Lack of resonance assistance in a classical intramolecular hydrogen bond: An exploration from quantum theory of atoms‐in‐molecules perspective

Abstract

AbstractA computational investigation has been performed to characterize the intramolecular hydrogen bond (IMHB) interaction in 7‐hydroxy‐2,3‐dihydro‐1H‐inden‐1‐one (7H1I). The results of natural bond orbital (NBO) analysis describe the formation of the IMHB in 7H1I in terms of hyperconjugative charge transfer from the lone electron pair of the acceptor oxygen atom (Oa) to the σ* antibonding orbital of the Od―H1 proton donating bond. However, the key motivation of the present work rests on characterization of the IMHB interaction by calculation of electron density (ρc), its Laplacian (∇2ρc), and electron energy density parameters at the bond critical point (BCP) based on the quantum theory of atoms‐in‐molecules (QTAIM). The topological parameters reveal the intriguing result of the lack of resonance assistance in the IMHB of 7H1I despite the presence of an apparently suitable route for the same, contrary to its aromatic (salicylic acid) and nonaromatic ((Z)‐3‐hydroxyacrylaldehyde) analogues. This inference is further supported by an analysis of the aromaticity of the conjoined benzene nucleus. The predominance of hyperconjugation as compared with rehybridization underlying the formation of IMHB in 7H1I is also argued in this context.