Intermolecular hydrogen bonds between 1,4-benzoquinones and HF molecule: Synergetic effects, reduction potentials and electron affinities.

Some biological activities of quinones can be attributed to the H-bonding ability of acceptor oxygen atoms. According to the results obtained from the quantum mechanical calculations performed on a wide variety of complexes between the 1,4-benzoquinone (BQ) derivatives and HF molecules, the interplay between H-bonds and individual H-bond interaction energies (EHB) can be affected by the substituents placed on the six-membered ring of BQ. The total binding energies of complexes become more negative by the electron donating substituents (EDSs) while the changes are reversed by the electron withdrawing substituents (EWSs). The mutual interplay between the X-BQ⋯(HF)n (n=1-3) interactions has been investigated using the geometrical parameters, synergetic energies (SE) and the EHB values. Hydrogen bonding decreases the reduction potentials (E0red) and increases the electron affinities (EA) of X-BQ derivatives. Linear relationships have been observed between the E0red (and EA) values and the Hammett constants of substituents.

π-Stacking effects on the hydrogen bonding capacity of methyl 2-naphthoate.

π-stacking effects of fused two-ring system of methyl 2-naphthoate (MNP) with benzene derivatives on the CO group, as a hydrogen bond acceptor, has been investigated by the quantum mechanical calculations at the M06-2X/6-311++G(d,p) level of theory. All substituents enhance the stacking interactions relative to the unsubstituted case, where enhancement is higher for electron-withdrawing substituents (EWSs). The hydrogen bonding ability of lone pairs of O* atom of stacked MNP decreases in the presence of strong electron-withdrawing substituents (NO2, NO and CN). The hydrogen bond ability of CO group of MNP is related to the sum of local minima of electrostatic potentials (∑ESPs) observed between stacked rings. The charge transfer (CT) is lower in the presence of EWSs. The study also shows that the interaction energies (ΔE) are linearly dependent on the combination of the sum of electron densities calculated at the bond critical points (BCPs) between the rings (∑ρBCP) and the sum of electron charge densities calculated at the ring critical points (∑ρRCP). There are good relationships between the Hammett constant σmeta and the global minimum of electrostatic potential around the O* atom (Vmin), the sum of local minima of the electrostatic potentials obtained between stacked rings, and the results of natural population analysis (NPA). An excellent correlation was found between the ΔE values and a combination of the electrostatic (σmeta), resonance/induction (σpara) and dispersion/polarizibility (molar refractivity, MR) substituent constant terms.