Four cocrystals of thymine with phenolic coformers: influence of the coformer on hydrogen bonding.

Cocrystals are molecular solids composed of at least two types of neutral chemical species held together by noncovalent forces. Crystallization of thymine [systematic name: 5-methylpyrimidine-2,4(1H,3H)-dione] with four phenolic coformers resulted in cocrystal formation, viz. catechol (benzene-1,2-diol) giving thymine-catechol (1/1), C5H6N2O2·C6H6O2, (I), resorcinol (benzene-1,3-diol) giving thymine-resorcinol (2/1), 2C5H6N2O2·C6H6O2, (II), hydroquinone (benzene-1,4-diol) giving thymine-hydroquinone (2/1), 2C5H6N2O2·C6H6O2, (III), and pyrogallol (benzene-1,2,3-triol) giving thymine-pyrogallol (1/2), C5H6N2O2·2C6H6O3, (IV). The resorcinol molecule in (II) occupies a twofold axis, while the hydroquinone molecule in (III) is situated on a centre of inversion. Thymine-thymine base pairing is common across all four structures, albeit with different patterns. In (I)-(III), the base pair is propagated into an infinite one-dimensional ribbon, whereas it exists as a discrete dimeric unit in (IV). In (I)-(III), the two donor N atoms and one carbonyl acceptor O atom of thymine are involved in thymine-thymine base pairing and the remaining carbonyl O atom is hydrogen bonded to the coformer. In contrast, in (IV), just one donor N atom and one acceptor O atom are involved in base pairing, and the remaining donor N atom and acceptor O atom of thymine form hydrogen bonds to the coformer molecules. Thus, the utilization of the donor and acceptor atoms of thymine in the hydrogen bonding is influenced by the coformers.