Name: using cyclic voltammetry, uv-vis-nir, and epr spectroelectrochemistry to analyze organic compounds
Uploaded: 2018-10-18T12:30:00.000+00:00
Duration: 11 min 44 s
Description: Watch this Scientific Journal Video about Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds at JoVE.com

The authors gratefully acknowledge the financial support of &#34;Excilight&#34; project &#34;Donor-Acceptor Light Emitting Exciplexes as Materials for Easy-to-tailor Ultra-efficient OLED Lightning&#34; (H2020-MSCA-ITN-2015/674990) financed by Marie Sk&#322;odowska-Curie Actions within the framework programme for research and innovations &#34;Horizon-2020&#34;.

The authors have nothing to disclose.

Electrochemical and spectroelectrochemical techniques have no limitations; one can analyze both solid state and liquid solutions in a broad range of temperature and other conditions with these techniques. The important thing in all of these cases is that compounds/materials are analyzed under the applied potential, replicating real world conditions for working organic electronics devices. The only difference is that in electrochemistry, the formation of charge carriers, is observed.
The methods presented here show the usefulness of the analysis of charged carriers generated in organic compounds that correlate with their applicability in organic electronics. Moreover, the electrochemical and spectroelectrochemical techniques are cheaper and less demanding than that of typical methods used in charge carrier analysis, but there are some critical steps and modifications to the protocol that are needed depending on the obtained results.
During electrochemical characterization, always start with a particular concentration. If a set of the compounds is compared, then all materials need to have the same molar concentration. The best is to start with 1 mM concentration and 50 mV/s scan rate as indicated in the protocol in this study, but it is good to know the concentration of the sample on the observed electrochemical behavior. Always try to measure at least three scans. The first two scans are usually different because the starting conditions (equilibrium) are different. The second and the third scans should be the same. If the second and third scans are the same, then there are probably no side reactions observed in this system (Figure 2a). In an oxidation process, a new peak at a lower potential appears showing that the conductive material was deposited on the WE18,19,24,25,29,30,31,32. If the height of the lower peak increases in successive scans, then probably the conjugated polymer was deposited18,19,24,25,29,30,31,32. If all the currents decrease in successive scans, then the nonconductive product of degradation was deposited on the electrode. If a very small peak is observed before the main oxidation or reduction peak (especially for polymers), then this is probably charge-trapping process19,23,31,34. If a very sharp dedoping peak of oxidation or reduction is observed, then this is probably caused by the decomposition of crystalline structures on an electrode formed through the electrocrystallization process during oxidation35.
Always check the behavior of the test compound before, during, and after redox peaks. It means that at least three CV scans should be registered: with upper (in the case oxidation) or lower vertex potential lower or higher, respectively, then the potential of peak maximum, with upper or lower vertex potential set to exactly on the peak maximum and with vertex potentials higher (oxidation) and lower (reduction) than potential of the peak maximum. The observed process may vary and sometimes two processes may be observed under one peak theoretically. Always compare the collected cyclic voltammograms of the electrolyte (step 2.6), the ferrocene (step 2.9), the compound (step 2.13), and the ferrocene with compound (step 2.19); there are several issues to be taken into account.
Always compare the CV signals of the electrolyte and the test compound, if any signals from the electrolyte is visible on the cyclic voltammogram of the measured compound, then the electrolyte must be changed because its electrochemical window is too low, or the electrolyte is contaminated. If the signal (redox couple) of ferrocene (step 2.9) and ferrocene with compound (step 2.19) are at the same position, then everything is performed properly. If the peaks are shifted between each other, then check the RE and repeat the measurement. If the signal (oxidation, reduction, or redox couple potential) of the test compound with added ferrocene (step 2.19) is at a higher potential than that of the pure compound (step 2.13), then consider the values (oxidation, reduction, or redox couple potential) from the cyclic voltammogram of the pure compound. The shift is caused by the higher amount of ferrocene in the solution. When two oxidation processes are observed, the first process (oxidation or reduction) which is always on the WE may affect the active surface; this may cause an increase in the oxidation potential of the second process (Figure 9).

