New insights into the interconversion mechanism between phenol and its isomers

We have studied the tautomerization reactions of 2,4-cyclohexadienone and 2,5-cyclohexadienone to phenol and the phenol dissociation to phenoxy radical using ab initio calculations. For the tautomerization mechanisms, the geometries were optimized at the CASSCF(8,8)/6-31G* level and the energetics were recalculated at the CASPT2(8,8)/cc-pVDZ level. For the dissociation reaction, the computational requirements for achieving comparable accuracy were stricter. In this case, the geometry optimizations were carried out at the CASSCF(10,9)/6-31G* level and the energetics were tuned with CASPT2(10,9)/aug-cc-pVTZ calculations. The tautomerization of 2,4-cyclohexadienone to phenol involved a one-step mechanism with a barrier of 53.3 kcal mol−1, while the interconversion from 2,4-cyclohexadienone to 2,5-cyclohexadienone involved a two-step mechanism, which disagreed with previous theoretical results. This interconversion occurs through a diradical intermediate. The highest barrier, between the reactant and the intermediate, was of 42.7 kcal mol−1. The best result for O-H bond dissociation enthalpy (BDE) of phenol to phenoxy was 86.2 kcal mol−1, which is within the interval proposed for this reaction by experimentalist authors. The height barrier corresponding to the rotation through the O-H bond was 3.3 kcal mol−1, which is very close to the experimental value of 3.4 kcal mol−1.