Seven 5-and 6-halogenated derivatives of uracil or 1-methyluracil (halogen = Cl, Br, I) were studied by single crystal X-ray diffraction. In contrast with pure 5-halouracils, where the presence of N-H…O and C-H…O hydrogen bonds prevents the formation of other intermolecular interactions, the general ability of pyrimidine nucleobases to provide electron donating groups to halogen bonding was confirmed in three crystals and cocrystals containing uracil with the halogen atom at the C6 position. In the latter compounds, among the two nucleophilic oxygen atoms in the C=O moiety, only the urea carbonyl oxygen O1 can act as halogen bond acceptor, being not saturated by conventional hydrogen bonds. The halogen bonds in pure 6-halouracils are all rather weak, as supported by Hirshfeld surface analysis. The strongest interaction was found in the structure of 6-iodouracil, which displayed the largest (13%) reduction of the sum of van der Waals (vdW) radii for the contact atoms. Despite this, halogen bonding plays a role in determining the crystal packing of 6-halouracils, acting alongside conventional hydrogen bonds.
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