The keto/enol equilibrium of phenol. An ab initio investigation

Ab initio 6-31G* basis set calculations have been carried out on phenol, 2,4-cyclohexadieneone and 2,5-cyclohexadieneone. Molecular geometries optimised at the restricted Hartree-Fock (RHF) level of theory indicate that all three structures are planar. Energy calculations were carried out using the RHF and second-order Moller-Plesset (MP2) formalisms, on the RHF optimised geometries. It was determined that phenol is significantly more stable than 2,4-cyclohexadieneone by 47.43 (RHF) and 72.51 (MP2) kJ mol(-1), and more stable than 2,5-cyclohexadieneone by 42.48 (RHF) and 70.61 (MP2) kJ mol(-1). From these results it is possible to estimate an equilibrium constant for phenol reversible arrow 2,4-cyclohexadieneone of 1.98 x 10(-13), which is in excellent agreement with experimental results.