Directionality of Cation/Molecule Bonding in Lewis Bases Containing the Carbonyl Group

Relationship between the C═O-X⁺ (X = H, Li, Na, K, Al, Cu) angle and covalent characteristic of the X⁺-M (M = CH₂O, CH₃CHO, acetone, imidazol-2-one (C₂H₂N₂O), cytosine, γ-butyrolactone) was investigated, theoretically. The calculated electron densities ρ at the bond critical points revealed that the covalency of the M-X⁺ interaction depended on the nature of the cation and varied as H⁺ > Cu⁺ > Al⁺ > Li⁺ > Na⁺ > K⁺. The alkali cations tended to participate in electrostatic interactions and aligned with the direction of the molecule dipole or local dipole of C═O group to form linear C═O-X geometries. Because of overlapping with lone-pair electrons of the sp² carbonyl oxygen, the H⁺ and Cu⁺ formed a bent C═O-X angle. Al⁺ displayed an intermediate behavior; the C═O-Al angle was 180° in [CH₂O/Al]⁺ (mainly electrostatic), but when the angle was bent (146°) under the effect of local dipole of an adjacent imine group in cytosine, the covalency of the CO-Al⁺ interaction increased. The C═O-X angles in M/X⁺ adduct ions were scanned in different O-X bond lengths. It was found that the most favorable C═O-X angle depended on the O-X bond length. This dependency was attributed to variation of covalent and electrostatic contributions with O-X distance. In addition, the structures of [CH₂S/X]⁺ and [CH₂Se/X]⁺ were studied, and only bent C═S-X and C═Se-X angles were obtained for all cations, although the dipole vectors of CH₂S and CH₂Se coincide with the C═S and C═Se bonds. The bending of the C═S-X and C═Se-X angles was attributed to the covalent characteristic of S-X and Se-X interactions due to high polarizability of S and Se atoms.