soluti on and in crystalline form are tentatively connected with a conformati onal change of the molecule with the physical state. The energy gap be tween a planar and a twisted configuration is very low and the intermo lecular interactions in the crystal, which are expected to be larger t han the modest intramolecular potential, must strongly reduce the dihe dral angle. As a flat molecule should give more favorable packing, it is reasonable that flavone should exist in this form in the crystal. E lectronic energy levels, charge distributions and bond orders were cal culated using semi-empirical methods. Electronic interactions of the r ings are discussed.