The structure of 1,2-dithioles studied by 13C 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 13C and 19F n.m.r. spectroscopy. By both methods it is found that these compounds are relatively polar. A correlation is established between the 13C 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 13C shifts depend on the positive charge taken by the 1,2‐dithiole ring and can then constitute a convenient evaluation of this charge.