Use of G4 Theory for the Assessment of Inaccuracies in Experimental Enthalpies of Formation of Aliphatic Nitro Compounds and Nitramines

Abstract

The gas-phase enthalpies of formation (ΔfH298o) of 57 aliphatic nitro compounds and nitramines (mono- and polynitro compounds including cyclic compounds and well-known explosives such as hexogen, octogen, and CL-20) were calculated using the Gaussian-4 (G4) theory applied to the atomization and isodesmic reaction energies. The ΔfH298o (g) values calculated from the atomization reactions were underestimated by an average of 10 kJ·mol–1, and they could not be used for the assessment of inaccuracies in the experimental enthalpies of formation. Much better agreement between theory and experiment was obtained using the isodesmic reaction procedure. Several isodesmic reactions were investigated for each compound. Evidence of high accuracy of most experimental data was provided by the agreement with theoretical results. The differences between the calculated and the experimental enthalpies of formation in the range from (8 to 53) kJ·mol–1 were assigned to possible errors in the experimental values for 17 compounds. The theoretical ΔfH298o (g) values were recommended for these compounds as being more reliable than the experimental values. As a result, a reference data set of internally consistent gas-phase enthalpies of formation of nitro compounds and nitramines was provided. Both experimental and calculated values are included in this data set. The enthalpies of sublimation or vaporization were evaluated for some compounds by taking into account the literature data on the condensed phase enthalpies of formation and the ΔfH298o (g) values recommended in our work. Thus, a set of self-consistent values of the enthalpy of formation in both condensed and gaseous phases and the enthalpy of sublimation or vaporization is presented for most of nitro compounds studied.