Synthon preference in a hydrated β-resorcylic acid structure and its cocrystal with thymine

Multicomponent crystals or cocrystals play a significant role in crystal engineering, the main objective of which is to understand the role of intermolecular interactions and to utilize such understanding in the design of novel crystal structures. Molecules possessing carboxylic acid and amide functional groups are good candidates for forming cocrystals. β-Resorcylic acid monohydrate, C7H6O4·H2O, (I), crystallizes in the triclinic space group P-1 with one β-resorcylic acid molecule and one water molecule in the asymmetric unit. The cocrystal thymine-β-resorcylic acid-water (1/1/1), C5H6N2O2·C7H6O4·H2O, (II), crystallizes in the orthorhombic space group Pca21, with one molecule each of thymine, β-resorcylic acid and water in the asymmetric unit. All available donor and acceptor atoms in (I) and (II) are utilized for hydrogen bonding. The acid and amide functional groups are well known for the formation of self-complementary acid-acid and amide-amide homosynthons. In (I), an acid-acid homosynthon is observed, while in (II), an amide-acid heterosynthon is present. In (I), the β-resorcylic acid molecule exhibits the expected intramolecular S(6) motif between the hydroxy and carbonyl O atoms, and an intermolecular R2(2)(8) dimer motif between the carboxylic acid groups; only the former motif is observed in (II). The water solvent molecule in (I) propagates the discrete dimers into two-dimensional hydrogen-bonded sheets. In (II), thymine and β-resorcylic acid molecules do not form self-complementary amide-amide and acid-acid homosynthons; instead, a thymine-β-resorcylic acid heterosynthon is observed. With the help of the water molecule, this heterosynthon is aggregated into a three-dimensional hydrogen-bonded network. The absence of thymine base pairing in (II) might be linked to the availability of additional functional groups and the preference of the donor and acceptor hydrogen-bond combinations.