Worldwide, researchers are continually searching for new organic materials that can be used in organic electronics with desirable performance or stability, which drops due to extended use. In the case of organic devices, it is important to understand the behavior of the charge carrier to fully know the rules driving the device behavior. Analysis of the effect of the molecular structure on the generation of the charge carrier and the dynamics and maintenance of the balance of injected charge carriers, both positive (holes) and negative (electrons), is crucial to improve the efficiency and stability of the organic devices. This ensures the effective recombination of these individual charges and consequently significantly improves the photoluminescence efficiency of the organic light emitting diodes (OLEDs)1,2. For organic photovoltaics (OPVs)3,4 as well as organic field effect transistors (OFETs)5,6, it is necessary to have materials with high charge carrier mobility. In addition to the analysis of charge carriers, several important parameters of organic electroactive materials help in predicting where the material could be used: ionization potential (IP), electron affinity (EA) energy levels, and band-gap between them7,8,9,10.
In this work, we present a method for the efficient measurement of cyclic voltammetry (CV) that can be used in the analysis of all types of electroactive materials. This technique provides information about redox properties, the doping/dedoping mechanism, the stability, the conversion and storage of energy, etc. It also allows for the estimation of electron affinity and ionization energy of the test compounds in a much cheaper and faster way compared to other high vacuum methods. The aforementioned parameters correlate with the energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).
The method presented in this article can be used to analyze all types of conjugated compounds such as those with delocalized &#960;-electrons in their structures. Conjugated compounds may be small molecules with large polymeric chains. Small molecules can also be monomers; during the initial reaction (photochemical, electrochemical, or chemical) monomers can form polymers. In OLED application, the energy level values are necessary to enable the use of the correct host for the emitter in a thermally activated delayed fluorescence (TADF) guest-host system or to decide with which compounds the exciplex donor-acceptor layer could be formed and what additional layers (electron transporting layer (ETL), hole transporting layer (HTL), electron blocking layer (EBL), and hole blocking layer (HBL)) will be necessary to synthesize stable efficiently charged balanced OLED devices11,12,13,14,15,16,17. Additional electrochemical measurements allow the investigation of possible side reactions during the process of degradation of the active layer and the formation of low mobile charge carriers (bipolarons)18,19,20,21,22.
Coupling electrochemical and spectroelectrochemical methods allows for easy, accurate, and reliable determination of the degree of oxidation or reduction of conjugated compounds and their degradation potential, which is crucial for stability23,24,25,26,27,28. Ultraviolet-visible and near-infrared (UV-Vis-NIR) spectroscopy coupled with electrochemistry can characterize the fundamental chromatic properties of all new conjugated compounds, such as the changing of the absorption band during doping18,19,20,21,22,23,24,25,26,27,28,29,30.
In a study related to the doping mechanism, it is important to define the type of charge carriers. In this process, two classes of charged quasiparticles take part, one with uncompensated spin (polarons) and the second being diamagnetic (bipolarons); electron paramagnetic resonance (EPR) spectroscopy provides invaluable assistance, which directly allows one to observe and track changes in populations of paramagnetic polarons29,30,31,32. In small molecules, it is difficult to form bipolarons, but these molecules can be quite conjugated and have bipolaron-inducing properties; it is important to check if and at which potential polarons and bipolarons are formed in the structure. Bipolarons are at least one order lower in mobility than that of polarons; therefore, if bipolarons are formed in working devices, then it could lead to an unbalanced ratio of the charge carriers, which would result in high current and overheating of the OLED device or may well be the centers of degradation33.
The method of measurement proposed in this study is cheap and faster and allows for the determination of the most valuable operative parameters for a large number of electroactive materials without the need for special devices that are based on newly synthesized materials to check its performance. By applying electrochemistry and spectroelectrochemistry, it is possible to select one material that is really promising from hundreds of new materials. In addition, it is possible to obtain detailed information regarding the processes of doping and their effects on the chemical structure of the test conjugated systems using electrochemical and spectroelectrochemical methods, which allows constructing more efficient organic electronics devices.

