Nitrogen NMR study of some mesoionic oxadiazoles and thiadiazoles

N-15 chemical shifts are reported for 10 mesoionic oxadiazoles and thiadiaz oles. Some supporting N-14 acid C-13 NMR data are also reported, together w ith some ab initio molecular orbital calculations and x-ray diffraction dat a. The relation between compound structure and N-15 chemical shifts is disc ussed. N-14 NMR measurements and ab initio molecular orbital calculations a re employed to identify the charge distributions within the molecules studi ed. Some (1)J(C4,C5), (2)J(N-15,C-13) and (1)J(N-15,C-13) spin coupling dat a for mesoionic oxadiazoles, thiadiazoles and acetylosydnonimines are given . X-ray diffraction data for the picrate of acetylsydnonimine and a 3,1,2,- thiadiazole are reported. The bond lengths within the mesoionic backbone ar e intermediate between the values for single and double bonds, suggesting a conjugated bond system. The arrangement of the exocyclic group observed in the solid state for acetylosydnonimine corresponds to the arrangement pred icted by solution NMR studies. Copyright (C) 2000 John Wiley & Sons, Ltd.