Carbon-13 chemical shifts of alkene carbons are observed in the ranges 78–109 ppm (Cα) and 154–164 ppm (Cβ) for a series of 11 2-acylidene-3,5-diaryl-2,3-dihydro-1,3,4-thiadiazoles, 3 2-acylidene-3-alkyl-2,3-dihydrobenzothiazoles, and 2 2-acylidene-3-alkyl-2,3-dihydrobenzoselenazoles of known geometry, indicating appreciable charge polarization in these compounds as in other push–pull olefins. Substitution that promotes more extensive charge delocalization results in the Cα signal shifting to the higher-frequency end of the chemical shift range. The observed shifts are compared with those calculated according to the Pretsch scheme. Key words: carbon-13 NMR, chemical shifts, push–pull olefins, 1,3,4-thiadiazoles, benzothiazoles, benzoselenazoles.
Understanding 125Te NMR chemical shifts in disymmetric organo-telluride compounds from natural chemical shift analysis