The nature of NO-bonding in N-oxide group

Abstract

The nature of the NO-bond in the N-oxide group was investigated by means of combined theoretical calculations (including QTAIM and NBO approaches) and statistical analyses of the contents of crystal structure databases. The N-O bond in the N-oxide group should be classified as the NO donating bond with an important contribution of ON back-donation (of the π-electron type, when available). The visualization of the Laplacian of electron density in the region of an oxygen valence sphere suggests the presence of two lone pairs for the imine-N-oxide group (characterized by effective ON back-donation). A detailed bonding analysis performed by means of natural resonance theory indicates that the N→O bond is of an order of magnitude clearly greater than 1. In addition, the stability of the N→O bond in various N-oxides was estimated. The analyses of the hydrogen- and halogen-bonded complexes of the N-oxides reveal strong Lewis basicity of the N-oxide group. The formation of H- and X-bonding leads to N→O bond elongation due to its structural, topological and spectroscopic characteristics. Moreover, in pyridine-N-oxide, the electron-withdrawing -NO2 group additionally stabilizes the N→O bond, whereas the opposite effect can be observed for the electron-donating-NH2 substituent. This is due to a substituent effect on the π-type ON back-donation. As a result, the oxygen atom in pyridine-N-oxide may change its availability during intermolecular interaction formation, as revealed in the interaction energy, which changes by about half of the estimated total interaction energy.

Non-covalent interactions of N-phenyl-1,5-dimethyl-1H-imidazole-4-carboxamide 3-oxide derivatives—a case of intramolecular N-oxide hydrogen bonds

“…Our recent studies have shown that the N-oxide group may act as an effective proton acceptor in hydrogen bonding [17][18][19]. Also, in case of the crystal structures investigated here, the Noxide group contributes to these kinds of interaction.…”

“…Various N-oxide interactions stabilizing crystal structures have been investigated [17]. On the basis of theoretical chemistry computations, it has been found that the N-oxide group may act as an effective Lewis base in both hydrogen and halogen bonds [18], even more effectively than the oxygen atom of a carbonyl or ether group. For example, the N-oxide group forms very strong hydrogen bonds with isocyanide with an energy close to 16 kcal/mol (DFT-B3LYP/aug-ccpVTZ).…”

“…Recently, the N-oxide group has been the subject of our studies in the context of its ability to act as an electron donor in various types of intermolecular interactions [17][18][19]. Various N-oxide interactions stabilizing crystal structures have been investigated [17].…”

Pyrimidopteridine N‐Oxide Organic Photoredox Catalysts: Characterization, Application and Non‐Covalent Interaction in Solid State

“…In the pyrimidopteridine N ‐oxides, the d(N−O) bond lengths are relatively short, with a mean value of about 1.26 Å. In comparison with typical N−O single bonds in hydroxylamines (1.45 Å) and N=O double bonds in nitrosoalkanes (1.27 Å), the crystallographic data suggests that this bond should be considered as a strong non‐polar dative N−O bond, stabilized by π‐type N−O back‐donation, rather than a polar covalent bond N + −O − . Thus, we propose that the NCI originates from lone pair–π‐hole rather than anion–π‐hole interactions.…”

Simple Trans-Platinum Complex Bearing 3-Aminoflavone Ligand Could Be a Useful Drug: Structure-Activity Relationship of Platinum Complex in Comparison with Cisplatin

“…The QTAIM analysis provides several important quantities such as for example the electron density (ρ) or the Laplacian of the electron density (∇ 2 ρ) calculated at specific points of the molecular space, the latter parameter describes local charge concentration and depletion. The QTAIM method is a powerful tool of structural chemistry allowing to characterize and define various types of bonding of both closed-and shared-shell character [90][91][92]. In the present study QTAIM calculations using the AIMAll program [93] were performed in order to obtain insight into the nature of interatomic bonding around the platinum center in the investigated systems.…”