1:1 crystal complexes of 2′,6′-dimethoxyflavone with trichloroacetic and chloroacetic acids.: Correlation between pKa values and x-ray hydrogen bond data

The structures of two solid inclusion compounds with the 2',6'-dimethoxyflavone host molecule (1) were investigated by single-crystal x-ray analysis. Both compounds, 1 trichloroacetic acid (1:1) and 1 chloroacetic acid (1:1) crystallize in non-centrosymmetric groups Pna2(1) and P2(1)2(1)2(1), respectively. The complexation involves an intermolecular hydrogen bond between the oxygen of the carbonyl group of the flavone and the acidic hydrogen of the substituted acetic acid. The results were used to improve a previous relationship between O ... O distances and pK(a) values. Copyright (C) 1999 John Wiley & Sons, Ltd.