One-electron standard reduction potentials of nitroaromatic and cyclic nitramine explosives

Abstract

Extensive studies have been conducted in the past decades to predict the environmental abiotic and biotic redox fate of nitroaromatic and nitramine explosives. However, surprisingly little information is available on one-electron standard reduction potentials ( E o( R-NO 2/ R-NO 2 −)). The E o( R-NO 2/ R-NO 2 −) is an essential thermodynamic parameter for predicting the rate and extent of reductive transformation for energetic residues. In this study, experimental (linear free energy relationships) and theoretical ( ab initio calculation) approaches were employed to determine E o( R-NO 2/ R-NO 2 −) for nitroaromatic, (caged) cyclic nitramine, and nitroimino explosives that are found in military installations or are emerging contaminants. The results indicate a close agreement between experimental and theoretical E o( R-NO 2/ R-NO 2 −) and suggest a key trend: E o( R-NO 2/ R-NO 2 −) value decreases from di- and tri-nitroaromatic (e.g., 2,4-dinitroanisole) to nitramine (e.g., RDX) to nitroimino compound (e.g., nitroguanidine). The observed trend in E o( R-NO 2/ R-NO 2 −) agrees with reported rate trends for reductive degradation, suggesting a thermodynamic control on the reduction rate under anoxic/suboxic conditions.