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Enthalpies of the C−N bond dissociation of nitrobenzene and twenty-seven mono substituted nitrobenzenes along with their formation enthalpies were calculated using different multilevel (G2, G3, G3B3, CBS-QB3) and density functional theory-based B3LYP and wB97XD techniques. The gas phase enthalpies of formation of mono substituted phenyl radicals were also calculated using these methods. The calculated values of reaction enthalpies were compared with available experimental data. We have concluded based on results of present and recent article, that B3LYP level of theory with 6-31G(d′f,p′) basis sets should proposed as the accurate method for calculating formation enthalpies of mono substituted nitrobenzenes as well as enthalpies of C–NO2 bond scission. Based on satisfactory correlation between experimental and calculated data we proposed revision of experimental activation energy of C–NO2 bond rupture in o-chloronitrobenzene, o- and m-nitrotoluene as wells as p-nitroaniline. Enthalpies of formation of some substituted phenyl radicals were estimated using experimental enthalpies of C–NO2 bond cleavage together with formation enthalpies of mono substituted nitrobenzenes and NO2 moiety.

Enthalpies of formation of twenty seven mono substituted nitrobenzenes were calculated employing various DFT and composite methods using atomization approach and two types of isodesmic reactions. The overall best agreement with experiment was achieved by using Gn multilevel techniques. It was found that the best performance among DFT methods within atomization approach demonstrates long range corrected wB97XD level theory. The average absolute deviation value calculated for B3P86/6-311G(d,p) which have been recently reported as an very accurate method for calculating formation enthalpies of poly and mono substituted nitrobenzenes exceeds 50 kcal/mol. Employing of isodesmic reactions tends to it improve estimations of DFT levels, though agreement between theoretical data obtained using this methodology with experiment depends strongly on the type of reaction and the quality of experimental Δ f H 0 ,298 values available for compounds involved in these bond balanced reactions.

Present theoretical study has been carried out with objective to reveal effect of chlorine and fluorine substituents on C–N bond strength in nitromethane. Gn, W1 and CBS-QB3 composite techniques were employed to compute accurate values of formation enthalpies of halogenated nitromethanes, corresponding radicals and ΔrH0(C–N) values. Calculations on DFT and ab initio levels were performed using B3LYP and QCISD methods. It was found that substitution of hydrogen by fluorine atoms causes only slightly decreasing of ΔrH0(C–N), whereas accumulation of chlorine atoms tends to decrease C–N bond strength significantly. Formation enthalpies of halogenated nitromethanes and corresponding radicals were calculated using both atomization approach and involving isodesmic reactions.

In recent paper we reported results concerning the gas phase enthalpies of formation of C1–C4 mononitroalkanes calculated using different multilevel and density functional theory (DFT)-based B3LYP techniques. The enthalpies of the C−N bond dissociation of these nitroalkanes had been also calculated. In present paper calculated activation enthalpies and enthalpies of reaction for HONO elimination from C2–C4 mononitroalkanes are represented. DFT/B3LYP, ab initio QCISD levels of theory together with G3, G3B3 G4 and CBS-QB3 were employed in this study. G4 calculated enthalpies of formation and reaction enthalpies of C–N bond rupture are performed in addition to activation and reaction enthalpies for HONO elimination. Comparison with available experimental activation energies showed that G3 and G3B3 estimations are of higher accuracy. The agreement of the CBS-QB3 calculations with experiment is also satisfactory. G4 calculated data demonstrated the worse agreement and did not produce correct trends for reaction enthalpies of C–N bond rupture.