Substitution is well-known to modulate the physico-chemical properties of molecules. In this study, a combined, multifactor approach was employed to determine a plethora of substitution patterns using -Br and -O-H in 1,4-naphthoquinone and its derivatives. On the basis of classical Density Functional Theory (DFT), 25 models divided into three groups were developed. The first group contains 1,4-naphthoquinone and its derivatives substituted only by -Br. The second group consists of compounds substituted by -Br and one -O-H group. As a result of the substitution, an intramolecular hydrogen bond was formed. The third group also contains -Br as a substituent, but two -O-H groups were introduced and two intramolecular hydrogen bonds were established. The simulations were performed at the ωB97XD/6-311++G(2d,2p) level of theory. The presence of substituents influenced the electronic structure of the parent compound and its derivatives by inductive effects, but it also affected the geometry of the 2 and 3 groups, due to the intramolecular hydrogen bonding and the formation of a quasi-ring/rings. The static DFT models were applied to investigate the aromaticity changes in the fused rings based on the Harmonic Oscillator Model of Aromaticity (HOMA). The OH stretching was detected for the compounds from groups 2 and 3 and further used to find correlations with energetic parameters. The evolution of the electronic structure was analyzed using Hirshfeld atomic charges and the Substituent Active Region (cSAR) parameter. The proton reaction path was investigated to provide information on the modulation of hydrogen bridge properties by diverse substitution positions on the donor and acceptor sides. Subsequently, Car-Parrinello Molecular Dynamics (CPMD) was carried out in the double-bridged systems (group 3) to assess the cooperative effects in double -O-H-substituted systems. It was determined that the -O-H influence on the core of the molecule is more significant than that of -Br, but the latter has a major impact on the bridge dynamics. The competitive or synergic effect of two -Br substituents was found to depend on the coupling between the intramolecular hydrogen bridges. Thus, the novel mechanism of a secondary (cooperative) substituent effect was established in the double-bridged systems via DFT and CPMD results comparison, consisting of a mediation of the bromine substitutions' influence by the cooperative proton transfer events in the hydrogen bridges.
Keywords: 1,4-naphthoquinone derivatives; CPMD; DFT; HOMA; Hirshfeld atomic charges; cSAR; substituent effect.