Anion-Anion Interactions in Aerogen-Bonded Complexes. Influence of Solvent Environment

Abstract

Ab initio calculations are applied to the question as to whether a AeX₅^- anion (Ae = Kr, Xe) can engage in a stable complex with another anion: F^-, Cl^-, or CN^-. The latter approaches the central Ae atom from above the molecular plane, along its C₅ axis. While the electrostatic repulsion between the two anions prevents their association in the gas phase, immersion of the system in a polar medium allows dimerization to proceed. The aerogen bond is a weak one, with binding energies less than 2 kcal/mol, even in highly polar aqueous solvent. The complexes are metastable in the less polar solvents THF and DMF, with dissociation opposed by a small energy barrier.

Keywords: AIM; energy decomposition; molecular electrostatic potential; π-hole.

Figure 1

MEP of XeF₅⁻ on its 0.001 au isodensity surface at the MP2/aug-cc-pVDZ level. Color scale ranges from −0.13 (blue) to −0.11 (red) au.

Figure 2

Optimized geometries of sample anion-anion complexes (a) KrF₅⁻···NC⁻, (b) XeCl₅⁻···Cl⁻, and (c) XeF₅⁻···F⁻.

Figure 3

Doubly occupied NBO (a) p and (b) s-orbitals of KrF₅⁻ monomer, and (c) ELF diagram.