Standard Enthalpies of Formation of Ortho- And Meta-Substituted Phenyl Nitroso Oxides From Homodesmotic and G4 Measurements

A combined approach, including the use of homodesmotic reference reaction (HDR) and G4 calculation of absolute enthalpies of its participants, was applied for theoretical evaluation of the gas-phase standard enthalpies of formation fH for the set of cis- and trans-isomers of ortho- and meta-substituted aromatic nitroso oxides R C6H4NOO. Compared with isomeric nitroaromatic compounds, nitroso oxides have a higher energy by 252 – 280 kJmol 1, which explains their high reactivity. A satisfactory correlation was established between the enthalpy of the HDR and the Hammett constants of meta-substituents R, which made it possible to derive a simple expression for fH evaluation of meta-substituted aryl nitroso oxides. A detailed analysis of the obtained results for the ortho-R C6H4NOO set (Hammett correlation, QTAIM and distortion/interaction analyses) allowed us to identify three main factors of R influencing on the enthalpy of formation of aryl nitroso oxide: the inductive-resonance effect, which is most strongly manifested in dimethylamino-substituted phenyl nitroso oxide, steric repulsion of the NOO and R fragments, and close-shell stabilization of the cis-isomers of syn-ortho-R C6H4NOO due to the interaction of terminal oxygen atom of nitroso oxide group with an appropriate atom of substituent.