Vibrational spectra and structure of 1,4-dinitrobenzene and its N-15 labelled derivatives: An ab initio and experimental study

Infrared and Raman spectra in solid state and solution of 1,4-dinitrobenzene, 1,4-dinitrobenzene-N-15,N-15 and 1,4-dinitrobenzene-N-14,N-15 have been studied and their fundamental frequencies have been assigned using isotopic frequency shifts and Raman depolarization ratios. Ab initio quantum chemical calculations have been carried out for the three 1,3-dinitrobenzene isotopomers at the 3-21G, 6-31G and 6-31G** basis set levels and the computed vibrational frequencies have been compared with the experimental ones. Best coincidence was achieved with the frequencies calculated at the 6-3IG lever: the mean deviation is 26.5 cm(-1). The calculated isotopic frequency shifts, induced by the N-15 labelling, are in very good accordance with the measured ones. Except the C-H bond lengths, all the geometry parameters, calculated for the isolated molecule, are in good agreement with the X-ray data, obtained for the 1,4-dinitrobenzene monocrystal. The nitro groups in the molecule hold charges of 0.390 e(-) each; there is practically no nitro group/phenylene ring conjugation. (C) 1997 Elsevier Science B.V.