Accurate thermochemical properties for energetic materials applications. II. Heats of formation of imidazolium-, 1,2,4-triazolium-, and tetrazolium-based energetic salts from isodesmic and lattice energy calculations

A computational approach to the prediction of the heats of formation (Delta H-f degrees's) of solid-state energetic salts from electronic structure and volume-based thermodynamics (VBT) calculations is described. The method uses as its starting point reliable Delta H-f degrees's for energetic precursor molecules and ions. The Delta H-f degrees's of more complex energetics species such as substituted imidazole, 1,2,4-triazole, and tetrazole molecules and ions containing amino, azido, and nitro (including methyl) substituents are calculated using an isodesmic approach at the MP2/complete basis set level. On the basis of comparisons to experimental data for neutral analogues, this isodesmic approach is accurate to < 3 kcal/mol for the predicted cation and anion Delta H-f degrees's. The Delta H-f degrees's of the energetic salts in the solid state are derived from lattice energy (U-L) calculations using a VBT approach. Improved values for the alpha and beta parameters of 19.9 (kcal nm)/mol and 37.6 kcal/mol for the U-L equation were obtained on the basis of comparisons to experimental U-L's for a series of 23 salts containing ammonium, alkylammonium, and hydrazinium cations. The total volumes are adjusted to account for differences between predicted and experimental total volumes due to different shapes of the ions (flat vs spherical). The predicted Delta H-f degrees's of the energetic salts are estimated to have error bars of 6-7 kcal/mol, on the basis of comparisons to established experimental Delta H-f degrees's of a subset of the salts studied. Energetic salts with the highest positive Delta H-f degrees's are predicted for azido-containing cations, coupled with heterocyclic anions containing nitro substituents. The substitution of functional groups on carbon versus nitrogen atoms of the heterocyclic cations has interesting stabilization and destabilization effects, respectively.