Ascorbic acid is a well-known antioxidant and radical scavenger. It can be oxidized by losing two protons and two electrons, but normally loses only one electron at a time. The reactivity of the ascorbate radical is unusual, in that it can either disproportionate or react with other radicals, but it reacts poorly with non-radical species. To explore the oxidation mechanism of ascorbic acid, the pK's and reduction potentials have been calculated using the B3LYP/6-31+G(d,p) and CBS-QB3 levels of theory with the SMD implicit solvent model and explicit waters. Calculations show that the most stable form of dehydroascorbic acid in water is the bicyclic hydrated structure, in agreement with NMR studies. The possible oxidation reactions at different pH conditions can be understood by constructing a potential-pH (Pourbaix) diagram from the calculated pK's and standard reduction potentials. At physiological pH disproportionation of the intermediate radical is thermodynamically favored. The calculations show that disproportionation proceeds via dimerization of ascorbate radical and internal electron transfer, as suggested by Bielski. In the dimer, one of the ascorbate units cyclizes. Then protonation and dissociation yields the fully reduced and bicyclic fully oxidized structures. Calculations show that this mechanism also explains the reaction of the ascorbic acid radical with other radical species such as superoxide. Ascorbate radical combines with the radical, and intramolecular electron transfer followed by cyclization and hydrolysis yields dehydroascorbic acid and converts the radical to its reduced form.
The response of electrochemical interfaces to applied voltages dictates both the chemical and physical properties of electrochemical systems. The capacitance of an interface intrinsically depends on its voltage-dependent, microscopic structural and compositional changes, yet detailed characterization of this structure/property relationship is often difficult. In this work, we employ constant potential molecular dynamics simulations to investigate the relationship between capacitance and interfacial structure of supercapacitor systems composed of pristine graphite electrodes combined with several organic solvents as well as [BMIm + ][BF 4 − ]/acetonitrile electrolytes at various concentrations. Specifically, we quantify how the total capacitance of acetonitrile, acetone, dichloroethane, and chloroform solvents depends on both inner layer and diffusion layer contributions and evaluate perfect screening of the Helmholtz layer when [BMIm + ][BF 4 − ] ions are added to the solvent. Surprisingly, we find that the inner layer capacitances for the organic solvents and [BMIm + ][BF 4 − ] solutions are very similar, regardless of the solvent type and largely independent of the ion concentration. This is because of the strong hydrophobic attraction of nonpolar alkyl groups of solvent molecules and ions with the graphene surface, which is similar for the different systems. When high voltage is applied, the electrostatic and hydrophobic interactions at the interface are modulated, leading to reorientation of interfacial ions and solvent molecules. The most significant structural rearrangements occur for acetone and pure [BMIm + ][BF 4 − ] ionic liquid, and these rearrangements are correlated to dielectric saturation of the inner layer capacitance at higher voltages. Our results imply that strong hydrophobic forces are an important influence on the double-layer capacitance of carbon-based supercapacitors, no matter the electrolyte composition.
Carbohydrates are essential moieties of many bioactive molecules in nature. However, efforts to elucidate their modes of action are often impeded by limitations in synthetic access to well‐defined oligosaccharides. Most of the current methods rely on the design of specialized coupling partners to control selectivity during the formation of glycosidic bonds. Reported herein is the use of a commercially available phenanthroline to catalyze stereoretentive glycosylation with glycosyl bromides. The method provides efficient access to α‐1,2‐cis glycosides. This protocol has been performed for the large‐scale synthesis of an octasaccharide adjuvant. Density‐functional theory calculations, together with kinetic studies, suggest that the reaction proceeds by a double SN2 mechanism.
The differential capacitance profile of electrochemical interfaces reflects the physical properties of the double layer. For carbon electrodes and ionic-liquid-based electrolytes, these capacitance profiles are not fully understood. In this work, we utilize constant voltage molecular dynamics simulations to compute differential capacitance profiles of ionic liquids [BMIm+][BF4 –] and [BMIm+][TFSI–] mixed with acetonitrile and 1,2-dichloroethane, at model graphene electrodes. We find that both pure and 10% mole fraction ionic liquid electrolytes exhibit camel-shaped capacitance profiles with two peaks on either side of a minimum centered at the potential of zero charge. This profile shape results from the electric-field-induced rearrangement of ion structure within the inner layer closest to the electrode interface. At a low potential, the ionic liquid inner layer is concentrated with nonpolar trifluoromethyl and butyl functional groups of the anions and cations, corresponding to the minimum of the capacitance profiles. With increasing voltage, electrostatic interactions of polar/charged functional groups with the electrode surface compete with these nonpolar interactions, leading to ion rearrangement that increases the inner-layer charge density and results in higher capacitance. After the ion restructuring is complete, the response saturates and capacitance diminishes. The presence of organic solvent significantly changes the composition of the inner layer. For example, strong nonpolar interactions between dichloroethane molecules and the graphene surface substantially block ion/electrode contact at moderate potentials. Overall, our simulations highlight the dynamic nature of the inner region of organic electrolyte double layers and the sensitive dependence on electrolyte composition and applied voltage.
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