Aromatic amine nitration mechanisms: reactivity governed by resonance-assisted hydrogen bonds

Abstract

The nitration of aromatic amines to N-nitroamines under strongly acidic conditions is a key strategy in energetic materials synthesis. Experimentally, the feasibility of this reaction varies considerably across different substrates. Conventional intramolecular hydrogen bonds (HBs) fail to fully explain the behavior of molecules containing resonance-assisted hydrogen bonds (RAHBs), which promote extended π-electron delocalization. To clarify the underlying mechanism, this study employs density functional theory (DFT) calculations to investigate the nitration of four aromatic amines: 3-aminotriazole (3-AT), 2,6-diamino-3,5-dinitropyrazine (ANPZ), 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Using 3-AT as a model substrate, we identify two main reaction pathways: a stepwise mechanism initiated by NO₂⁺ involving σ-complex formation and a synergistic mechanism initiated by N₂O₅ that simultaneously introduces the nitro group and removes a proton via a single transition state. Extension of the two mechanisms to other substrates reveals that RAHB not only thermodynamically disfavors σ-complex formation in the stepwise pathway but also raises the kinetic barrier of the synergistic pathway, rendering nitration infeasible for LLM-105 and TATB. For ANPZ, however, the kinetic barrier of the synergistic pathway on the side without an apparent HB constraint is below the threshold, making nitration feasible. Accordingly, disrupting the RAHB in TATB is predicted to restore nitration reactivity, in agreement with experimental observations. This work clarifies the nitration mechanisms of aromatic amines and offers theoretical guidance for improving the success rate of such transformations.