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 (Δ f H 298 o ) 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 Δ f H 298 o (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 Δ f H 298 o (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 Δ f H 298 o (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.

Enthalpy of formation of guanidine and its amino and nitro derivatives

“…Figure 3 shows that the G4 calculation applied to atomization reaction overestimates the enthalpy of formation of guanidine; the addition of one or two NH 2 groups has a small influence on the D f H 298 (g) value, while that of three amino groups increases the overestimation effect. Earlier it was found that G4 theory underestimates the D f H 298 (g) values for nitro compounds [22,23]. The same trend, as can be seen from Fig.…”

“…The same trend, as can be seen from Fig. 3, is also observed for nitroguanidines; however, the presence of guanidine fragment decreases this underestimation somewhat compared to aliphatic and aromatic nitro compounds [22,23]. The largest underestimation is observed for dinitroguanidines; the addition of NH 2 groups somewhat compensates for the effect of NO 2 groups.…”

“…This value is close to that calculated from atomization reaction using G4(MP2) energies (8.8 kJ/mol), whereas the same value calculated using G4 energies (2.4 kJ/mol) is underestimated. A similar trend in G4 and G4(MP2) values was also observed for different nitro compounds [23] for which the values determined from G4 isodesmic reaction calculations were in best agreement with experiment. Additional support for recommended value was provided when H 2 NNO 2 was involved as a reference molecule to calculate the enthalpy of formation of nitroguanidine by isodesmic reaction approach.…”

“…The G4 method was found to be accurate in calculating the enthalpies of formations of different classes of compounds. However, as it was found in our previous works [22,23], the G4 theory underestimates the D f H 298 values of the most nitro compounds by up to 20 kJ/mol. A good agreement with experiment was obtained only when the isodesmic reactions were used instead of the atomization reaction.…”

“…For isodesmic reaction, the resulting enthalpy of formation was calculated combining the G4 enthalpy of reaction with the enthalpies of formation of reference molecules. The accuracy of experimental D f H 298 values for reference species involved in isodesmic and other balanced reactions was supported by G4 calculations applied to atomization reactions in our previous works [23][24][25].…”