C-13 NMR study of substituent effects in 1,2,4-oxadiazole and 1,2,4-thiadiazole derivatives

Agirbas H., Kara Y. S.

PHOSPHORUS SULFUR AND SILICON AND THE RELATED ELEMENTS, vol.179, no.7, pp.1435-1443, 2004 (SCI-Expanded) identifier identifier


C-13 chemical shifts of C=N, C=O and C=S carbons of 3,4-distibstituted-1,2,4-oxadiazole-5-ones(thiones) and 3,4-disubstituted-1,2,4-thiadiazole-5-ones have been determined ill CDCl3 solution. Exceptionally good Hammett correlations of C-13 NMR chemical shifts of these carbons with sigma were obtained. The negative p valites observed (inverse substituent effects) indicate pi-polarization of the C=N, C=O and C=S bonds. As expected, the long distance C=O and C=S C-13 chemical shifts were found less susceptible to substituent-induced electronic changes.