MOLECULAR AND ELECTRONIC-STRUCTURES AND CONFORMATIONAL-ANALYSIS OF DERIVATIVES OF 9-NITROANTHRACENE, ANIONIC SIGMA-COMPLEXES

Potassium 10-methoxy-9-nitroanthracenide was studied by X-ray diffract ion analysis. The central ring of the anthracene fragment adopts an as ymmetrically flattened boat conformation. The bond lengths in the C(Ar )-C(NO2)-C(Ar) fragment are indicative of the presence of conjugation between the pi-systems of the benzene rings and the nitro group owing to a substantial contribution of the aci form to the structure of the nitro group. Quantum-chemical calculations of anionic sigma-complexes of 9-nitroanthracene derivatives were performed. In all cases, the cen tral ring adopts two conformations with the pseudoaxial and pseudoequa torial orientations of the substituents at the saturated C(10) atom, r espectively. The relative stability of the conformers and the factors determining the stability were considered. The minima on the potential energy surface have a flattened shape in spite of relatively high bar riers to the conformational transition (2-4 kcal mol(-1)). The deviati on of the C(Ar)-C(Ar)-C(sp(3))-C(Ar) torsion angle from the equilibriu m value by +/- 20 degrees causes an increase in the energy of the anio n by less than 1 kcal mol(-1). The effect of the substituents on the c harge distribution was considered.