Molecular Structure of Nitrobenzene in the Planar and Orthogonal Conformations A Concerted Study by Electron Diffraction, X-Ray Crystallography, and Molecular Orbital Calculations

The molecular structure and ring distortions of nitrobenzene have been determined by gas-phase electron diffraction and ab initio molecular orbital (MO) calculations as well as from the structures of six derivatives studied by X-ray crystallography. The experimental value of the ring angle at the ipso position is alpha = 123.4 +/- 0.3 degrees in the free molecule; this is about 1.5 degrees less than the hitherto reported values. Regression analysis of the ring angles in the six derivatives studied by X-ray crystallography yields alpha = 122.7(1)degrees for nitrobenzene in a crystalline environment. The small difference in the two values of is interpreted as an effect of intermolecular interactions in the crystal. The value produced by the MO calculations, alpha = 122.3 degrees at the 6-31G* (5D) level, is smaller than either of the experimental results. As regards the ring angles at the meta and para positions, the three techniques of structure determination consistently indicate that these are larger than 120 degrees by a few tenths of a degree. Other important geometrical parameters from the electron diffraction study are (r(g)(C-C)) = 1.399 +/- 0.003 degrees, r(g)(C-N) = 1.486 +/- 0.004 angstrom, r(g)(N-O) = 1.223 +/- 0.003 angstrom, and angle O-N-O = 125.3 +/- O.2 degrees X-ray diffraction experiments on 3,5-dimethyl-4-nitrobenzoic acid and 3,5-dimethylbenzoic acid and ab initio MO calculations provide solid evidence thet the geometry of nitrobenzene is little affected when the nitrogroup is twisted by 90 degrees out of the planar equilibrium conformation. This indicates that the extent of pi-electron transfer from the benzene ring to the nitro group is small. The barrier to rotation is estimated to be 17 +/- 4 kJ mol(-1) from the electron diffraction data.