N-15 NMR ANALYSIS OF 1,2,3-THIADIAZOLES

The N-15 nmr spectra of a series of 1,2,3-thiadiazoles reveal the stro ng influence of substituents at C-5 on the N-2 resonance. Upon methyla tion, the two thiadiazole nitrogen resonances are shielded, but the mo st dramatic shift is observed for the methylated nitrogen, DELTAdelta > 140 ppm. The N-15 chemical shifts of some mesoionic thiadiazoles wer e also determined and explained by the dual effect of 5-substitution a nd salt formation. By disconnecting these effects, the N-15 chemical s hifts of 10 and 11 were found to be unusual and to reflect a thiapenta lene character.