Hydrogen bonding of phenols or their radical cations with water or ammonia: Substituent effects and the influence on phenol oxidation

We did UB3LYP/6-31++g** and ROMP2/6-311++g** calculations on the hydrogen bonding of para-substituted phenols and their radical cations with water and ammonia. It was found that the magnitudes of the proton affinities increase in the order water (165 kcal/mol) < ammonia (204 kcal/mol) approximate to phenoxyl radicals (193-235 kcal/mol) < phenolate anions (321-352 kcal/mol). The slopes of the proton affinities against the substituent sigma(p) constants are about 22 and 15 kcal/mol for phenoxyl radicals and phenolate anions. It was also found that the, slopes of the binding energies against the substituent sigma(p) constants decrease in the order phenol-water complex (1.1 kcal/mol) < phenol-ammonia complex (1.4 kcal/mol) < phenol radical cation-water complex (4.1 kcal/mol) < phenol radical cation-ammonia complex (9.3 kcal/mol). The structure of the substituted phenol radical cation-ammonia complex was found to rely on the proton affinity of the corresponding phenoxyl radical. When the proton affinity is larger than 214 kcal/mol, the non-proton-transferred form is the only minimum on the potential energy surface. When the proton affinity is smaller than 210 kcal/mol, the proton-transferred form is the only minimum. The only complex for which both the proton-transferred and non-proton-transferred forms are minima was found for p-hydroxylphenol radical cation. On the other hand, all the phenol radical cation complexes with water have the non-proton-transferred form as the only minimum on the potential surface. Hydrogen bonding to ammonia was found to lower the adiabatic oxidation potentials of phenols by 0.5-1.2 eV. Hydrogen bonding to water was found to lower the adiabatic oxidation potentials of phenols by 0.4-0.6 eV. In general, a phenol substituted with a more electron-withdrawing group shows larger reduction in the adiabatic oxidation potential when complexed to water or ammonia.