Electronic effects on 13C NMR chemical shifts of substituted 1,3,4‐thiadiazolium salts

A series of 3‐ and 4‐X‐cinnamoyl‐1,3,4‐thiadiazolium‐2‐phenylamine chlorides were prepared and fully characterized by spectroscopic techniques. The 13C NMR chemical shifts of the α and β side‐chain carbons were compared with those of the corresponding cinnamic acid precursors and more significant transmission electronic effects were observed for mesoionic compounds. The ratio ρR/ρI for the α and β side‐chain carbons in mesoionic derivatives reflects a more important inductive contribution whereas the value of ρR/ρI for the β side‐chain carbons indicates a higher resonance contribution. The chemical shifts of C‐2 and C‐5 of the heterocyclic ring were correlated with substituent constants and showed that the more extensive conjugation of the side‐chain with an exocyclic moiety is favoured by electron‐withdrawing groups. The correlation analysis of chemical shifts with dual parameters indicated a significant contribution of a resonance effect for C‐2 (ρR/ρI = 1.70) when compared with C‐5 (ρR/ρI = 1.03).