<table><tbody><tr><td>Potentiostat</td><td>Metrohm</td><td>Autolab PGSTAT100</td><td></td></tr><tr><td>EPR</td><td>JEOL</td><td>JES-FA200</td><td></td></tr><tr><td>UV-Vis detector</td><td>Oceanoptics</td><td>QE6500</td><td></td></tr><tr><td>NIR detector</td><td>Oceanoptics</td><td>NIRQuest</td><td></td></tr><tr><td>Dichloromethane (DCM)</td><td>Sigma-Aldrich</td><td>106048</td><td></td></tr><tr><td>Tetrabutylammonium tetrafluoroborate (Bu&lt;sub&gt;4&lt;/sub&gt;NBF&lt;sub&gt;4&lt;/sub&gt;)</td><td>Sigma-Aldrich</td><td>86896</td><td></td></tr><tr><td>2-propanol, 99.9%</td><td>Sigma-Aldrich</td><td>675431</td><td></td></tr><tr><td>Acetone, 99.9%</td><td>Sigma-Aldrich</td><td>439126</td><td></td></tr><tr><td>Ultrasonic Bath</td><td>Elma</td><td>S30H</td><td></td></tr><tr><td>Tetrahydrofuran &gt;99.9%</td><td>Sigma-Aldrich</td><td>401757</td><td></td></tr><tr><td>ferrocene &gt;98%</td><td>Sigma-Aldrich</td><td>F408</td><td></td></tr><tr><td>decamethylferrocene &gt;97%</td><td>Sigma-Aldrich</td><td>378542</td><td></td></tr></tbody></table>

using cyclic voltammetry, uv-vis-nir, and epr spectroelectrochemistry to analyze organic compounds

Cyclic voltammetry (CV) is a technique used in the analysis of organic compounds. When this technique is combined with electron paramagnetic resonance (EPR) or ultraviolet-visible and near-infrared (UV-Vis-NIR) spectroscopies, we obtain useful information such as electron affinity, ionization potential, band-gap energies, the type of charge carriers, and degradation information that can be used to synthesize stable organic electronic devices. In this study, we present electrochemical and spectroelectrochemical methods to analyze the processes occurring in active layers of an organic device as well as the generated charge carriers.

