Metal cation binding to phenol: DFT comparison of the competing sites

The two metal ion binding regions of phenol were characterized and compared by density functional theory (DFT). The monocations of sodium, magnesium, aluminum, and the first-row transition series were considered, using mainly the B3LYP and MPW1PW91 functionals. Calculations on the model ligands water and benzene were carried out at the same level of theory. The MPW1PW91 functional is more favorable than the B3LYP functional toward binding to the aromatic ring, while no strong difference is seen for oxygen binding. Comparison with experimental data and higher level computational results for water and benzene indicate that MPW1PW91 is better than B3LYP in predicting the differential between ring and oxygen binding energies, and seems to do an excellent job of making this comparison. Except for Na+, for which the ring and oxygen neighborhoods have similar binding energies, the ring site of phenol was favored for all the metal ions. The differential was quite small (2-3 kcal mol(-1)) for Mg+, for which the O site may have significant thermal population at ordinary temperatures. Al+, Cr+, and Mn+ showed ring/O differential binding energies of 5-6 kcal mol(-1), which probably rules out significant thermal populations of the oxygen site. The other transition metal ions all showed very large ring/O binding energy differentials. The ring/O binding differentials for phenol were accurately mirrored by the differentials between binding of the same metal ions to benzene versus water.