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Crystallization of 4′-diethylamino-3-hydroxyflavone (1) yields two polymorphic crystals. One is monoclinic, space group P21/n (1a), and the other triclinic, P1 (1b). In both crystals, the two adjacent molecules form a dimeric unit by two intermolecular O–H···O hydrogen bonds. The hydrogen-bonded dimer in crystal 1a is nearly planar, whereas that in crystal 1b is centrically twisted, having cis-like and trans-like conformations for the terminal carbon atoms at the diethylamino group.
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Hino et al. (2013) studied this question.
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