Use of G4 Theory for the Assessment of Inaccuracies in Experimental Enthalpies of Formation of Aromatic Nitro Compounds

Abstract

The gas-phase enthalpies of formation (ΔfH298°) of 101 aromatic nitro compounds were calculated using the Gaussian-4 (G4) theory applied to the atomization and isodesmic reaction energies. The ΔfH298° (g) values calculated from the atomization reactions were underestimated by an average of 13 kJ·mol–1 and they could not be used for the assessment of inaccuracies in the experimental enthalpies of formation. A good agreement with the most available experimental data was obtained using the isodesmic reaction procedure. From 5 to 26 isodesmic reactions with different reference species were constructed for each compound. About 15 aliphatic nitro compounds and 100 different C/H/N/O/Cl compounds were used as the reference species in these reactions; the accuracy of enthalpies of formation of all reference compounds was preliminarily confirmed by theoretical calculations. Evidence of accuracy of experimental data was provided by the agreement with calculated values. The differences between the calculated and the experimental enthalpies of formation in the range from (8 to 46) kJ·mol–1 were assigned to possible errors in the experimental values for 21 compounds. The theoretical ΔfH298° (g) values were recommended for these compounds as being more reliable than the experimental values. On the basis of theoretical results, a reference data set of internally consistent gas-phase enthalpies of formation of aromatic nitro compounds was provided. Both experimental and calculated values are included in this data set. The recommended ΔfH298° (g) values of aromatic nitro compounds are consistent with each other and with ΔfH298° (g) values of about 115 different C/H/N/O/Cl compounds including aliphatic nitro compounds. More accurate condensed phase enthalpies of formation and enthalpies of sublimation (or vaporization) were recommended in some cases based on a critical analysis of reported experimental data supplemented by quantum chemical calculations.