Observation of Conformational Isomers of the Tetraisopropylnitrobenzene Anion Radical by Electron Spin Resonance Spectroscopy

Abstract

The electron spin resonance spectrum of the anion radical obtained by electroreduction of 2,3,5,6-tetraisopropylnitrobenzene has been studied extensively. The spectrum is interpreted in terms of two discrete conformational isomers existing in thermodynamic equilibrium. The isomeric difference arises from concerted interaction of the ring substituents which allows the nitro group to assume two different angular orientations with respect to the plane of the ring. These orientations are manifested as two different 14N isotropic coupling constants. In acetonitrile solution these values are 23.6 and 22.0 G for the more- and less-twisted conformers, respectively. An apparent equilibrium constant for interconversion is defined and evaluated under a wide variety of solvent and temperature conditions. The free energy of conversion from the more- to the less-twisted conformation is estimated as ΔG298° = −5±1×102 cal mole−1 in acetonitrile. A necessary condition for the proposed equilibrium is that both species decay at the same rate and this condition is observed experimentally. 2,4,6-Tri-isopropylnitrobenzene anion radical was also studied. The nitrogen coupling constant was 20.6 G in acetonitrile, but no conformational isomerism was observed presumably because of the absence of buttressing 3,5-substituents.