Binding of Na+, Mg+, and Al+ to the π Faces of Naphthalene and Indole: Ab Initio Mapping Study

Binding energy maps have been calculated for the binding of Na +, Mg +, and Al + to the entire π-facial region of indole and to the parts of the naphthalene face near the long central axis. Binding sites exist for all three cations over the phenyl ring and over the pyrrole ring of indole, although for Na + the pyrrole site almost disappears. The phenyl binding site of indole is in all cases about 4 kcal mol -1 more stable than the pyrrole site and about 6 kcal mol -1 more stable than the similar site in naphthalene. The Hartree−Fock calculations suggested an additional stable binding site for the open-shell Mg + ion over the nitrogen atom of indole, but this site was not reproduced by density-functional calculations and may well be a computational artifact. A differential electrostatic picture is considered in which the π face of indole is equivalent, in its cation interactions, to the π face of naphthalene, with the addition of a differential electrostatic field equal to the difference in the electrostatic fields surrounding naphthalene and indole. The differential electrostatic picture is highly successful for Na +, almost as successful for Al +, and almost as successful for Mg + . This gives quantitative support to the validity of an electrostatic approach to understanding, comparing, and predicting cation binding properties of different π-facial molecules.