Enthalpy of Formation of the Energetic Materials Possessing Dinitromethyl and Trinitromethyl Functional Groups: Combined Quantum Chemical Composite and Isodesmic Reaction Approach.

Dinitromethyl and trinitromethyl moieties are promising functional groups in the development of energetic compounds with enhanced oxygen balance, density, and performance. The reliable data of energy content are crucial thermodynamic parameters in determining their performance and application prospects toward energetic materials (EMs). Direct experimental measurements of enthalpy of formation through bomb calorimetry are complicated due to their unstable nature and higher heat release, while the large size of these molecules limits the use of highly accurate quantum chemical methods that suffer from scaling problems. The isodesmic reaction method is commonly applied to determine the heat of formation of large EMs by cleaving them into simple molecules with known accurate experimental heats of formation and conserving the number and type of bonds on both sides of the reaction. However, the unavailability of experimental heats of formation for dinitromethane, 1,1-dinitroethane, trinitromethane, and 1,1,1-trinitroethane raises the use of nitromethane in isodesmic reactions, leads to a significant change in bond environments, and involves multiple compounds in isodesmic reactions. In this work, we calculated the gas-phase enthalpies of formation for dinitromethane, 1,1-dinitroethane, trinitromethane, and 1,1,1-trinitroethane compounds using an isodesmic reaction scheme and validated the results with quantum composite (Gaussian-4 and CBS-QB3) methods. The heats of formation obtained from the Gaussian-4 (G4) method are used to predict the energy content of EMs with dinitromethyl and trinitromethyl functional groups to reduce the error associated with different bonding environments in isodesmic reactions. We believe that the combined use of isodesmic reaction and the composite method can significantly reduce the error in the prediction of heats of formation of target dinitromethyl- and trinitromethyl-substituted EMs.