17O NMR spectra of some Meisenheimer adducts

The O-17 NMR spectra of O-17 isotope-enriched 1,3,5-trinitrobenzene and its Meisenheimer adducts with hydride ion (1), acetone anion (2) and methoxide ion (3) were measured. The adducts 1-3 show O-17 signals shifted upheld compared with those of 1,3,5-trinitrobenzene. A higher upheld shift is exhibited by the oxygen atoms of the 4-nitro group (similar to 90 ppm) and a lower shift by those of the 2- and 6-nitro groups (similar to 40 ppm). The shift is practically independent of the structure or bulkiness of the nucleophile bound at the tetrahedral centre C-1 in the adduct and also, within an experimental error, of the counter ion and solvent (acetonitrile or a mixture of DMSO-d(6) and methanol). The measured delta(O-17) values indicate that in the Meisenheimer adducts the negative charge at the oxygen atoms is larger in 4-NO2 than in 2- and 6-NO2 groups. Copyright (C) 2000 John Wiley & Sons, Ltd.