Topological analysis of the electron localisation function (ELF), natural bond orbital and Wiberg bond index calculations have been applied to study the electronic structure of the oxaziridine molecule with a special focus on the nitrogen-oxygen bond. The calculations have been performed at the DFT(B3LYP, CAM-B3LYP, ωB97XD, M06-L, M06-2X) and post-Hartree-Fock (CCSD(T) and CASSCF) computational levels with applied aug-cc-pVTZ basis set. Nature of N-O bonding has been characterised by two resonance forms, N⁺O⁻ and N⁻O⁺, owing to a very small population (< 0.60e) of the bonding basin V(N,O), localised in the ELF field for the N-O region. The importance of electron correlation effects for the description of the N-O bonding has been observed in the CASSCF calculations. The orbital description (Wiberg, NBO) differs from topological characterisation, indicating a single N-O bond.

Local nature of the boron-nitrogen (BN) bonding with different formal multiplicities (B≡N, B=N, B-N) have been investigated for 25 experimentally established organoboron molecules in both real and the Hilbert space, using topological analysis of electron localization function (ELF), electron density (AIM), and natural bond orbital (NBO) method. Each BN bond has been represented (ELF) by the bonding disynaptic attractor V(B,N), with the basin electron population between 5.72e and 1.83e, confirming possible existence of all the three bond types. A covalent character of bonding can be associated with the dative mechanism due to the V(B,N) bonding basin formed mainly (91–96%) by the N electron density. Similarly, the NBO method shows 2-center natural orbitals, consisting largely of the hybrids from the N atom. The AIM analysis yields the features typical for shared (H_(3,−1)(r) < 0) and closed-shell (∇²ρ_(3,−1)(r) > 0) interactions. The delocalization indices, describing electron exchanges between B and N quantum atoms, are smaller than 1.5, even for formally very short triple B≡N bonds.

A prototypical reaction between ammonia and formaldehyde has been investigated at the DFT(M06)/6-311++G(d,p) computational level using the Bonding Evolution Theory (BET). BET is a very useful tool for studying reaction mechanisms as it combines topological analysis of electron localisation function with the catastrophe theory. Each of two studied reactions: H₂C=O + NH₃ ↔ HO–C(H₂)–NH₂ (hemiaminal) and HO–C(H₂)–NH₂ ↔ HN = CH₂ (Schiff base) + H₂O consists of six steps. Formation of hemiaminal starts from a nucleophillic attack of nitrogen lone pair in NH₃ on the carbon atom in H₂C=O and is subsequently followed by hydrogen transfer within the N–H^..O bridge. A Schiff base is formed via the dehydration reaction of the hemiaminal, where the C–O bond is broken first, followed by hydrogen transfer towards the [HO]^δ− moiety, resulting in water and methanimine. The present paper focuses on differences in reaction mechanisms for the processes described above. The results have been compared to the reaction mechanism for stable hemiaminal synthesis from benzaldehyde and 4-amine-4H-1,2,4-triazole studied previously using the BET theory.

The mechanisms of reaction of benzaldehyde (ald) with 4-amine-4H-1,2,4-triazole (4at), leading to Schiff base (Sch) and water, were investigated using topological analysis of the electron localisation function and catastrophe theory. Two reactions (synthesis of hemiaminal and synthesis of Schiff base) are represented by one catastrophe sequence: ald+4at: 1-14-[FF^†F^†FF^TS1FF^†F^†FF^†F^†CF^†]-2-9-[C^†FF^TS3F^†F^†FFF]-0:Sch+H₂O with only fold (F) and cusp (C) catastrophes. The first reaction, in which a molecule of the hemiaminal is formed, consists of 14 steps separated by 13 catastrophes. The mechanism is non-concerted. The covalent bond C–N is formed after the formation of the O–H bond is terminated. The Schiff base formation through the water molecule elimination in the second reaction requires nine steps with eight catastrophes. The mechanism is non-concerted because first the C–O bond is broken and then the proton transfer occurs that results in the O–H bond creation.