Normal Vibrations of Trinitro Toluene: Need for a Fresh Study

In view of certain infirmities in the available data on the normal coordinate analysis of TNT, a fresh study from both theoretical and experimental viewpoints has been necessitated. Wilson's GF matrix method with Urey-Bradley force field has been used to calculate normal frequencies. Valence charge distribution using INDO/MO calculations lend support to the force field used. The experimental data have been obtained from the Fourier transform infrared absorption spectra. This spectrum is a superposition of the two types of molecular structures present in TNT. The changed valence charge distribution in trinitrobenzene on the introduction of the methyl group is indicated as a possible reason for the explosive nature of TNT. is too large to be ignored. Further, Nash et aJ have used a semi-empirical force field whereas in the present communication the Urey-Bradley force field used takes into account 1-3 and 1-4 nonbonded interactions in addition to bonded interactions. Moreover, the frequency assignments for the various functional groups and NO2 group, in particular, result in a divergence between the experimental and calculated values up to 37 per. cent, as acknowledged by the authors4 themselv,es. In view o:f these facts, it was considered necessary to undertake a fresh and more exhaustive study of the normal modes of vibration of TNT. Our present calculations provide an agreement ranging from 91 to 99 per cent between the observed and calculated frequencies. I