Both the Quantum Theory of Atoms in Molecules (QTAIM) and the Electron Localization Function (ELF) method have been applied to analyze the H…O interactions. Numerous relationships between ELF and QTAIM parameters were found. It is interesting that the CVB index based on the ELF method as well as the total electron energy density at the bond critical point of the proton–acceptor distance (Hbcp) may be treated as universal descriptors of the hydrogen bond strength, they are also useful to estimate the covalent character of this interaction. There are so-called resonance-assisted hydrogen bonds (RAHBs) among the species analyzed. It was found that there are not any distinct differences between RAHBs and the other intramolecular hydrogen bonds.
Dependencies between the QTAIM parameters and CVBI; for Gbcp, the linear correlation is presented, while for Hbcp and Vbcp, the second order polynomial relationships are presented.
The ELF analysis of interactions in complexes of S(CN)2, Se(CN)2, SFCl and SeFCl with F– and Cl– anions was performed in our lab. The sulphur and selenium atoms act in these complexes as Lewis acid centres interacting with fluorine and chlorine anions. The arrangement of sub-units in complexes is in agreement with the σ-hole concept; particularly it is a result of contacts between positive and negative electrostatic potentials´ sites. The interactions in complexes analyzed may be classified as very strong charge assisted chalcogen bonds and they possess numerous characteristics typical for covalent bonds. The QTAIM and ELF approaches allow to predict the directions of nucleophilic attack.
Isosurfaces of the Laplacian of electron density (∇2 ρ = 0.253 au) of the S(CN)2 monomer. Isolines of ∇2 ρ are included – black and red contours correspond to negative and positive Laplacian values, respectively (range set to ±0.2 au with step equal to 4 x 10-2 au)
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