The most common application of CV analysis is the estimation of IP and EA. Even though there are a couple ways to obtain CV data, it is strongly recommended to calculate them based on the onset of redox peaks (Figure 3). This approach allows unifying the calculation procedure. Not all of the tested materials undergo reversible oxidation/reduction processes (Figure 3); in such situations, it is not possible to calculate based on average potentials (average from potentials correspond to a maximum of cathodic-reduction and anodic-oxidation peaks). However, it is almost always possible to run the tangent to a peak as shown in Figure 3. With the intersection with the background line and use of Equations 1 and 2, the IP and EA values are estimated as 5.35 eV and &#8722;2.90 eV, respectively. There are also several different scales used to evaluate IP and EA based on CV measurements. The most commonly used scales for organic materials are &#8722;4.8 and &#8722;5.1 eV as equivalent to 0.00 V versus Normal Hydrogen Electrode (NHE). However, all of the scales are only an approximation; remember this when comparing different results. The crucial thing is to state what parameters were considered for calculations. In this case, the value &#8722;5.1 eV has been chosen as it should correspond to the formal potential of the ferrocene redox couple; in the Fermi scale, it is 0.40 V versus Saturated Calomel Electrode (SCE) in acetonitrile, which is in agreement with the previous measurement.
There are many articles published regarding the analysis of CV. Herein, we show when the process is not going to be as it is expected. The analysis is based on thiophene derivative: NtVTh (structure shown in Figure 4), which undergoes degradation upon oxidation (Figure 5).
NtVTh has two oxidation potentials: the first at 0.70 V and the second at 0.84 V (Figure 5). During the first cycle, the reduction peak is not observed, indicating an irreversible process. Electrochemical characteristics of NtVTh show that polymerization does not occur and after the first oxidation potential, some electro-inactive layer of reaction products deposits at the electrode surface, thus hindering the polymerization process. What is visible is the reaction with the radical cation on the vinyl bond, where the molecule is losing its conjugation and form dimers on the electrode.
While it is difficult to extract information about charge carriers from the CV, it is possible to distinguish between polarons and bipolarons when supported by a UV-Vis-NIR spectrometer. The neutral poly(OiPrThEE) was characterized by two wide absorption &#960;|&#960;* transitions bands with peaks maxima at &#955;max1 = 363 nm and &#955;max2 = 488 nm, related to the aromatic form of the undoped polythiophene derivative. During the oxidative doping, new polaronic and bipolaronic bands are generated. The UV-Vis spectrum obtained during the poly(OiPrThEE) oxidation revealed the diminishing the neutral polymer absorption band (300-550 nm) (Figure 6) together with the formation of a new absorption band (550-950 nm) of the radical cations of bithiophene and p-phenylenevinylene with maxima at 692 nm. The isosbestic point of the oxidation process was located at 604 nm. The bipolaronic band appeared between the 950 nm and 1700 nm with a maximum located at 1438 nm.
EPR spectroscopy is the technique that detects materials with an unpaired electron, this includes organic radicals39. There are several parameters that could be extracted from EPR spectra, but one of the most interesting is to estimate where the radicals are localized. Electrons, similar to protons, possess spin. By placing an electron in an external magnetic field, this spin can be split two ways: parallel and antiparallel to the magnetic field, giving two energy levels. This phenomenon is known as the Zeeman effect40,41. In case of organic radicals, the unpaired electron interacts not only with the external magnetic field but also with magnetic nuclei (nuclei which have a nonzero spin; I&#8800;0). A number of degenerate energy levels are equal to 2I + 1, where I is the spin quantum number of the nucleus with which the unpaired electron interacts42. The interaction of the unpaired electron with a larger number of magnetic nuclei leads to further splitting of the energy levels and to hyperfine structure of EPR spectra registration43 (Figure 7).
For molecules where the unpaired electron interacts with an even larger number of nuclei, the individual spectral line could overlap, which results in registration of a single, broad signal44,45,46 (Figure 8). This is typical for conjugated polymers, where the generated radical ion during a redox process is delocalized47,48.
The combination of EPR spectroscopy with electrochemical methods allows the characterization of charge carriers (radical ion) generated during the redox process as well as the determination of the mechanism of these processes49,50. If well-resolved (peaks are separated; not in the form of one broad peak) spectra are registered, as in the case of electrochemical reduction of s-tetrazine derivative (Figure 7), then the analysis of the hyperfine structure of spectra leads to conclusions about the localization of unpaired electron. One way to analyze this kind of spectra is to conduct simulation with special software and to fit simulated spectra with the experimental one51. This is especially helpful when the hyperfine structure is complex due to the interaction of the unpaired electron with large numbers of protons. In case of the s-tetrazine derivative shown in Figure 7, simulation of EPR spectra (red line) indicates the interaction of the unpaired electron with four nitrogen atoms of s-tetrazine.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56656/56656fig1.jpg" /> 
Figure 1: Electrochemical and spectroelectrochemical cells used for measurements. The figure presents the scheme setup of electrochemical/spectroelectrochemical cells using cyclic voltammetry, ultraviolet-visible and near-infrared (UV-Vis-NIR), and electron paramagnetic resonance (EPR) spectroelectrochemical measurements. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56656/56656fig2.jpg" /> 
Figure 2: Cyclic voltammetry (CV) of properly measured compound COPO1 (a) and the CV with an unstable reference potential dibenzothiophene-S,S-dioxide with ferrocene (b)52. The figure shows two cyclic voltammograms. (a) presents correctly registered CV and (b) shows a voltammogram registered using a reference electrode with no stable potential. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/56656/56656fig3.jpg" /> 
Figure 3: Cyclic voltammetry (CV) of compound COPO1 in a wide range of potentials. Estimation of the onset potentials for EA and IP calculations of the COPO1 compounds52. EA=&#8722;2.90 eV and IP = 5.35 eV. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/56656/56656fig4.jpg" /> 
Figure 4. EPR six spectral line of manganese standard. The paramagnetic signal of manganese used for calibration of the signal shift. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/56656/56656fig5.jpg" /> 
Figure 5: Cyclic Voltammetry (CV) of compound NtVTh. Cyclic voltammograms of 15 mM NtVTh in 0.1 M Bu4NBF4/CH3CN and relative to ferrocene standard presenting the degradation process involved on working electrode. Scan rate 0.05 V/s: (a) was taken in the range &#8722;1.4 V to 0.8 V, and (b) in the range &#8722;1.4 V to 0.9 V. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/56656/56656fig6.jpg" /> 
Figure 6: Ultraviolet-visible and near-infrared (UV-Vis-NIR) spectroelectrochemistry of poly(OiPrThEE) derivative. UV-Vis-NIR spectra presenting the evolution of absorption band through the generation of charge carriers on polymer structure. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/56656/56656fig7.jpg" /> 
Figure 7: EPR spectroelectrochemical analysis of tetrazine derivative. (a) Structure of s-tetrazine derivative; (b) EPR spectra registered during electrochemical reduction of s-tetrazine derivative (black line-experimental and red line simulated spectrum). <a href="https://www.jove.com/files/ftp_upload/56656/56656fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/56656/56656fig8.jpg" /> 
Figure 8: EPR spectroelectrochemical polaron signal of the conjugated polymer. Electron paramagnetic resonance (EPR) spectra registered during the first step of oxidation of conjugated polymer (EPR spectra of polaron species). <a href="https://www.jove.com/files/ftp_upload/56656/56656fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 9" class="xfigimg" src="/files/ftp_upload/56656/56656fig9.jpg" /> 
Figure 9: Cyclic voltammetry (CV) of ferrocene and decamethylferrocene. Comparison of two electrochemical standards as pure and as mixture showing the shift of the potential. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig9large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds at JoVE.com

Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds

In this article, we describe electrochemical, electron paramagnetic resonance, and ultraviolet-visible and near-infrared spectroelectrochemical methods to analyze organic compounds for application in organic electronics.

Cyclic voltammetry (CV) is a technique used in the analysis of organic compounds. When this technique is combined with electron paramagnetic resonance ...

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26.3K Views.  Silesian University of Technology. In this article, we describe electrochemical, electron paramagnetic resonance, and ultraviolet-visible and near-infrared spectroelectrochemical methods to analyze organic compounds for application in organic electronics.

Video: Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds

Analysis of Volatile and Oxidation Sensitive Compounds Using a Cold Inlet System and Electron Impact Mass Spectrometry

This video presents a protocol for the mass spectrometrical analysis of volatile and oxidation sensitive compounds using electron impact ionization. The analysis of volatile and oxidation sensitive compounds by mass spectrometry is not easily achieved, as all state-of-the-art mass spectrometric methods require at least one sample preparation step, e.g., dissolution and dilution of the analyte (electrospray ionization), co-crystallization of the analyte with a matrix compound (matrix-assisted laser desorption/ionization), or transfer of the prepared samples into the ionization source of the mass spectrometer, to be conducted under atmospheric conditions. Here, the use of a sample inlet system is described which enables the analysis of volatile metal organyls, silanes, and phosphanes using a sector field mass spectrometer equipped with an electron impact ionization source. All sample preparation steps and the sample introduction into the ion source of the mass spectrometer take place either under air-free conditions or under vacuum, enabling the analysis of compounds highly susceptible to oxidation. The presented technique is especially of interest for inorganic chemists, working with metal organyls, silanes, or phosphanes, which have to be handled using inert conditions, such as the Schlenk technique. The principle of operation is presented in this video.

