Relationships between nitro group reduction potentials and torsion angles in di-ortho-substituted nitrobenzenes; a crystallographic and oxygen-17 NMR study

Abstract

A series of 3-nitro-4-alkylbenzamides has been prepared, and the effects of nitro group torsion angle on reduction potential studied. Nitro and carboxamide group torsion angles have been determined by ¹⁷O NMR spectroscopy and X-ray crystallography, and one-electron reduction potentials by pulse radiolysis. ¹⁷O Chemical shifts indicated similar amide torsion angles (from 35° to 45°) as the alkyl group varied from hydrogen to tert-butyl, but widely differing nitro group torsion angles; from 36°(hydrogen) to 92°(tert-butyl). Crystal structures of the isopropyl and tert-butyl derivatives indicate amide group torsion angles (50° and 64°) somewhat larger than those predicted by ¹⁷O NMR, and nitro group torsion angles (59° and 65° respectively) considerably smaller than those predicted by ¹⁷O NMR (75° and 92° respectively). These results support earlier data that ¹⁷O chemical shifts predict for erroneously large nitro group torsion angles in non-rigid but sterically crowded molecules, because of additional contributions to the shift from van der Waals repulsions. The drop in reduction potential of 90 mV between the unsubstituted and tert-butyl derivatives is too large to be accounted for by the electronic effects of the alkyl groups, and indicates that increasing the nitro group torsion angle significantly lowers reduction potential.