X-ray structures and dipole moments of tropones annulated with furan, benzene, and oxepin

The precise X-ray structures of furo[3,4-d]tropone (2), 2,4-dimethyl-8H-3-oxaheptalen-8-one (5), and 4,5-benzotropone (6) were determined. Their tropone rings show definite bond alternation and nearly planar conformation, though slightly boat-shaped like polycyclic tropone. The ground-state dipole moments of 2, 5, and 6, and also 8H-3-oxaheptalen-8-one (3) and 2-methyl-8H-3-oxaheptalen-8-one (4) were measured in benzene at 25°C. Analyses of the ground-state dipole moments by means of ab initio calculations demonstrate that the polar structures of 2 and 6 having an annulated aromatic ring are based on the local dipoles of penta-1,4-dien-3-one and the induced dipole of the annulated ring, whereas those of 3 and 5 are based not only on the local dipoles but also on the partial intramolecular CT interaction from the olefinic oxepin ring to the pentadienone moiety. According to the McRae equation, the dipole moments of the two lowest excited states of 2, 3, 5, and 6 were evaluated by using the frequency shifts of the absorption maxima in a number of polar and nonpolar solvents. They are all oriented in the same direction as that of the ground state, but are greater in magnitude.