Journal of heterocyclic chemistry vol:30 issue:2 pages:301-305
The N-15 nmr spectra of a series of 1,2,3-thiadiazoles reveal the strong influence of substituents at C-5 on the N-2 resonance. Upon methylation, the two thiadiazole nitrogen resonances are shielded, but the most dramatic shift is observed for the methylated nitrogen, DELTAdelta > 140 ppm. The N-15 chemical shifts of some mesoionic thiadiazoles were also determined and explained by the dual effect of 5-substitution and salt formation. By disconnecting these effects, the N-15 chemical shifts of 10 and 11 were found to be unusual and to reflect a thiapentalene character.