Key Techniques For Coupling Electrochemistry With Nuclear Magnetic Resonance Spectrometry

In this paper,to overcome the shortcomings of the present liquid-phase electrochemical nuclear magnetic resonaance(EC-NMR)electrolytic cell in the literature,an in situ EC-NMR electrolytic cell based on palisade gold film working electrode is proposed,by which the electrochemical cyclic voltammetry(CV)curves can be readily obtained,and high-resolution in situ NMR spectra can also be acquired simultaneously for quantitative and qualitative ananlysis.Liquid NMR spectra,simultaneously recorded with CV or constant potential electrolysis,have been obtained to investigate the electrochemical redox behaviors of electrochemical systems under different reaction conditions and are used to characterize the dynamic structure and mechanism of electrochemical process.In situ potential-dependent liquid NMR measurement are introduced in this dissertation to reveal molecular changes with potentials scanning.The quantitative analysis and determination of the reactants in an in situ EC-NMR reaction can be well achieved with the assitance of an external standard located in this electrolytic cell.The design and implementation of field frequency interlocking system being suitable for in situ EC-NMR specialization spectrometer is also introduced.The main research results are shown as follows.1.Unlike the general concept in EC-NMR,a circular symmetry working electrode,usually being a glass tube with thin metal film deposited on the outer surface,still has great influence on the radio frequency field homogeneity in the coil of NMR spectrometer probe,despite that its thickness is much less than the metal skin depth.In this paper,a novel palisade gold film(PGF)depositing outside of capillary tube is introduced to serve as the working electrode.Experimental results show that the uniformity of radio frequency field is greatly improved by the nutation behavior of proton NMR.Another advantage of such working electrode is that an external reference standard adapted to the analytes could be chosen for the purpose of quantitative NMR(qNMR)research.A three-electrode electrochemical cell including PGF working electrode(EC-qNMR cell)was successfully fabricated.Its electrochemical and in situ qNMR performance was validated in a standard 5 mm NMR probe by acquiring voltammograms and high-resolution NMR spectra simultaneously.The evolution of in situ EC-qNMR spectra monitoring the redox of p-benzoquinone and hydroquinone with time demonstrated the feasibility of the EC-qNMR cell,including monitoring reaction quantitatively and studying their reaction mechanisms.2.In spite of the unique advantages in the joint of high sensitivity NMR with electrochemical analysis,there are still tremendous technical difficulties of electromagnetic compatibility needed to be overcome,which immensely impede the accuracy quantitative determination of the reactant.The paper presents the repercussions of gradual varying conductivity of the electrolyte in a real time electrochemical reaction on the spatial distribution of the radio frequency(RF)field amplitude and phase in the NMR probe.To compensate such non-linear trend of the spatial dependent distribution,both the NMR signal peak integration intensity of external standard and substance under test were previously calibrated to improve the precision of the quantitative determination of the substances.The evolution of in situ EC-qNMR spectra monitoring the redox of p-benzoquinone and hydroquinone with time demonstrated the feasibility of the EC-qNMR cell,including monitoring reaction quantitatively and studying their reaction mechanisms.3.The design and the performance of a two-chamber thin-layer electrochemical device for in situ potential-dependent liquid NMR measurement are introduced in this dissertation.Liquid NMR spectra,simultaneously recorded with CV,have been obtained to reveal molecular changes with potentials scanning.As a proof of concept,redox properties of 1,4-benzoquinone based systems have been investigated,and a ? dimerization has been identified by combining both in situ and ex situ NMR analyses.The alternately decreased and increased NMR signals at different chemical shift around the reduction/oxidation waves indicated the transform among quinone,quinone dianion or semiquinone radicals,confirming a single step two-electron process or two successive one-electron transfer steps in the presence or absence of high concentration water.In order to confirm the hypothesis that the ? dimerization occurred in the process,ex situ NMR measurements,including HSQC(1H-13C)2D NMR,1D NMR while mimicing the interactions between 1,4-benzoquinone and quinone dianion and diffusion coefficient(D)measurement through DOSY-1H NMR,were involved to provide the evidence to further support the production of the ? dimerization form.The above descriptions demonstrate that,based on in situ NMR spectra recorded with potentials scanning,molecular information on real-time intermediates can be monitored and identified for the understanding of electrochemical processes in a reliable way.This in situ EC-NMR technology provides a new approach for spectroelectrochemistry,which will contribute to developing EC-NMR as an important tool for the analysis of electrochemical process at a molecular level.4.Homogeneous and stable magnetic field is essential for a high-resolution NMR spectrum.The field-frequency lock technique can effectively improve the stability of the magnetic field.The whole design scheme of the field frequency interlock system for specialized EC-NMR spectrometer is presented in this paper.The digital control is implemented with Xilinx Virtex-4 FPGA,XC4VFX12-FF668.The signal received from the probe is detected by the quadrature detector,and the resonance frequency drift is locked by the quasi-feedback loops.In the lock control board of the lock system,Wind River's VxWorks real-time operating system is chosen to fulfill the strict requirement for the performance of real-time data acquisition and control,as well as the stability of operation system.Online debugging with the Varian 500 MHz spectrometer shows that the lock system achieves the goal of compensating for the intrinsic drift of a superconducting magnet field.