The structure of 1,2‐dithioles studied by 13 C n.m.r. spectroscopy

The charge separation between the dithiole ring and oxygen in 3‐(1′,2′‐dithiole‐3′‐ylidene)‐6‐methyl‐2,3‐dihydropyran‐2,4‐diones and 3‐(1′,2′‐dithiole‐3′‐ylidene)‐2,3‐dihydrobenzo[ b ] pyran‐2,4‐diones has been determined by 13 C and 19 F n.m.r. spectroscopy. By both methods it is found that these compounds are relatively polar. A correlation is established between the 13 C chemical shift of some carbon atoms and the S2p binding energies measured by e.s.c.a. in the 1,2‐dithiole derivatives. This correlation shows that these 13 C shifts depend on the positive charge taken by the 1,2‐dithiole ring and can then constitute a convenient evaluation of this charge.