Peculiarities of quasi-aromatic hydrogen bonding

Abstract

The analysis of the calculated topological and energetic characteristics is presented for the hydroxyaryl, alkyl Schiff and Mannich bases. The quantum theory of the atoms-in-molecules methodology has been used to explore the topology of the electron density at the bond and ring critical points in the studied compounds with intramolecular hydrogen bonding. The dependencies between the electron-topological, aromatic, and energetic parameters under tautomeric equilibrium have been analysed. The calculated non-adiabatic potential curves show the difference between the unconjugated and p-conjugated compounds. The calculated dependency between the binding energy of hydrogen bonding (DE HB ) and the aromaticity of the chelate chain (the HOMA aromaticity index) reveals a difference between the two types of compounds. The results obtained have made it possible to figure out a basic difference between unconjugated and p-conjugated compounds.

Quasi-aromaticity—what does it mean?

“…by analysis of malonaldehyde itself (the energy corresponding to proton transfer barrier does not exceeds the value of 10 kcal/mol, depending on substituent), 93 but also for ketoenamine-enolimine tautometic equilibrium studies performed for a series of various hydroxyaryl, alkyl Schiff and Mannich bases. 94 As a consequence, the stronger H-bonds are observed for more delocalized chelate rings, which is in good agreement with Leffler-Hammond postulate, according to which systems of stronger H-bonds are structurally closer to transition states corresponding to proton transfer reaction. 95,96 When the chelate ring is substituted in position 1 (or 3), the symmetric relation between both ground state energies and geometries is perturbed and one of ground state structures is energetically preferred.…”

“…It is worth mentioning that the structural and electron density-topological parameters of H-bridge in malonaldehyde derivatives were found to be relatively well correlated with indications of various aromaticity indices, including those based on structural and electronic properties of the chelate ring. 93,94,98 …”

Intramolecular Hydrogen Bonds in Selected Aromatic Compounds: Recent Developments

“…The papers [28] and [29] dwell on calculations of the hydroxyaryl Schiff and Mannich bases by the Density Functional Theory (DFT) method, illustrating the influence of proton transfer in quasi-aromatic hydrogen bonding on the shape of the potential energy curve. Ref.…”

“…In Ref. [29], a complete analysis of the electron density changes at the bonds' and rings' critical points obtained by calculation of the potential energy curve for proton transfer within the adiabatic approximation for 2-((1-methylo-imino)ethyl)phenol (Figure 3) is described.…”

“…In Ref. [29], a complete analysis of the electron density changes at the bonds' and rings' critical points obtained by calculation of the potential energy curve for proton transfer within the adiabatic approximation for 2-((1-methylo-imino)ethyl)phenol (Figure 3) is described. For a description of the electron density changes occurring in the molecule under proton transfer in the hydrogen bridge, the method of "Atoms in Molecules" (AIM) [30] was employed.…”