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

S-(2-chloroethyl)-1,4-dithianium, a persistent solid degradation product of mustard and the major constituent of ''mustard heels", exhibits motional l ine-broadening in its C-13 CP-MAS NMR spectrum. Variable temperature examin ation of the compound and a series of related S-alkyl-1,4-dithianium and th ianium salts demonstrates the generality of the motion in these cyclic sulf onium salts. The underlying motional process for the carbons in the six-mem ber rings cannot be attributed to ring rotation, the classical interpretati on for cyclohexane, but is consistent with conformational motion. Solid dit hiane, and the low temperature phases of cyclohexane and thiane were also s tudied by variable temperature C-13 MAS NMR to contrast the behavior of the se neutral rings with the cyclic sulfonium salts. Distinguishing characteri stics of the conformational mechanism include smaller E-a' s (2.3-6.1 kcal/ mol) and line-broadening (400-600 Hz), compared to the ring-rotation mechan ism operative in solid dithiane (12.9 kcal/mol, 1300 Hz) and the low-temper ature phase of cyclohexane (11.4 kcal/mol, 925 Hz). The values for the low- temperature phase of thiane (3.8 kcal/mol, 525 Hz) are more consistent with conformational motion rather than hindered-rotation. (C) 1999 Elsevier Sci ence B.V. ALI rights reserved.