This video presents a protocol for the mass spectrometrical analysis of volatile and oxidation sensitive compounds using electron impact ionization. The presented technique is especially of interest for inorganic chemists, working with metal organyls, silanes, or phosphanes which have to be handled using inert conditions, such as the Schlenk technique.

This video presents a protocol for the mass spectrometrical analysis of volatile and oxidation sensitive compounds using electron impact ionization. The ...

Electrochemical Impedance Spectroscopy as a Tool for Electrochemical Rate Constant Estimation

Electrochemical impedance spectroscopy (EIS) was used for advanced characterization of organic electroactive compounds along with cyclic voltammetry (CV). In the case of fast reversible electrochemical processes, current is predominantly affected by the rate of diffusion, which is the slowest and limiting stage. EIS is a powerful technique that allows separate analysis of stages of charge transfer that have different AC frequency response. The capability of the method was used to extract the value of charge transfer resistance, which characterizes the rate of charge exchange on the electrode-solution interface. The application of this technique is broad, from biochemistry up to organic electronics. In this work, we are presenting the method of analysis of organic compounds for optoelectronic applications.

Electrochemical impedance spectroscopy (EIS) of species that undergo reversible oxidation or reduction in solution was used for determination of rate constants of oxidation or reduction.

Electrochemical impedance spectroscopy (EIS) was used for advanced characterization of organic electroactive compounds along with cyclic voltammetry (CV). ...

Raman and IR Spectroelectrochemical Methods as Tools to Analyze Conjugated Organic Compounds

In the presented work, two spectroelectrochemical techniques are discussed as tools for the analysis of the structural changes occurring in the molecule on the vibrational level of energy. Raman and IR spectroelectrochemistry can be used for advanced characterization of the structural changes in the organic electroactive compounds. Here, the step-by-step analysis by means of Raman and IR spectroelectrochemistry is shown. Raman and IR spectroelectrochemical techniques provide complementary information about structural changes occurring during an electrochemical process, i.e. allows for the investigation of redox processes and their products. The examples of IR and Raman spectroelectrochemical analysis are presented, in which the products of the redox reactions, both in solution and solid state, are identified.

A protocol of step-by-step Raman and IR spectroelectrochemical analysis is presented.

In the presented work, two spectroelectrochemical techniques are discussed as tools for the analysis of the structural changes occurring in the molecule ...

EPR Monitored Redox Titration of the Cofactors of Saccharomyces cerevisiae Nar1

Electron Paramagnetic Resonance (EPR) monitored redox titrations are a powerful method to determine the midpoint potential of cofactors in proteins and to identify and quantify the cofactors in their detectable redox state.
The technique is complementary to direct electrochemistry (voltammetry) approaches, as it does not offer information on electron transfer rates, but does establish the identity and redox state of the cofactors in the protein under study. The technique is widely applicable to any protein containing an electron paramagnetic resonance (EPR) detectable cofactor.
A typical titration requires 2 ml protein with a cofactor concentration in the range of 1-100 &#181;M. The protein is titrated with a chemical reductant (sodium dithionite) or oxidant (potassium ferricyanide) in order to poise the sample at a certain potential. A platinum wire and a Ag/AgCl reference electrode are connected to a voltmeter to measure the potential of the protein solution. A set of 13 different redox mediators is used to equilibrate between the redox cofactors of the protein and the electrodes. Samples are drawn at different potentials and the Electron Paramagnetic Resonance spectra, characteristic for the different redox cofactors in the protein, are measured. The plot of the signal intensity versus the sample potential is analyzed using the Nernst equation in order to determine the midpoint potential of the cofactor.

