Molecular-Structures and Conformations of 9,9'-Anthroin ('α' and 'β' Forms), 9,9'-Anthroin Acetate, and Some Related Benzoin Derivatives

Abstract

The molecular conformations of 9,9′-anthroin (two forms, 'α' and 'β') and its acetate, as well as those of the acetates of benzoin and p- anisoin , have been studied by X-ray diffraction methods and found to display similar characteristics in respect of the COCO moiety. The conformation of 1,2-diaryl acyloins is characterized by a small torsion angle (10-24°) about the central four-atom fragment O=C-C-O, and the intramolecular distance between the two oxygen atoms does not exceed 2.8 Ǻ. The torsion angle about the three-bond segment Ar-C-C-Ar is in all cases smaller than 90°. In benzoin and its derivatives, the arylcarbonyl group deviates from planarity by 4-12°. In the cases of β 9,9′-anthroin and its acetate, the carbonyl group is twisted out of the plane of the anthracene moiety by 75°, and the two anthracene π-systems are in an overlapping arrangement; for α-anthroin, despite the presence of disorder in the COCOH moiety, a similar disposition can be inferred, indicative overall that, for such species, hydrogen bonding is not necessarily dominant in determining conformation.