Evaluation of thermochemical characteristics of salts with pentazenium cation

Abstract

Context

Using quantum-chemical and crystal modeling, the solid-state enthalpies of three hypothetical high-energy salts with the pentazenium cation N₅⁺ were estimated: nitrate NO₃⁻, dinitramide N(NO₂)₂⁻, and azide N₃⁻, yielding the cationic contribution of pentazenium. Supplementing the lattice energy mixing method with an additive approach to determining the enthalpies of salts, values are given for salts with the perchlorate anion N₅⁺ClO₄⁻; four salts with the halogen anions N₅⁺I⁻, N₅⁺Br⁻, N₅⁺Cl⁻, and N₅⁺F⁻; and two experimentally existing structures: pentazenium tetrafluoroborate N₅⁺BF₄⁻ and pentazenium hexafluorophosphate N₅⁺PF₆⁻. Calculations of salt structures were performed using a wide range of density functional theory (DFT) methods with varying functionals and bases, as well as composite methods for calculating the enthalpies of gas phase formation, to show the low variability of the final result and the universality of the approach. The calculations of the explosive characteristics of salts with oxygen–nitrogen anions showed that their detonation velocity is in the range of 7.1–7.6 km s⁻¹. Of the three salts considered, the azide salt has the lowest density equal to 1.1 g cm⁻³, while the nitrate and dinitramide are at the level of 1.6 g cm⁻³ and 1.7 g cm⁻³, respectively. The heat of detonation of pentazenium salts is about 800–850 cal g⁻¹.

Methods

In this work, broad set of DFT calculations were conducted through the software Gaussian 09: B3LYP/6-31G(d,p), B3LYP/aug-cc-PVDZ + GD2, M052X/aug-cc-pVTZ, and M062X/6–311 +  + G(d,p). For crystal structure optimization, the atom–atom potential methods with PMC program (Packing of Molecules in Crystal) were used. Charges for molecular electrostatic potential were fitted by FitMEP, and enthalpies of formation in gas phase were assessed by CBS-4 M, G3B3, G4, and W1BD.