Vibrational coupling between the nitro groups in 1,4-dinitrobenzene and 1,3-dinitrobenzene molecules: ab initio and normal coordinate analyses treatments

Ab initio quantum chemical computations have been performed for 1,4-dinitrobenzene and 1,3-dinitrobenzene and their (15)N isotope-labeled derivatives at the 3-21G, 6-31G and 6-31G(*) basis set levels. The results have been compared with the experiment: the best coincidence has been obtained by the 6-31G method for 1,4-dinitrobenzene and by the 6-31G(*) method for 1,3-dinitrobenzene. The isotopic frequency shifts of the nitro group bands, induced by (14)N-(15)N labeling and the vibrational coupling between the nitro groups, calculated with GAMESS software, have been in accordance with the measured ones. A normal coordinate analysis has been carried out for the 1,4-dinitro- benzene molecule using a slightly modified force field of 1,3-dinitrobenzene, reasonable fit between the observed and calculated frequencies has been obtained. The results support the conclusion that the electronic contribution to the vibrational coupling between the two nitro groups in the studied dinitrobenzenes is negligible: thus, it may be suggested that the dynamical coupling will predominate.