Hydrogen-bonding interactions in the crystalline-phase structures of cinnamic acid derivatives

Secondary interactions responsible for the crystalline-phase structures of various cinnamic acids and cinnamic acid esters drawn from the Cambridge Structural Database were studied with the help of the Cerius package implemented on an SGI workstation. Primarily hydrogen-bonding interactions were sought, but deviation from planarity was also measured. For the acids, the main structural feature is the strong hydrogen bonding between the carboxyl groups. The dimers are interconnected by C-H ... O intermolecular hydrogen bonds. In most cases the C atom of the C-H unit was a member of the aromatic ring. Intramolecular (olefinic) C-H ... O bonds were found to be frequent, fixing synperiplanar and antiperiplanar C=C-C=O conformations with about the same abundance in the acids and exclusively synperiplanar conformations in the esters. The carbonyl group of the ester is always involved in C-H ... O hydrogen bonding. Here, the C atom of the C-H unit was either a member of the aromatic ring or the olefinic group, or was attached to the alcoholic O atom of the ester group. The beta-phenyl and the carboxyl or the ester groups are almost coplanar. The crystals have a layered structure and in the most frequent parallel arrangement the phenyl groups are offset by varing amounts (but always to a small extent) in neighbouring layers. The common hydrogen bonds are mostly within a layer.