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

Abstract

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.

Cation−π Interactions: A Theoretical Investigation of the Interaction of Metallic and Organic Cations with Alkenes, Arenes, and Heteroarenes

“…[60][61][62][63][64][65][66][67] In this context, a number of calculations have been carried out on several cation-π complexes. [60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79] However, the focus of most of these investigations has been the evaluation of optimal geometries or interaction energies of these complexes. While attempts have been made in some of these investigations to rationalize the large variation in the strength of these interactions, 60,64,67,[76][77][78] little effort has been expended in understanding the origin of these interaction energies using rigorous quantum chemical methodologies.…”

Absolute Potassium Cation Affinities (PCAs) in the Gas Phase

“…[61] In the Na ± indole complex, the cation can bind to either the benzo-p or pyrrolo-p face of the ligand. [64] For the larger potassium cation, only one type of K ± indole complex is found, in which the cation binds to the benzo-p face, with an estimated PCA of 89 kJ mol À1 (species o, Figure 2). Thus, even though the PCA of pyrrole is larger than that of benzene, it appears that K binds exclusively to the benzo-p face of indole.…”

Dissociations of Complexes Between Monovalent Metal Ions and Aromatic Amino Acid or Histidine

“…With free indole as a ligand, metal ions typically show a preference for binding on the aryl ring of the bicyclic system [21][22][23]; however, in metal ion-Trp complexes [16,[24][25][26], or in other environments where competing binding sites are available [27,28], additional geometrical constraints imposed by chelation with oxygens, nitrogens, or other electron-rich binding sites become important and the lowest-energy isomers, tridentate chargesolvated structures, often have the metal ion positioned away from the center of the aryl ring and toward the pyrrole ring.…”