Bent and planar molecules in polymorphs of the tricyclic carbon sulfide C6S8

Abstract

The binary sulfur-carbon compound C₆S₈ (3H,7H-bis[1,2]dithiolo[3,4-b:3′,4′-e][1,4]dithiine-3,7-dithione) was synthesized and characterized in 1993 by Rauchfuss et al. The crystal structure determination (monoclinic, space group P2₁/c) revealed an essentially planar molecular shape (α-C₆S₈). Repeating the synthetic procedure of Rauchfuss we obtained red crystals of C₆S₈ in a second crystallographic form (β-C₆S₈, tetragonal, space group P2₁c). Additionally, the formation of a black, insoluble, amorphous sulfur-carbon polymer with the tentative formula (C₆S₈)_n was observed. The molecules in β-C₆S₈ are angled along the S–S axis of the central 1,4-dithiine ring. The dihedral angle between the two planar C₃S₅ moieties is 133°. The molecular geometry, IR, Raman and UV spectra of C₆S₈ are analyzed theoretically at the B3LYP/cc-pVTZ level and are compared with experimental data. Assignments of vibrations and electronic transitions are made. A C₂ symmetric, bent molecular structure is found for C₆S₈, in agreement with the crystal structure of the β-form, which is only a few kJ mol⁻¹ more stable than the planar structure found in the α-C₆S₈ polymorph. C₆S₈ is a rare example of a substituted 1,4-dithiine that is structurally characterized in both the planar and bent forms.