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

Abstract

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.

Review of advances in coupling electrochemistry and liquid state NMR

“…Experiments were carried out in presence and absence of proton donor showing two different mechanisms for the electrochemical reduction of BQ. Association of in situ NMR and in situ ESR spectroelectrochemistry confirmed both mechanisms by revealing paramagnetic and diamagnetic species involved in the proposed mechanisms [2]. The manufacture and the implementation of this type of electrochemical cell in situ a modern NMR instrument seems to be more accessible to novice users.…”

“…First, Dunsh and co-workers [2,3] proposed an overview of in situ spectroelectrochemistry dealing with NMR spectroscopy among other powerful techniques such as ESR (Electron Spin Resonance) and IR (Infra-Red). Second, Jahn and Karst [4] addressed a particular attention to EC-NMR coupling in a review dedicated to EC-(LC-)MS coupling (electrochemistry-liquid chromatography-mass spectrometry).…”

Solution-phase electrochemistry-nuclear magnetic resonance of small organic molecules

“…Early on, Albert et al, comparing two different EC-NMR set-ups, failed to record 13 C-NMR spectra with his in-probe EC-NMR but succeeded with his hyphenated EC-NMR [21]. For in-situ EC-NMR, both successes [15] and failures [25] have been reported in this respect.…”

“…In addition, EC frequently generates paramagnetic radicals that are NMR silent. In-situ electron-spin resonance (ESR) spectroelectrochemistry, which detects only paramagnetic species, has been suggested as a highly orthogonal tool [25,26]. Also, ultrafast exchange phenomena involving the radicals and diamagnetic species, which lead to signal broadening for the latter, can be employed to detect the presence of the radicals indirectly.…”

“…Quinone-hydroquinone systems, especially 1,4-benzoquinone, have been most popular for the assessment of method performance [8,12,[20][21][22][23]25]. The earliest publication studied the reduction of a ketone (trans-1-phenyl-1-buten-3-one) [19].…”