Interplay of aromatic energy and hydrogen bonding energy in stabilizing naphthoquinone conformers was shown; overall stability is dictated by the aromatic stabilization instead of hydrogen bonding energy. Maximum hardness principle and minimum electrophilicity principle supports the stability of naphthoquinone conformers. The total energy partition analysis describes electrostatic interaction as the major force in stabilizing these systems.

Abstract

Intramolecular hydrogen-bonded naphthoquinone derivatives are investigated using both DFT (density functional theory) and MP2 (Møller–Plesset perturbation theory) method. Three different sets of naphthoquinone derivatives, each set consisting of syn and anti conformers with respect to the substitution by (–OH/–NH₂) at peri and para position of the ring, are considered for investigation. In all cases, the syn conformer is found to be energetically more stable compared with the anti conformer. However, the hydrogen bond strength of the anti conformer is found to be more as obtained from NBO (natural bond orbital) analysis and QTAIM (quantum theory of atoms in molecules) theory. The O···H–O interactions are associated with covalent character while O···H–N interactions are noncovalent in nature. The NCI (noncovalent interaction) isosurface clearly indicates the formation of O···H–O and O···H–N intramolecular hydrogen bond. The energy partition analysis indicates electrostatic interaction as the dominant force in stabilizing these systems. The aromaticity calculation by HOMA (harmonic oscillator model of aromaticity) index and Bird index indicate the syn conformer with higher aromaticity compared to the anti conformer. The overall stability of the syn conformers is thus dominated by the aromatic stabilization energy instead of hydrogen bonding energy. The global reactivity descriptors further support the higher stability of the syn conformers according to maximum hardness principle (MHP) and minimum electrophilicity principle (MEP).