Photochemical initial reaction mechanism of the ε-CL-20 explosive: a molecular-level study based on TDDFT

Hexanitrohexaazaisowurtzitane (CL-20) is a pure explosive with the highest energy density, and its environmental release and photochemical transformation have attracted considerable attention. However, the initial reaction mechanism under photoexcitation remains unclear. Time-dependent density functional theory (TDDFT) was employed to systematically investigate the photochemical reaction pathways, bond cleavage sequence, and reaction mechanisms of ε-CL-20 in this study. Electron density difference analysis reveals that the ground-state to first-excited-state transition for all five polymorphs (α, β, ε, ξ, and γ) is predominantly characterized by n(O) → π*(NO₂), which weakens N-O bonds and promotes homolytic cleavage of N-NO₂ bonds to generate ˙NO₂ radicals in this process. Within the framework of static electronic structure calculations, a plausible stepwise homolysis sequence of N-NO₂ bonds in the cage structure of ε-CL-20 is proposed: N21-N22 → N15-N16 → N13-N14 → N19-N20 → N23-N24 → N17-N18. Reaction potential energy surface calculations reveal that under photoexcitation, ε-CL-20 undergoes six consecutive denitration reactions, exhibiting a novel alternating photochemical-thermal preferred mode (odd-numbered steps are photochemical, while even-numbered steps are thermal); the C₆H₆N₆ cage core formed after complete denitration is the most stable due to its high symmetry and aromaticity. Regarding laser technology, the UV excitation energy for the first denitration is high (S₁ state ∼601 kJ mol^-1). An 810 nm femtosecond laser and a 1064 nm picosecond laser require five- and six-photon absorption, respectively, to reach the excited state; multiphoton efficiency is extremely low and single-photon absorption in this band is weak, resulting in very low photochemical initiation efficiency. In subsequent denitration steps, even-numbered steps are dominated by thermochemistry, although photochemical channels remain open for odd-numbered steps, and near-infrared excitation still demands high-order multiphoton processes. Therefore, the thermal pyrolysis pathway is more practical for laser processing. Regarding environmental photochemistry, the UV component of natural sunlight has photon energies below the first denitration excitation energy, making direct single-photon triggering of photochemical denitration difficult. However, it can indirectly promote ground-state thermal pyrolysis via photothermal conversion, providing a theoretical basis for assessing the environmental persistence of CL-20 and developing photocatalytic remediation technologies. The photolysis mechanism of CL-20 at the molecular level was revealed in this study, which has important guiding significance for predicting the environmental fate of energetic materials and enabling safe laser processing.