The mechanism of the radical-anion reduction of 2,4,6-trinitrotoluene: a theoretical insight

摘要

The results of a computational study to model some aspects of the degradation of 2,4,6-trinitrotoluene (TNT), an explosive material, are reported. Experimental work has indicated the presence of reduction products, either 4-amino-2,6-dinitrotoluene (4-ADNT) or 2-amino-4,6-dinitrotoluene (2-ADNT), with large amounts of 4-ADNT found in plant tissue. The reduction mechanism we have studied is of the radical-anion type. In the first step, TNT accepts an electron, which is delocalized over the nitro groups. In the second step, protonation occurs either at the 2- or 4-position. Several further steps, including electron addition, lead to the reduction of a nitro group to an amino group. Depending on the initial protonation position, either 4-ADNT or 2-ADNT is formed. We have calculated the molecular structures and energies of the TNT anion and the different intermediates and products formed using ab initio methods (HF/6-31G~(**) and density functional theory (DFT) with diffuse functions). The most significant result from the HF/6-31G~(**) calculations is that the preference of reduction at the 4-position arises from the preferred protonation at this site (a 3.7 kcal mol~(-1) difference) compared with the 2-position. Once this isomer forms, the site of reduction is controlled, even though the subsequent intermediates leading to 2-ADNT are slightly more stable than their 4-position counterparts; further, 2-ADNT is calculated to be slightly more stable than 4-ADNT. We extended the investigations of the preferred site of protonation by inclusion of electron correlation at the MP2 level and DFT. Model compounds corresponding to the species formed at these crucial steps were also studied with the MP2, DFT, and MP4 approximations.