Enhancing the Reduction Potential of Quinones via Complex Formation

Abstract

Quantum calculations are used to study the manner in which quinones interact with proton-donating molecules. For neutral donors, a stacked geometry is favored over a H-bond structure. The former is stabilized by charge transfers from the N or O lone pairs to the quinone's π* orbitals. Following the addition of an electron to the quinone, the radical anion forms strong H-bonded complexes with the various donors. The presence of the donor enhances the electron affinity of the quinone. This enhancement is on the order of 15 kcal/mol for neutral donors, but up to as much as 85 kcal/mol for a cationic donor. The increase in electron affinity is larger for electron-rich quinones than for their electron-deficient counterparts, containing halogen substituents. Similar trends are in evidence when the systems are immersed in aqueous solvent.

Unveiling electrostatic portraits of quinones in reduction and protonation states

“…The E value (via Eq. Li + is also reflected in cation binding energies to this anionic state than o-BQ (4.3 times) noticed by Nepal et al 17 Keeping the above in mind, a complete picture of the electronic distribution in reduction and protonation states of BQ is further confirmed with the interaction of a test electrophile, viz., Li + , in terms of the respective binding energies, E. The corresponding optimized structures along with their Li + binding distances are shown in Figure 5. (Figure 5a) and −9.17 kcal mol −1 above the ring plane (Figure 5b) Further, it is observed that addition of an electron and a proton to BQ, i.e., BQH • state follows the trend going from more negative than the BQ one MESP minimum values at carbonyl (which is having most negative MESP minimum value) towards more positive side at the hydroxyl oxygen and then above the ring in C=C regions.…”

“…As shown by Nepal et al, 17 a cationic species viz., CNH 2 (NHCH 3 ) + 2 forms a complex with o-BQ. The deeper negative electronic region of o-BQ •− , around the oxygen atom, than that in o-BQ supports the reported higher binding energy (∼ 2.1 times) of cationic species with o-BQ •− than the latter one.…”

“…We have also performed MESP topographical analysis on a topological isomer of para-BQ i.e. orthobenzoquinone (o-BQ) and its radical semiquinone anion (o-BQ •− ), as another test case that has been studied by Nepal et al 17 The MESP isosurfaces with respective minima of o-BQ and o-BQ •− are shown in the Figure 4(a) and (b), respectively. The most negative MESP minimum is observed near carbonyl oxygen of o-BQ having value −51.3 kcal mol −1 , indicating the more electron localization (see Figure 4(a)).…”

“…They observed that the interaction energies of BQ •− and BQ 2− -water monomer/dimer complexes are more favourable than the respective neutral BQ complexes by ∼ 2.6 and ∼ 6.0 kcal mol −1 due to the increase in H-bond strength (by ∼ 2 kcal mol −1 ) in the reduced species. Nepal et al, 17 recently studied, using the DFT method, the H-bonding interactions of oquinone and its radical semiquinone anion with several proton donor molecules viz., dimethylamine, methanol, ethanol, dimethylurea and a strong cationic species, viz., CNH 2 (NHCH 3 ) + 2 . They visualized the MESP regions and qualitatively assessed the H-bonding ability of oquinone substituted with the electron-donating NH 2 and electron-withdrawing Cl groups.…”

Highly Selective Halide Receptors Based on Chalcogen, Pnicogen, and Tetrel Bonds

“…[68,69] This basis set and functional have been applied previously to similar sorts of systems to good effect. [14,[33][34][35][70][71][72][73][74][75] Relativistic effects are expected to be most significant for atoms below the third row of the periodic table. Minima on each potential energy surface were assured by the absence of any negative frequencies.…”