Aromaticity balance, π-electron cooperativity and H-bonding properties in tautomerism of salicylideneaniline: the quantum theory of atoms in molecules (QTAIM) approach

Abstract

Topological analysis based on DFT calculations regarding proton transfer reaction in salicylideneaniline (SA) was performed to scrutinize possible changes in the intramolecular H-bond, π-electron delocalization and aromaticity levels of certain fragments. Quantum chemical calculations and natural bond orbital (NBO) analyses were carried out over a tautomeric ensemble whose members correspond to the molecules at different stages in tautomeric interconversion of SA. The elaboration of intramolecular hydrogen bonding in terms of the relevant topological parameters and the interpretation of certain dependencies regarding its strength were examined. The results show that delocalization index (DI) between donor and acceptor atom δ(O,N) is a useful topological parameter for describing H-bond strength, which is influenced by π-delocalization level within quasiaromatic chelate ring, indicating its resonance-assisted character. NBO analyses reveal that lone-pair (LP) population on N center also affects the strength of intramolecular H-bond in SA. Furthermore, π-electron transfer accompanying intramolecular proton migration in SA is brought into being through formally vacant non-Lewis type LP* orbital on the tautomeric proton. As a result of this, tautomeric protons in molecular entities near TS have hypovalent character due to the lack of electron population in the bonding orbital relative to that in LP* orbital. While H-bonds in the tautomeric ensemble of SA are predominantly partial covalent, molecular entities close to transition state have the strongest covalent H-bonds. The most important result is also that there are linear correlations between the orders of bonds (hydroxyl and amine) involving intramolecular H-bond and electron density values at the relevant BCPs due to partially covalent character of these bonds, contrary to exponential behavior as for purely covalent bonds. Quasiaromatic chelate ring formation is established not only to compel a reduced aromaticity of salicylidene ring but also to decrease in LP-population on N.

Hydrogen-bridged chelate ring-assisted π-stacking interactions

“…Aromatic Schiff bases are one of the most promising chemical species to be considered for this purpose because of their structural and electronic properties mentioned below. The structures of ortho-hydroxy aromatic Schiff bases, in particular salicylideneaniline (SA) derivatives, have received much attention owing to their keto-phenol (or NH OH) tautomerism (Fabian et al, 2004;Amimoto & Kawato, 2005;Filarowski, 2005;Filarowski et al, 2005;Petek et al, 2008Petek et al, , 2010Karabiyik et al, 2009Karabiyik et al, , 2011. Depending on the position of the tautomeric proton, two types of intramolecular hydrogen bonds are possible, O-HÁ Á ÁN and OÁ Á ÁH-N, in phenol (OH) and in cisketo (NH) tautomers, respectively.…”

Quasi-aromaticity—what does it mean?

“…al 129. studied aromaticity balance, π-electron cooperativity and H-bonding properties in tautomerism of salicylideneaniline and found that delocalization in chelate chain estimated by PDI130 correlates with H-bond strength.…”

Effects of pnictogen and chalcogen bonds on the aromaticities of carbazole-like and dibenzofuran-like molecular skeletons: Cambridge Crystallographic Data Centre (CCDC) Study

“…This is an intriguing aspect implying aromaticity balance between six‐membered and five‐membered rings of dibenzothiophene as compared with dibenzofuran. Aromaticity balance is a well‐known mechanism involving intramolecular electronic charge transfer . Inclusion of selenium as endocyclic heteroatom in dibenzofuran skeleton makes central five‐membered ring considerably aromatic, while inclusion of tellurium leads to a deficiency in the aromaticity of five‐membered ring without considerable changes in aromaticities of two flanking benzenes.…”

“…Aromaticity balance is a well-known mechanism involving intramolecular electronic charge transfer. [44,45] Inclusion of selenium as endocyclic heteroatom in dibenzofuran skeleton makes central five-membered ring considerably aromatic, while inclusion of tellurium leads to a deficiency in the aromaticity of five-membered ring without considerable changes in aromaticities of two flanking benzenes. Selenium and tellurium are distinguished from other chalcogens by the presence of fully occupied dorbitals.…”