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Structural and computational analysis of intermolecular interactions in a new 2- thiouracil polymorph

The crystallization and characterization of a new polymorph of 2-thio­uracil by single-crystal X-ray diffraction, Hirshfeld surface analysis and periodic density functional theory (DFT) calculations are described. The previously published polymorph (A) crystallizes in the triclinic space group P1, while that described herein (B) crystallizes in the monoclinic space group P21/c. Periodic DFT calculations showed that the energies of polymorphs A and B, compared to the gas-phase geometry, were −108.8 and −29.4 kJ mol−1, respectively. The two polymorphs have different inter­molecular contacts that were analyzed and are discussed in detail. Significant differences in the mol­ ecular structure were found only in the bond lengths and angles involving heteroatoms that are involved in hydrogen bonds. Decomposition of the Hirshfeld fingerprint plots revealed that O...H and S...H contacts cover over 50% of the noncovalent contacts in both of the polymorphs ; however, they are quite different in strength. Hydrogen bonds of the N—H...O and N—H...S types were found in polymorph A, whereas in polymorph B, only those of the N— H...O type are present, resulting in a different packing in the unit cell. QTAIM (quantum theory of atoms in mol­ecules) computational analysis showed that the inter­ action energies for these weak-to-medium strength hydrogen bonds with a noncovalent or mixed inter­action character were estimated to fall within the ranges 5.4–10.2 and 4.9–9.2 kJ mol−1 for polymorphs A and B, respectively. Also, the NCI (noncovalent inter­action) plots revealed weak stacking inter­actions. The inter­ action energies for these inter­actions were in the ranges 3.5–4.1 and 3.1–5.5 kJ mol−1 for polymorphs A and B, respectively, as shown by QTAIM analysis.

2-thiouracil polymorph ; X-ray diffraction ; DNA model system ; QTAIM and NCI plot analysis

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