Directionality of Cation/Molecule Bonding in Lewis Bases Containing the Carbonyl Group
journal contributionposted on 05.09.2017, 13:49 by Younes Valadbeigi, Jean-François Gal
Relationship between the CO–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 CO group to form linear CO–X geometries. Because of overlapping with lone-pair electrons of the sp2 carbonyl oxygen, the H+ and Cu+ formed a bent CO–X angle. Al+ displayed an intermediate behavior; the CO–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 CO–X angles in M/X+ adduct ions were scanned in different O–X bond lengths. It was found that the most favorable CO–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 CS–X and CSe–X angles were obtained for all cations, although the dipole vectors of CH2S and CH2Se coincide with the CS and CSe bonds. The bending of the CS–X and CSe–X angles was attributed to the covalent characteristic of S–X and Se–X interactions due to high polarizability of S and Se atoms.