Intramolecular hydrogen bonding in some ortho-nitrodiphenylamines and related compounds. A proton magnetic resonance study

Abstract

.Proton magnetic resonance spectra of a wide variety of substituted 2-nitro- and 2,4-dinitro-diphenylamines and related compounds reveal that long-range coupling occurs between the NH proton and the 5-proton of the nitrophenyl ring in non-polar or weakly polar solvents. However, in the case of the 2,4-dinitrodiphenyl- amines, this coupling is absent in highly polar solvents, except when the non-nitrated ring carries powerful electron-donating substituents, e.g. methoxyl or dimethylamino, ortho or para (but not meta) to the NH group. An examination of these phenomena leads to the conclusion that such long-range coupling provides evidence that intramolecular hydrogen bonding occurs between the NH and the 2-nitro groups. N-Benzyl-2,4-dinitroaniline also exhibited similar long-range coupling but, in dimethyl sulphoxide solut'ion, this coupling was temperature-dependent. The chemical shift data of the ortho-nitrodiphenylamines suggest that they adopt a non-planar skew conformation in solution. A six-bond H-F coupling was observed between the fluorine atom and the 6-proton of 2'-fluoro-2,4-dinitrodiphenylamine.