NMR Spectroelectrochemistry Research Articles

In Situ Polymerization Synthesis of Graphdiyne Nanosheets as Electrode Material and Its Application in NMR Spectroelectrochemistry.

In situ NMR spectroelectrochemistry is extremely powerful in studying redox reactions in real time and identifying unstable reaction intermediates. In this paper, in situ polymerization synthesis of ultrathin graphdiyne (GDY) nanosheets was realized on the surface of copper nanoflower/copper foam (nano-Cu/Cuf)-based electrode with hexakisbenzene monomers and pyridine. Palladium (Pd) nanoparticles were further deposited onto the GDY nanosheets by the constant potential method. By using this GDY composite as electrode material, a new NMR-electrochemical cell was designed for in situ NMR spectroelectrochemistry measurement. The three-electrode electrochemical system consists of a Pd/GDY/nano-Cu/Cuf electrode as the working electrode, a platinum wire as the counter electrode, and a silver/silver chloride (Ag/AgCl) wire as a quasi-reference electrode, which can be dipped into a specially constructed sample tube and adapted for convenient operation in any commercial high-field, variable-temperature FT NMR spectrometer. The application of this NMR-electrochemical cell is illustrated by monitoring the progressive oxidation of hydroquinone to benzoquinone by controlled-potential electrolysis in aqueous solution.

NMR spectroelectrochemistry in studies of hydroquinone oxidation by polyaniline thin films

In this paper, we developed a direct way for the study of hydroquinone oxidation through in situ electrochemistry-combined nuclear magnetic resonance (EC-NMR). Electro-polymerization-induced nano-polyaniline film was utilized as the catalyst in the process of electrochemical oxidation of hydroquinone. In situ EC-NMR provides a powerful tool to probe products distribution and reaction rate, which could be useful to investigate electro-catalytic mechanism and evaluate the electro-catalytic capacity. To study the influence of both protic and aprotic media in the electrocatalytic process, we mixed water and dimethylsulfoxide to mimic possible real-life electrochemical environments. The proposed in situ EC-NMR is capable of quantitatively monitoring the generation of products under varied solvent composition and pH values. A positive effect to electro-catalysis capacity is observed when the concentration of DMSO or water is increased from the volume equivalent point, implying that there should be a competition between protic and aprotic media, which can be characterized by EC-NMR technique.

(19)F NMR-, ESR-, and vis-NIR-spectroelectrochemical study of the unconventional reduction behaviour of a perfluoroalkylated fullerene: dimerization of the C70(CF3)10(-) radical anion.

The most abundant isomer of C70(CF3)10 (70-10-1) is a rare example of a perfluoroalkylated fullerene exhibiting electrochemically irreversible reduction. We show that electrochemical reversibility at the first reduction step is achieved at scan rates higher than 500 V s(-1). Applying ESR-, vis-NIR-, and (19)F NMR-spectroelectrochemistry, as well as mass spectrometry and DFT calculations, we show that the (70-10-1)(-) radical monoanion is in equilibrium with a singly-bonded diamagnetic dimeric dianion. This study is the first example of (19)F NMR spectroelectrochemistry, which promises to be an important method for the elucidation of redox mechanisms of fluoroorganic compounds. Additionally, we demonstrate the importance of combining different spectroelectrochemical methods and quantitative analysis of the transferred charge and spin numbers in the determination of the redox mechanism.

In situ NMR spectroelectrochemistry for the structure elucidation of unstable intermediate metabolites

In situ NMR spectroelectrochemistry is presented in this study as a useful hybrid technique for the chemical structure elucidation of unstable intermediate species. An experimental setting was designed to follow the reaction in real time during the experimental electrochemical process. The analysis of (1)H NMR spectra recorded in situ permitted us (1) to elucidate the reaction pathway of the electrochemical oxidation of phenacetin and (2) to reveal the quinone imine as a reactive intermediate species without using any trapping reaction. Phenacetin has been considered as hepatotoxic at high therapeutic amounts, which is why it was chosen as a model to prove the applicability of the analytical method. The use of 1D and 2D NMR experiments led to the elucidation of the major species produced from the oxidation process. We demonstrated that in situ NMR spectroelectrochemistry constitutes a fast way for monitoring unstable quinone imines and elucidating their chemical structures.

Charged states in diphenylamino endcapped thiophenes with a 1,4-phenylene core: In situ electron spin resonance/ultraviolet–visible–near infrared and nuclear magnetic resonance spectroelectrochemistry and quantum chemical study

The three diphenylamino-endcapped thiophene compounds 1,4-bis-(2-diphenylamino-thiophen-5-yl)-benzene (A), 1,4-bis-[2-(phenothiazin-10-yl)-thiophen-5-yl]-benzene (B), and 1,4-bis-(5-diphenylamino-[2,2′]bithiophen-5′-yl)-benzene (C) with a central phenylene core were investigated by in situ ESR/UV–vis–NIR and in situ NMR spectroelectrochemistry and quantum chemical studies. The computed vertical ionization potentials are 4.85eV for A, 5.39eV for B and 4.80eV for C while the adiabatic ionization potentials are slightly lower 4.45eV (for A), 4.96eV (for B) and 4.47eV (for C). The minimal difference between computed ionization potentials of A and C and the lower oxidation ability of B are in good agreement with the experimental electrochemical findings with the first half-wave anodic potentials 0.69V (for A), 0.81V (for B) and 0.67V (for C), all vs. decamethylferrocene (DmFc+/DmFc). For the radical monocations A+ and C+ the spin delocalisation on the central phenylene moiety was confirmed both experimentally and by theoretical calculations. For the radical cation B+, the presence of a phenothiazinyl moiety is responsible for the delocalization of unpaired electron in the lateral part of this molecule. The stability of the charged states was increased substantially by the incorporation of two further 2,5-thienylene moieties between the central moiety and the side groups in C. Detailed spectroelectrochemical studies of the cation radical, dication, trication and tetracation of C are in good agreement with the calculated electronic transitions and experimental values for all redox states. In situ NMR spectroelectrochemical studies at different temperatures indicate dimerisation reactions of charged 1,4-bis-(5-diphenylamino-[2,2′]bithiophen-5′-yl)-benzenes.

