Interplay between Charge Density Distribution, Crystal Structure Energetic Features, and Crystal Morphology of 6‑Methyl-2-thiouracil

This study provides a detailed charge
density distribution analysis
of 6-methyl-2-thiouracil crystals together with comprehensive energetic
investigations. The findings constitute a link between crystal network
features, electronic structure, molecular motif interaction energy,
and crystal morphology. Our results revealed agreement between the
topologically estimated hydrogen bond energy and the computational
values. It also occurred that the methyl group and sulfur atom significantly
affect crystal packing and contribute to a variety of intermolecular
contacts. Nevertheless, the strongest interactions observed in the
crystal structure are of electrostatic type. It is well reflected
in the generated molecular electrostatic potential surfaces of the
complementary character. Additionally, simple analysis of the interslab
interaction energies and surface energy values led to a rough explanation
of the crystal elongation direction and preferably created facets,
such as the well-developed {010} crystal form. It should be also noted
that complex investigations of the charge density distribution revealed
significant residual peaks next to the sulfur atom, neither reliably
modeled, nor justified by theory. These could be explained by the
presence of a small percentage of the oxo-thiol tautomeric form in
the crystal or by the S···S interactions across the
crystal lattice (S–S bridge formation, etc.); however, no binding
conclusions were drawn.