Multinuclear magnetic resonance studies of 2‐aryl‐1,3,4‐selenadiazoles

Abstract

2-Aryl-1,3,4-selenadiazoles were studied by (1) H, (13) C, (15) N and (77) Se NMR spectroscopy. The results (chemical shifts and coupling constants) were correlated with Hammett substituent parameters as well as calculated chemical shifts and bond lengths.

Multinuclear magnetic resonance studies of 2‐aryl‐1,3,4‐thiadiazoles

“…The chemical shifts of N‐4 are very close to the value reported for unsubstituted 1,3,4‐thiadiazole (−7.9 ppm) . The 15 N NMR data obtained for the molecules studied are very similar to those presented previously for selena analogs, the signal of the selenium containing compounds is less than 5 ppm shifted . The ranges of the chemical shifts of N‐3 and N‐4 in the molecules studied are 16.74 and 2.87 ppm, respectively.…”

“…The observed chemical shifts of C 5 –H proton (8.99–9.30 ppm; Table ) take values intermediate between those of 1,3,4‐oxadiazoles (8.32–9.30 ppm) and those of 1,3,4‐selenadiazoles (9.77–10.05 ppm) . The value reported for unsubstituted 1,3,4‐thiadiazole (7.55 ppm) is distinctly smaller than that observed for 2‐aryl‐substituted ones.…”

“…In both cases, the ‘goodness of fit’ of dual substituent parameter correlation, judged by f parameter ( f = SD/RMS), indicates the good correlation ( f = 0.10 and 0.13 for C‐2 and C‐5, respectively). As for selenium and oxygen containing analogs, the opposite effects of the substituent electronic character on C‐2 and C‐5 chemical shifts may be explained by the redistribution of the C═N bond electron density caused by its polarization and, in consequence, an increase in the diamagnetic effects on C‐5. This mechanism is more effective in highly delocalized electron systems of aromatic thiadiazole and selenadiazole rings, in comparison with non‐aromatic oxadiazoles …”

“…If the values computed for N‐3 correlate well with the experimental ones, then the correlation obtained for N‐4 is very poor ( r 2 < 0.6). The reasons for such a difference are not clear, such effects were not observed for oxa and selena analogs …”

“…In our previous papers, we have presented 1 H, 13 C, 15 N, 17 O and 77 Se NMR studies of substituted 1,3,4‐oxa(selena)diazoles . This paper is a concise report on the NMR properties of their thia analogs.…”