N-15, O-17 NMR and X-ray diffraction study of mesoionic 1,2,3,4-thiatriazolium-5-olate and its ethylated derivative

Two mesoionic compounds with oxygenous exocyclic groups: 3-phenyl-1,2,3,4-t hiatriazolium-5-olate 1 and its ethylated derivative 2 were investigated by means of N-15, O-17 NMR and X-ray diffraction techniques. The exocyclic C5 -O6 bond of thiatriazole 1 [1.224(3) Angstrom] has a strong double bond cha racter. Bond lengths in the thiatriazole ring are intermediate between sing le and double bond values except for S1-C5 [1.800(2) Angstrom] which is clo se to a single Csp(3)-S bond. The C5-O6 bond is significantly longer for th e ethylated derivative 2 [1.314(4) Angstrom]. The ethyl group attached to O 6 is located in the trans position in relation to the ring S1 atom. The experimental data are compared with the results of ab initio molecular orbital calculations. The calculated absolute nuclear shieldings, chemical shifts and charge densities, in spite of some limitations, can be useful as an aid to signal assignments and for an understanding of the NMR parameter s. (C) 1999 Elsevier Science B.V. All rights reserved.