Structure and conformational analysis of a bidentate pro-ligand, C ₂₁H₃₄N₂S₂, from powder synchrotron diffraction data and solid-state DFTB calculations

The molecular and crystalline structure of ethyl 1′,2′, 3′,4′,4a′,5′,6′,7′- octahydrodispiro[cyclohexane-1,2′-quinazoline-4′, 1′′-cyclohexane]-8′-carbodithioate (I) was solved and refined from powder synchrotron X-ray diffraction data. The initial model for the structural solution in direct space using the simulated annealing algorithm implemented in DASH [David et al. (2006). J. Appl. Cryst. 39, 910-915] was obtained performing a conformational study on the fused six-membered rings of the octahydroquinazoline system and the two spiran cyclohexane rings of (I). The best model was chosen using experimental evidence from ¹H and ¹³C NMR [Contreras et al. (2001). J. Heterocycl. Chem. 38, 1223-1225] in combination with semi-empirical AM1 calculations. In the refined structure the two spiran rings have the chair conformation, while both of the fused rings in the octahydroquinazoline system have half-chair conformations compared with in-vacuum density-functional theory (DFT) B3LYP/6-311G^*, DFTB (density-functional tight-binding) theoretical calculations in the solid state and other related structures from X-ray diffraction data. Compound (I) presents weak intramolecular hydrogen bonds of the type N - H⋯S and C - H⋯S, which produce delocalization of the electron density in the generated rings described by graph symbols S(6) and S(5). Packing of the molecules is dominated by van der Waals interactions.