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

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journal contribution
posted on 05.09.2017, 13:49 by Younes Valadbeigi, Jean-François Gal
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.