Thermochemistry of methyl and ethyl nitro, RNO₂, and nitrite, RONO, organic compounds

Computational quantum theory is employed to determine the thermochemical properties of n-alkyl nitro and nitrite compounds: methyl and ethyl nitrites, CH₃ONO and C₂H₅ONO, plus nitromethane and nitroethane, CH3NO₂ and C₂H₅NO₂, at 298.15 K using multilevel G3, CBS-QB3, and CBS-APNO composite methods employing both atomization and isodesmic reaction analysis. Structures and enthalpies of the corresponding ad-tautomers are also determined. The enthalpies of formation for the most stable conformers of methyl and ethyl nitrites at 298 K are determined to be -15.64 ± 0.10 kcal mol^-1 (-65.44 ± 0.42 kJ mol^-1) and -23.58 ± 0.12 kcal mol^-1 (-98.32 ± 0.58 kJ mol^-1), respectively. δ_fH ^o(298 K) of nitroalkanes are correspondingly evaluated at -17.67 ± 0.27 kcal mol^-1 (-74.1 ± 1.12 kJ mol^-1) and -25.06 ± 0.07 kcal mol^-1 (-121.2 ± 0.29 kJ mol ^-1) for CH₃NO₂ and C₂H ₅NO₂. Enthalpies of formation for the aci-tautomers are calculated as -3.45 ± 0.44 kcal mol^-1 (-14.43 ± 0.11 kJ mol^-1) for aci-nitromethane and -14.25 ± 0.44 kcal mol ^-1 (-59.95 ± 1.84 kJ mol^-1) for the aci-nitroethane isomers, respectively. Data are evaluated against experimental and computational values in the literature with recommendations. A set of thermal correction parameters to atomic (H, C, N, O) enthalpies at 0 K is developed, to enable a direct calculation of species enthalpy of formation at 298.15 K, using atomization reaction and computation outputs.