Critical assessment of the intramolecular hydrogen bonding (IMHB) interaction in two isomeric hydroxynicotinic acids: A computational approach

Density Functional Theory (DFT) based computational study has been performed to characterize intramolecular hydrogen bonding (IMHB) interaction in two isomeric hydroxynicotinic acids viz. 3-hydroxyisonicotinic acid (3HINA) and 4-hydroxynicotinic acid (4HNA). The IMHB interaction has been analyzed 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 assistance of resonance in the IMHB interaction present in these molecular systems is also delineated from quantum chemical calculations. The comparatively greater IMHB energy for 4HNA with respect to its isomer has also been attempted to address critically along with the argument about the superiority of quantum chemical criteria over geometrical criteria for the assessment of IMHB interaction in the studied compounds.