Predicting reduction rates of energetic nitroaromatic compounds using calculated one-electron reduction potentials

The evaluation of new energetic nitroaromatic compounds (NACs) for use in green munitions formulations requires models that can predict their environmental fate. Previously invoked linear free energy relationships (LFER) relating the log of the rate constant for this reaction (log(k)) and one-electron reduction potentials for the NAC (E¹_NAC) normalized to 0.059 V have been re-evaluated and compared to a new analysis using a (nonlinear) free-energy relationship (FER) based on the Marcus theory of outer-sphere electron transfer. For most reductants, the results are inconsistent with simple rate limitation by an initial, outer-sphere electron transfer, suggesting that the linear correlation between log(k) and E¹_NAC is best regarded as an empirical model. This correlation was used to calibrate a new quantitative structure-activity relationship (QSAR) using previously reported values of log(k) for nonenergetic NAC reduction by Fe(II) porphyrin and newly reported values of E¹_NAC determined using density functional theory at the M06-2X/6-311++G(2d,2p) level with the COSMO solvation model. The QSAR was then validated for energetic NACs using newly measured kinetic data for 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), and 2,4-dinitroanisole (DNAN). The data show close agreement with the QSAR, supporting its applicability to other energetic NACs.