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Topological Study of the Electron Density of Cation-π-Anion Complexes
* 1 , 1 , 2
1  Departamento de Química Física, Facultade de Ciencias, Universidade de Santiago de Compostela. Campus de Lugo. Avenida Alfonso X El Sabio s/n, Lugo 27002, SPAIN.
2  Centro Singular de Investigación en Química Biológica e Materiais Moleculares (CIQUS), Universidade de Santiago de Compostela, Rúa Jenaro de la Fuente s/n, Santiago de Compostela 15782, SPAIN.

Abstract: The central role that non-covalent interactions play in many fields of science and technology has been widely recognized for many years. Between this kind of interactions, those that involve π‑systems deserve a particular interest because their great importance in biological environments. The participation of tyrosine, phenylalanine, tryptophan and histidine residues in the function of active sites of enzymes is only one of the many examples that support this interest. Because of this significance, many studies about π–π, cation‑π, anion‑π and, more recently, cation‑anion‑π complexes have been carried out, both experimentally and theoretically. The major forces identified as responsible of the energetic and geometric features of the complexes traditionally have electrostatic nature, mainly in the cases where a cation and a π system are involved. However, there are evidences that in many of these systems, induction and dispersion contributions are important also, and sometimes can even be the leading forces. With the aim of gaining understanding on the fundamental causes acting behind the non-covalent interactions that include π‑systems, in the present work, the electron density of various cation‑anion‑π complexes has been studied using the Bader’s Atoms in Molecules theory. The geometry of the ternary complexes was previously optimized using the MP2 and M062X methods of calculation, and then the correlated wavefunction was analyzed looking for bond paths and critical points, at which the electron density and its Laplacian were computed. Indole, and methyl-indole, both structured molecules with relevance in biological environments were chosen as models of π‑systems. The cations (Na+, NH4+, C(NH2)3+ and N(CH3)4+) and the anions (Cl, NO3, HCOO and BF4) included in the study also have biological interest and vary from monoatomic to structured species.
Keywords: Cation-π-Anion, AIM theory, Cation-π complexes