GIAO/DFT ¹³C NMR Chemical Shifts of 1,3,4-Thiadiazoles

Abstract

¹ H, ¹³ C and ¹⁵ N NMR spectra of 2-acetylamino-1,3,4-thiadiazole and its 5-substituted derivatives have been measured and assigned based on reference data, as well as homo- and heteronuclear 2 D NMR experiments. In addition, the GIAO/DFT approach at the B3LYP level of theory using the 6-311G basis set was used to calculate the ¹³ C NMR chemical shifts. Although this method gives reliable results for 2-arylhydrazones of 1,3-diphenylpropanetrione, 2-phenacylpyridines, (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines, 4-fluoroanilines, (1Z,3Z)-1,4-di(pyridin-2-yl)buta-1,3-dienediols and their tautomeric forms, the calculated chemical shifts for the 1,3,4-thiadiazoles studied are less satisfactory. Presence of the sulfur atom(s) seems to be responsible for such behavior.

Keywords:

• 1,3,4-Thiadiazoles
• ab initio calculations
• NMR chemical shifts

Acknowledgments

We are very much indebted to the CYFRONET Regional Computing Centre of S. Staszic University of Mining and Metallurgy in Cracow for supply of computer time and providing programs (Grant KBN/SGI-ORIGIN-2000/ATRBydg/072/2000). Special Laboratory Technician Reijo Kauppinen is acknowledged for his help in NMR measurements. Also we are grateful to the University of Isfahan Research Council for the partial support of this work.

Notes

^a B3LYP/6-311G//RHF/3-21G.

^b Assignment is based on the ¹⁵N NMR shift of unsubstituted 1,3,4-thiadiazole -7.9 ppm in DMSO-d.16 In PFG ¹H,¹⁵N HMBC H-5 of 1 shows cross-peaks with both ring nitrogens, the cross-peak with the signal at -22.8 ppm being more intensive supporting the above assignment.

^c Not observed because H-5 is missing in these compounds and side chain NH is too far and its signal is too broad to give any cross-peaks with the ring nitrogens.

^a Chemical shifts obtained with HF/6-31G(d,p) and HF/6-31G(2d,p) methods of geometry optimization are given in regular font and italic, respectively.

^b Basis set used in calculation of chemical shifts.