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.