X-ray crystal structure analysis of 4'-bromo-5-hydroxyflavone and 4'-bromo-3-hydroxyflavone was carried out to investigate the structures of pyrone ring and hydrogen bonding which may have some relation to the fluorescence emission. The bond lengths of the carbonyl groups in 4'-bromo-5-hydroxyflavone and 4'-bromo-3-hydroxyflavone were found to be 1.245 and 1.232 A respectively. This is consistent with the fact that 5-hydroxyl group forms a stronger hydrogen bond than 3-hydroxyl group. On the other hand, as described in the previous paper, the carbonyl absorption bands of type I compounds occurred in shorter wavelength region than those of type II, and a similar phenomenon was also observed in 4'-bromo-5-hydroxyflavone and 4'-bromo-3-hydroxyflavone. The serious inconsistency between the bond lengths measured by X-ray analysis and the stretching frequencies observed in infrared spectra for the carbonyl group is considered to be caused by a coupling effect. The differences in planarity and aromaticity between type I and type II were not considered to be significant, but the aromaticity of pyrone ring in type II is presumed to be increased remarkably in their metal chelates, which explains the strong fluorescence emission observed in their metal chelates. The 4'-bromo-3-hydroxyflavone forms an intermolecular hydrogen bonding only in crystalline state.
