Unraveling the direct effect of cationic methylation on molecular structure, electronic structure, and stability of pentazolate salts in an external electric field

Abstract

Polynitrogen compounds, particularly cyclo-pentazolate (c-N₅ˉ) salts, hold significant potential as high-energy-density materials but are hindered by sensitivity issues. Here, we report the synthesis and characterization of two novel methyl-functionalized pentazolate salts: methoxyammonium pentazolate (MOA⁺N₅ˉ) and dimethylbiguanidinium pentazolate (DMBG⁺N₅ˉ). Both salts exhibit onset decomposition temperatures ≥ 82 °C and low mechanical sensitivities (IS ≥ 30 J, FS ≥ 300 N). In addition to material synthesis, this study integrates cation methylation with external electric field (EEF) modulation to explore structural and electronic responses. First-principles and quantum chemical calculations demonstrate that methylation mitigates field-induced polarization across varying EEF intensities. The N–N trigger bonds within the c-N₅ˉ rings show remarkable structural resilience. Frontier orbital analysis reveals that the HOMO–LUMO gap responds controllably to field polarity: the gap widens under moderate positive EEF (0.002–0.006 Ry a.u.) and narrows under equivalent negative fields (-0.006 to -0.002 Ry a.u.). Electrostatic potential analysis further indicates that methylation maintains stable, large positive regions (49% for MOA⁺N₅ˉ and 61% for DMBG⁺N₅ˉ) even under field perturbations. Simulated impact sensitivities under EEF confirm enhanced mechanical stability. This work highlights the coupling effect of methylation and EEF regulation, offering new insights into the design of structurally resilient, field-tolerant energetic materials and advancing the understanding of pentazolate chemistry.