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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.