Evaluating mutual influences of cation-π interactions and H-bonding: Cases of indole and BN-indole

Complexes of two indoles, both organic and inorganic, with a Li cation and a water molecule are reported. The mutual strengthening of both types of interactions (cation-π and H-bonding) is estimated by theoretical techniques. [Display omitted] An understanding of how various cations and water molecules form complexes with BN-indoles, inorganic counterparts of indole, may provide vital insights for its applications in biology as a motif of unnatural tryptophan. In this paper, we study mutual influences of the hydrogen bond (H-bond) and cation–π interactions in various complexes of indole and BN-indole with Li+ and H2O. Using the second-order symmetry-adapted perturbation theory (SAPT2), we determine the strength of both interactions and obtain their energy constituents. It has been established that the substitution of a CC bond with a BN one in the indole molecule does not lead to a significant drop of Li+ interaction energy (Eint). Moreover, Eint values for Li+ and indole + H2O and BN-indole + H2O complexes further increase, and the stabilization reaches 6.41–8.09 kcal/mol. Eint for a H2O molecule and Li+@indole and Li+@BN-indole complexes also increases, and the enhancement is calculated to be 5.93–7.50 kcal/mol. The quantum theory of atoms in molecules (QTAIM) approves the existing of H-bonds, and using topological parameters we calculate H-bonding energies, which are in a good accordance with the SAPT2 results. The complementary independent gradient model (IGM) technique helps to visualize 3D regions of non-covalent interactions and serves as an additional indicator of both types of the interplay. Finally, the basic regularities in mutual strengthening of H-bonds and cation–π interactions have been established. We believe that some theoretical aspects, derived in the present work, will be useful for the employment of biologically relevant molecules with new physical and chemical properties introduced via BN-CC substitution.