THE STRUCTURE OF NITROBENZENE AND THE INTERPRETATION OF THE VIBRATIONAL FREQUENCIES OF THE C-NO2 MOIETY ON THE BASIS OF AB-INITIO CALCULATIONS

The vibrational spectra of nitrobenzene and its para-d1, d5, O-16O-18, O-18(2) and N-15 isotopic modifications are evaluated using the RHF/6-31G* ab initio harmonic force field. A rigorous interpretation of the experimental C-NO2 moiety bands is carried out. Systematic deficiencies of the SCF method are effectively removed by applying scale factors optimized previously for a number of aliphatic nitro compounds. Fully optimized geometries are also reported for planar and orthogonal nitrobenzene conformations at the RHF and MP2 computational levels using the standard 6-31G* and 6-31G** basis sets. Theoretical geometries and barriers to internal rotation are compared with available experimental data. The calculations suggest that steric factors affect appreciably the structural parameters of the C-NO2 fragment in the equilibrium planar conformation and consequently the potential function for internal rotation in nitrobenzene.