13C CP-MAS NMR studies of S-alkyl-1,4-dithianium salts: conformational motion in the solid state

Abstract

S-(2-chloroethyl)-1,4-dithianium, a persistent solid degradation product of mustard and the major constituent of “mustard heels”, exhibits motional line-broadening in its 13C CP-MAS NMR spectrum. Variable temperature examination of the compound and a series of related S-alkyl-1,4-dithianium and thianium salts demonstrates the generality of the motion in these cyclic sulfonium salts. The underlying motional process for the carbons in the six-member rings cannot be attributed to ring rotation, the classical interpretation for cyclohexane, but is consistent with conformational motion. Solid dithiane, and the low temperature phases of cyclohexane and thiane were also studied by variable temperature 13C MAS NMR to contrast the behavior of these neutral rings with the cyclic sulfonium salts. Distinguishing characteristics of the conformational mechanism include smaller Ea’s (2.3–6.1kcal/mol) and line-broadening (400–600Hz), compared to the ring-rotation mechanism operative in solid dithiane (12.9kcal/mol, 1300Hz) and the low-temperature phase of cyclohexane (11.4kcal/mol, 925Hz). The values for the low-temperature phase of thiane (3.8kcal/mol, 525Hz) are more consistent with conformational motion rather than hindered-rotation.