An ab initio molecular orbital study of nitrosophenol/quinone monooxime equilibria

Abstract

The nitrosophenol/quinone monooxime tautomeric equilibrium was studied by ab initio MO calculations using the Hartree–Fock method at the 6-31G and 6-31G∗ levels of theory. The 13 examined structures were based on benzene, naphthalene and phenanthrene ring systems with ortho and para substitution patterns of nitroso and hydroxy groups. Results show that the quinonoid form becomes increasingly favored with increasing ring system size. For 2-nitrosophenol and 1-nitroso-2-naphthol the phenolic forms are more stable by 10.3 and 0.5kcal/mol, respectively, but for 9,10-nitrosophenanthrol the quinonoid form is more stable by 4.6kcal/mol. Also, with larger ring systems the geometries of both tautomeric forms become increasingly similar. The most stable ortho structures possess an intramolecular H-bond that appears to be stronger in quinonoid forms. Results of the calculations are accordant with experimental data.