A combination of in situ ESR and in situ NMR spectroelectrochemistry for mechanistic studies of electrode reactions: the case of p‐benzoquinone

A combined in situ NMR and in situ ESR spectroelectrochemical study of a reaction mechanism is presented detecting and describing the whole number of paramagnetic and diamagnetic intermediates and final products in an electrode reaction. While in situ NMR spectroelectrochemistry provides a powerful method for the study of structural or electronic changes of diamagnetic molecules in any electrochemical reaction mechanism, in situ ESR spectroelectrochemistry is the method of choice to detect paramagnetic structures and to characterise their electronic state via the g-value and hyperfine splitting in redox reactions. To demonstrate the power of this combination of magnetic spectroscopies in electrochemistry, the reduction of p-benzoquinone to hydroquinone is followed by both these spectroelectrochemical methods at selected pH values, thus considering the influence of the proton on the reaction mechanism. The results of both in situ spectroelectrochemical methods at the same redox system are used to get the complete reaction mechanism of p-benzoquinone at electrodes in aqueous solutions.

Recent Advances in in situ multi-spectroelectrochemistry

To consider the past, present and future of in situ spectroelectrochemistry, a review on the recent state of modern spectroelectrochemistry and trends in the development of spectroelectrochemcial techniques is presented for the combined application of different in situ spectroelectrochemcial methods like ESR spectroelectrochemistry, NMR spectroelectrochemistry, Raman spectroelectrochemistry or IR spectroelectrochemistry to electrode systems. Starting with a discussion of the first steps in spectroelectrochemistry in the past, the main part of this review is focused on the advantages of the combined application of spectroelectrochemical techniques in the analysis of electrode reactions. The spectroelectrochemical methods are demonstrated to be successful in electrode reactions both for solid structures like polymers or carbon nanotubes and for molecular structures like fullerenes and oligothiophenes. The final outlook is attributed to future developments in spectroelectrochemistry.

In Situ NMR Spectroelectrochemistry of High Sensitivity: A New Inside into the Mechanism of Electrode Reactions

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Charge stabilisation by dimer formation of an endcapped thiophene tetramer—an in situ NMR spectroelectrochemical study

The role of dimers in the charge stabilisation of 5,5′′′-dicyano-3,3′′-dibutyl 2,2′:5′,2′′:5′′,2′′′-quaterthiophene (DCNDBQT) via an electrochemical reaction is studied in detail by in situ NMR spectroelectrochemistry. Among the two oxidation steps the first was found to be reversible while the second is completely irreversible. By in situ NMR spectroelectrochemistry we are able to demonstrate the formation of a π-dimer via the cation generated in the first electron transfer as well as the formation of a σ-dimer by a follow-up reaction of the dication generated in the second oxidation step. As the thiophene oligomer is blocked in the 2- and 5-positions by the cyanide group the monomer undergoes a dimerisation in the 4′-position. In situ NMR spectroelectrochemistry delivers proof of the coupling reaction of the thiophene rings in the electrochemistry of α,ω-endcapped oligothiophenes although such endcapped structures are expected to be prevented from chemical follow-up reactions. Furthermore, substitution by cyano groups opens the route to a cathodic reduction of the structure. The dicyanodibutylquaterthiophene gives a single reduction step which turns out to be quasi-reversible but the anionic structure formed in the reduction was not detectable by in situ NMR spectroelectrochemistry. This and the follow-up reactions are discussed in detail.

High-field NMR spectroelectrochemistry of spinning solutions: simultaneous in situ detection of electrogenerated species in a standard probe under potentiostatic control

This report summarizes the design and performance of the first practicable in situ electrogenerative high-field solution-NMR cell, designed for simultaneous NMR spectroelectrochemistry in a conventional 10- or 16-mm 300 MHz probe. The axially symmetric three-electrode assembly is stationary and self-aligning. It dips into a standard spinning 10-mm sample tube and is adapted for convenient operation in any commercial high-field, multi-nuclear, variable-temperature FT NMR spectrometer. The tubular, rf-transparent working electrode (presently consisting of a gold film, coated on glass) completely spans the detection region; the recommended coating thickness is ≤0.01 δ, where δ is the rf-absorbing conductor skin depth. With careful shimming, very narrow 1H signals (∼1 Hz) can be obtained. The performance of the in situ NMR-electrochemical cell is illustrated by monitoring the progressive reduction of ‘ p-quinone’ (1,4-benzoquinone) to its dihydro-form by controlled-potential electrolysis in an acidic medium.