RELATIONSHIPS BETWEEN NITRO-GROUP REDUCTION POTENTIALS AND TORSION ANGLES IN DI-ORTHO-SUBSTITUTED NITROBENZENES - A CRYSTALLOGRAPHIC AND O-17 NMR-STUDY

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 O-17 MNR spectroscopy and X-ray crystallography, and one-electron reduction potentials by pulse radiolysis. O-17 Chemical shifts indicated similar amide torsion angles (from 35 degrees to 45 degrees) as the alkyl group varied from hydrogen to tert-butyl, but widely differing nitro group torsion angles; from 36 degrees (hydrogen) to 92 degrees (tert-butyl). Crystal structures of the isopropyl and tert-butyl derivatives indicate amide group torsion angles (50 degrees and 64 degrees) somewhat larger than those predicted by O-17 NMR, and nitro group torsion angles (59 degrees and 65 degrees respectively) considerably smaller than those predicted by O-17 NMR (75 degrees and 92 degrees respectively). These results support earlier data that O-17 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.