Oxidation Products of Organic Trisulfanes: Molecular Structures and Energies of Various Isomers of the Dimethyltrisulfane Oxides Me 2 S 3 O and Me 2 S 3 O and of 1,3– t Bu 2 S 3 O 2

Abstract Ab initio MO calculations (MP 2/6–311 G ** //HF/6–311 G ** ) have been performed for several isomers (including rotamers) of Me 2 S 3 O and Me 2 S 3 O 2 . MeS(O)SSMe exists as five rotamers; the most stable form ( 1a ) has a helical backbone CSSSC with S‐S bond lengths of 206.0 (S II S II ) and 212.4 pm (S II S II ). The most stable rotamer of MeS‐S(O)SMe ( 2a ), is less stable than 1a by 10.7 kJ mol ‐1 ; it is of C s , symmetry, while a rotamer of C s symmetry ( 2b ) is less stable than 2a by only 1.4 kJ mol ‐1 . Both 2a and 2b are stabilized by O ⃛ H hydrogen bonds. The S‐S bond lengths of 2a are 210.0 and 212.0 pm; the CSSSC chain is not helical (CSSS torsion angles 166.3 and –75.4°). The 1,3‐dioxide MeS(O)‐SS(O)Me ( 3 ) has two equivalent chiral centers and exists as diastereomers. The most stable isomer 3a ( RR )/( SS ) is of C 2 symmetry with methyl groups trans to each other; the SO bonds form an angle of about 90°. The meso form 3b is less stable than 3a by 17.2 kJ mol ‐1 and the rotamers 3c and 3d are less stable by 25.6 kJ mol ‐1 and 28.4 kJ mol ‐1 , respectively. The trisulfane‐1.2–dioxide MeS(O)‐S(O)SMe has two nonequivalent chiral centers and exists as five isomers. The most stable form, the ( RS )/( SR ) form 4a , is less stable than 3a by 21.4 kJ mol ‐1 and is characterized by SS bonds of 220.9 (S III S III ) and 208.3 pm (S III S II ). The rotamer 4b is less stable by 5.9 kJ mol ‐1 . The isomers 4c , 4d , and 4e are all of ( SS )/( RR ) configuration and are less stable than 4a by 6.3, 12.7, and 12.0 kJ mol ‐1 . For comparison, ab initio MO calculations (HF/6–311 G * ) for t BuS(O)SS(O)‐ t Bu yielded two diastereomers of practically identically energy that both contain helical CSSSC backbones. The ( RS ) form is less stable than the ( RR )/( SS ) form by 1.8 kJ mol ‐1 .