Ab initio investigation of internal rotation in conjugated molecules and the orientation of NO2 in nitroaromatics: nitrobenzene, o-monofluoro- and o,o '-difluoro-nitrobenzenes

Calculation of the barriers and associated one-dimensional torsional potent ial are performed for internal rotation of the nitro group in nitrobenzene as well as for o-fluoro- and o,o'-difluoro-nitrobenzenes. It is shown that the presence of substituents in the ortho-positions force the nitro group t o rotate about the C-N bond, out of the plane of the benzene ring. In these conjugated molecules, the inclusion of electron correlation is shown to be necessary for reliable barrier prediction. It is found that the pi-stabili sation (resonance) has a relatively small contribution to the structure of the nitroaromatics, whereas the steric repulsion between the ortho-substitu ents and the oxygen atoms of the NO2 group are rather significant. The inte rplay between them accounts for the fact that for o,o'-C6H3F2NO2 the barrie r height at 0 degrees is larger than that at 90 degrees when electron corre lation is included. (C) 1999 Elsevier Science B.V. All rights reserved.