The electrochemical behavior of several phenols, quinones and hydroquinone in acetonitrile (CH3CN) with varying amounts of water were investigated to understand the effect of hydrogen-bonding on their voltammetric responses. Karl Fischer coulometric titrations were performed to obtain an accurate reading of the water concentrations. The solvent/electrolyte mixture was carefully dried using 3 Å molecular sieves to obtain an initial water content that was close to the substrate concentration (∼1 × 10–3 M), and higher water contents were then achieved via the addition from microliter syringes. It was found that small changes in what is often considered “trace” amounts of water were sufficient to substantially change the potential and in some cases the appearance of the voltammetric waves observed during the oxidation of the phenols/hydroquinones and reduction of the quinones. Density functional theory calculations were performed on the reduced/oxidized species in the presence of varying numbers of water molecules to better understand the hydrogen-bonding interactions at the molecular level. The results highlight the importance of accurately knowing the trace water content of organic solvents when used for voltammetric experiments.
Monostibine-protected ionic Au13 nanoclusters, namely, [Au13(L)8(Cl)4][Cl] (L = SbPh3, 2a·Cl; Sb(p-tolyl)3, 2b·Cl) were prepared by the direct reduction of Au(L)Cl with NaBH4 in dichloromethane.
The phenol, α-tocopherol, can be electrochemically oxidised in a -2e(-)/-H(+) process to form a diamagnetic cation that is long-lived in dry organic solvents such as acetonitrile and dichloromethane, but in the presence of water quickly reacts to form a hemiketal. Variable scan rate cyclic voltammetry experiments in acetonitrile with carefully controlled amounts of water between 0.010 M-0.6 M were performed in order to determine the rate of reaction of the diamagnetic cation with water. The water content of the solvent was accurately determined by Karl Fischer coulometric titrations and the voltammetric data were modelled using digital simulation techniques. The oxidation peak potential of α-tocopherol measured during cyclic voltammetry experiments was found to shift to less positive potentials as increasing amounts of water (0.01-0.6 M) were added to the acetonitrile, which was interpreted based on hydrogen-bonding interactions between the phenolic hydrogen atom and water. Several other phenols were examined and they displayed similar voltammetric features to α-tocopherol, suggesting that interactions of phenols with trace amounts of water were a common occurrence in acetonitrile. The H-bonding interactions of α-tocopherol with water were also examined via NMR and UV-vis spectroscopies, with the voltammetric and spectroscopic studies extended to include other coordinating solvents (dimethyl sulfoxide and pyridine).
Voltammetric experiments with 9,10-anthraquinone and 1,4-benzoquinone performed under controlled moisture conditions indicate that the hydrogen-bond strengths of alcohols in aprotic organic solvents can be differentiated by the electrochemical parameter ΔEp (red) =|Ep (red(1)) -Ep (red(2)) |, which is the potential separation between the two one-electron reduction processes. This electrochemical parameter is inversely related to the strength of the interactions and can be used to differentiate between primary, secondary, tertiary alcohols, and even diols, as it is sensitive to both their steric and electronic properties. The results are highly reproducible across two solvents with substantially different hydrogen-bonding properties (CH3 CN and CH2 Cl2 ) and are supported by density functional theory calculations. This indicates that the numerous solvent-alcohol interactions are less significant than the quinone-alcohol hydrogen-bonding interactions. The utility of ΔEp (red) was illustrated by comparisons between 1) 3,3,3-trifluoro-n-propanol and 1,3-difluoroisopropanol and 2) ethylene glycol and 2,2,2-trifluoroethanol.
The reduced forms of quinones (Q •−/2− ) are well-known for their binding affinities toward electrophiles. The ability to modify and add substituents onto quinones to alter their electronic and steric properties allows the optimization of their structures for the highest interactions with electrophiles. Three reduced naphthoquinones with different methyl substitutions of their quinone ring were investigated for their suitability as electrocatalysts for CO 2 capture and conversion. In the aprotic organic solvent acetonitrile and in the absence of dissolved molecular oxygen, the quinones can be reduced in consecutive one-electron steps to form first the monoanion radicals (Q •− ) and then at more negative potentials the dianions (Q 2− ). When CO 2 (g) is purged into the solution, the two one-electron reduction processes merge into one two-electron chemically reversible reduction process at the same potential as the first one-electron reduction process observed in an Ar(g) atmosphere. It is proposed that a complex is formed between the reduced quinone and nCO 2 molecules, [Q(CO 2 ) n ] 2− , that allows the dianion to be formed at a lower energy (voltage) compared to under an Ar(g) atmosphere. The binding is completely chemically reversible so that purging the solutions of [Q(CO 2 ) n ] 2− with Ar(g) results in the carboxylated complex dissociating according to two major pathways. Pathway (A) involves the generation of Q 2− (or Q •− ) and CO 2 (g), while pathway (B) results in the negative charge transferring to the CO 2 molecules to form the carboxyl radical anion, CO 2•− , and the neutral Q.
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