Conformational properties of asymmetrically substituted mono-, di- and trisulfides: solid and liquid phase Raman spectra

The Raman spectra of diallyl trisulfide, allyl methyl sulfide, allyl methyl disulfide and allyl methyl trisulfide in the liquid and solid states are interpreted in terms of their conformational behavior. In both the liquid and solid states, the allyl moieties in all molecules are in a gauche conformation in which the sulfur atoms and the terminal vinyl carbons form a dihedral angle of between 105° and 120°. For diallyl trisulfide and allyl methyl trisulfide in both the liquid and solid states, only a near-cis SS rotational conformer exists with a CSSS dihedral angle of approximately 25°. Furthermore, the rotational isomerism of both SS bonds of these molecules is the same. For allyl methyl disulfide in the solid and liquid phases only the gauche SS conformer exists, with a CSSC dihedral angle of approximately 70°. In the liquid state, two CS rotational conformers occur in each of the four molecules examined. The gauche conformer corresponds to a CCSC (allyl methyl sulfide) or CCSS (allyl methyl disulfide, allyl methyl trisulfide and diallyl trisulfide) dihedral angle of 70°, while the trans conformer corresponds to a CCSC or CCSS dihedral angle of 180°. For allyl methyl sulfide in the solid state only the trans CS conformer exists, while for allyl methyl disulfide and diallyl trisulfide only the gauche CS conformers exist in the solid phase. For allyl methyl trisulfide in the solid state both CS conformers exist, with the gauche CS conformer being more stable. We suggest that the gauche CS conformers for the di- and trisulfide systems are stabilized by attractive 1,4 C··S interactions analogous to those proposed for other sulfur containing molecules (H.E. Van Wart, L.L. Shipman and H.A. Scheraga, J. Phys. Chem., 79 (1975) 1428; ibid., 79 (1975) 1436.).