A computational acumen into the relative applicability of geometrical and quantum chemical criteria in assessing intramolecular hydrogen bonding (IMHB) interaction: 5-Halosalicylic acids as representative examples

Abstract

Density functional theory (DFT) based computational study has been performed to characterize intramolecular hydrogen bonding (IMHB) interaction in a series of salicylic acid derivatives having different halogen substitution at the 5-position on the benzene ring. The molecular systems studied are salicylic acid (SA), 5-fluorosalicylic acid (5FSA), 5-chlorosalicylic acid (5ClSA) and 5-bromosalicylic acid (5BrSA). Particular emphasis has been given on the analysis of IMHB interaction by the calculation of electron density ρ(r) and Laplacian ∇2ρ(r) at the bond critical point using Atoms-In-Molecule (AIM) theory. Topological features and energy densities based on ρ(r) through the perturbation of the IMHB distances suggest that at equilibrium geometry the IMHB interaction develops certain characteristics typical of a covalent interaction. Concomitantly, the role of charge transfer interaction in the IMHB has been critically addressed under the provision of Natural Bond Orbital (NBO) analysis. The formation of Resonance Assisted Hydrogen Bond (RAHB) in the studied molecular systems is also delineated from quantum chemical calculations. The interplay between aromaticity and RAHB is discussed in this context using both geometrical and magnetic criteria as the aromaticity descriptors. The optimized geometry features, analysis of the molecular electrostatic potential map have also been found to produce a consensus view in relation with the formation of RAHB in the studied systems. The lack of a specific pattern of IMHB energy in the studied molecules with the electronegativity of the substituent has also been attempted to address critically. Another major aspect of the present study is the argument about the superiority of quantum chemical criteria over geometrical criteria for the assessment of IMHB interaction in the studied compounds.