17O NMR spectra of some Meisenheimer adducts

Abstract

The 17O NMR spectra of 17O 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 17O signals shifted upfield compared with those of 1,3,5-trinitrobenzene. A higher upfield shift is exhibited by the oxygen atoms of the 4-nitro group (∼90 ppm) and a lower shift by those of the 2- and 6-nitro groups (∼40 ppm). The shift is practically independent of the structure or bulkiness of the nucleophile bound at the tetrahedral centre C1 in the adduct and also, within an experimental error, of the counter ion and solvent (acetonitrile or a mixture of DMSO-d6 and methanol). The measured δ(17O) 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 © 2000 John Wiley & Sons, Ltd.