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

The nitrosophenol/quinone monooxime tautomeric equilibrium was studied by a b initio MO calculations using the Hartree-Fock method at the 6-31G and 6-3 1G* levels of theory. The 13 examined structures were based on benzene, nap hthalene and phenanthrene ring systems with ortho and para substitution pat terns of nitroso and hydroxy groups. Results show that the quinonoid form b ecomes increasingly favored with increasing ring system size. For 2-nitroso phenol and 1-nitroso-2-naphthol the phenolic forms are more stable by 10.3 and 0.5 kcal/mol, respectively, but for 9,10-nitrosophenanthrol the quinono id form is mon stable by 4.6 kcal/mol. Also. with larger ring systems the g eometries of both tautomeric forms become increasingly similar. The most st able ortho structures possess an intramolecular H-bond that appears to be s tronger in quinonoid forms. Results of the calculations are accordant with experimental data. (C) 2000 Elsevier Science B.V. All rights reserved.