Quinuclidinone thiosemicarbazone hydrate: Insights into crystal structure from an energetic perspective

Abstract (English)

The hydrate form of quinuclidinone thiosemicarbazone was synthesized following established procedures and successfully isolated in pure form. The compound crystallizes in the orthorhombic P212121 space group, with the asymmetric unit containing one organic molecule (A) and one water molecule (W). During the refinement process, both the conventional independent atom model (IAM) and quantum crystallographic method Hirshfeld atom refinement (HAR) were used. HAR provided notable improvements in both geometry and refinement statistics, particularly for hydrogen atoms, which were refined with anisotropic displacement parameters and without constraints. Interaction energies between the nearest neighbors of A and W molecules in the crystal structure were calculated using the hybrid method in CrystalExplorer as well as DFT with a def2-TZVP basis set and M06-2X functional. Seven interactions with energies stronger than −10 kJ mol⁻¹ were identified. The strongest interaction is found between molecules A and A, mediated through two hydrogen bonds (N–H···N and N–H···S) with a total energy of −39 kJ mol⁻¹. The second strongest interaction in the crystal structure, with E = −30 kJ mol⁻¹, occurs between A and W molecules, via N–H···O. Weaker interactions (−25 kJ mol⁻¹ and −22 kJ mol⁻¹) are also mediated by hydrogen bonds (O–H···N and O–H···S) between W and A. All these interactions are primarily influenced by electrostatic energy contributions, indicating that hydrogen bonds play a critical role in both the geometric arrangement and energetic stabilization of the crystal packing. Additionally, three weaker interactions occur between A molecules, with approximately equal contributions from electrostatic and dispersion components to the total energy.