15N, 17O 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-thiatriazolium-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 character. Bond lengths in the thiatriazole ring are intermediate between single and double bond values except for S1-C5 [1.800(2) Angstrom] which is close to a single Csp(3)-S bond. The C5-O6 bond is significantly longer for the ethylated derivative 2 [1.314(4) Angstrom]. The ethyl group attached to O6 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 parameters. (C) 1999 Elsevier Science B.V. All rights reserved.