Developing a Theoretical Approach for Accurate Determination of the Density and Thermochemical Properties of Energetic Ionic Liquids

Energetic ionic liquids (EILs) are novel explosives and propellants which are useful for a variety of military and industrial applications. The physicochemical properties of EILs play an important role in determining their performance in practical applications. In this study, a combination of ab initio and empirical methods has been developed to accurately predict the key properties of EILs: a) temperature-dependent heat of formation (ΔHf), entropy (S) and heat capacity (Cp) of cation/anion pairs in the gas phase; b) 298 K density (ρ) and heat of formation (ΔHf) of EILs in the condensed phase. Ab initio methods were selected based on comprehensive validations of the thermochemical properties of closed shell molecules (glyoxal), open shell radicals (vinylperoxy), triplet carbines (oxoethylidene), and the densities of various simple salts (LiF, NaF, KF, MgF2, CaF2, LiCl, NaCl, KCl, MgCl2, CaCl2). The CBS-APNO/G3/G4//M06-2X/6-311++G(d,p) level of theory was selected for the calculation of geometries, frequencies, energies, etc., and the CBS-APNO level of theory was chosen for calculating the original volumes of cations and anions. The proposed methods/approach calculated: a) the gas-phase thermochemistry of the cation/anion pairs of simple salts (Li+, Na+, K+, Mg2+, Ca2+, F− and Cl−) and six triazolium-based energetic salts (ESs) or EILs representatives (3-azido1,2,4-triazolium, 1-methyl-3-azido-1,2,4-triazolium, 1,4-dimethyl-3-azido-1,2,4-triazolium, nitrate and perchlorate), b) the condensed-phase density, lattice energy and heat of formation of ESs/EILs (3-azido1,2,4-triazolium nitrate, 3-azido-1,2,4-triazolium perchlorate, 1-methyl-3-azido-1,2,4-triazolium nitrate, 1-methyl-3-azido-1,2,4-triazolium perchlorate, 1,4-dimethyl-3-azido-1,2,4-triazolium nitrate and 1,4-dimethyl-3-azido-1,2,4-triazolium perchlorate). In comparison with experimental and theoretical results in literature, excellent agreement was observed for all properties. Overall, discrepancies were less than 10 %, a clear indication of the reliability of proposed methods/approach.