EPR Monitored Redox Titration of the Cofactors of Saccharomyces cerevisiae Nar1

The goal of this protocol is to use electron paramagnetic resonance (EPR) monitored redox titrations to identify different cofactors of Saccharomyces cerevisiae Nar1. Redox titrations offer a very robust way to obtain midpoint potentials of different redox active cofactors in enzymes and proteins.

Electron Paramagnetic Resonance (EPR) monitored redox titrations are a powerful method to determine the midpoint potential of cofactors in proteins and to ...

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Ultraviolet-Visible (UV-Vis) Spectroscopy

Source: Laboratory of Dr. B. Jill Venton - University of Virginia
Ultraviolet-visible (UV-Vis) spectroscopy is one of the most popular analytical techniques because it is very versatile and able to detect nearly every molecule. With UV-Vis spectroscopy, the UV-Vis light is passed through a sample and the transmittance of light by a sample is measured. From the transmittance (T), the absorbance can be calculated as A=-log (T). An absorbance spectrum is obtained that shows the absorbance of a compound at different wavelengths. The amount of absorbance at any wavelength is due to the chemical structure of the molecule.
UV-Vis can be used in a qualitative manner, to identify&#160;functional groups or confirm the identity of a compound by matching the absorbance spectrum. It can also be used in a quantitative manner, as concentration of the analyte is related to the absorbance using Beer&#39;s Law. UV-Vis spectroscopy is used to quantify the amount of DNA or protein in a sample, for water analysis, and as a detector for many types of chromatography. Kinetics of chemical reactions are also measured with UV-Vis spectroscopy by taking repeated UV-Vis measurements over time. UV-Vis measurements are generally taken with a spectrophotometer. UV-Vis is also a very popular detector for other analytical techniques, such as chromatography, because it can detect many compounds.
Typically, UV-Vis is not the most sensitive spectroscopy technique, because not a lot of light is absorbed over a short path length. Other spectroscopy techniques such as fluorescence have higher sensitivity, but they are not as generally applicable, as most molecules are not fluorescent. UV-Vis has a similar sensitivity to other absorbance measurements, such as infrared spectroscopy.

Ultraviolet-Visible UV-Vis Spectroscopy: Principle and Uses

Source: Laboratory of Dr. B. Jill Venton - University of Virginia
Ultraviolet-visible (UV-Vis) spectroscopy is one of the most popular analytical ...

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Analytical Chemistry

Nuclear Magnetic Resonance (NMR) Spectroscopy

Source: Laboratory of Dr. Henrik Sund&#233;n &#8211; Chalmers University of Technology

Nuclear magnetic resonance (NMR) spectroscopy is a vital analysis technique for organic chemists. With the help of NMR, the work in the organic lab has been facilitated tremendously. Not only can it provide information about the structure of a molecule but also determine the content and purity of a sample. Compared with other commonly encountered techniques for organic chemists &#8212; such as thermal analysis and mass spectrometry (MS) &#8212; NMR is a non-destructive method that is valuable when recovery of the sample is important.
One of the most frequently used NMR techniques for an organic chemist is proton (1H) NMR. The protons present in a molecule will behave differently depending on its surrounding chemical environment, making it possible to elucidate its structure. Moreover, it is possible to monitor the completion of a reaction by comparing NMR spectra of the starting material to that of the final product.
This video exemplifies how NMR spectroscopy can be used in the everyday work of an organic chemist. The following will be shown: i) preparation of an NMR sample. ii) Using 1H NMR to monitor a reaction. iii) Identifying the product obtained from a reaction with 1H NMR. The reaction that will be shown is the synthesis of an E-chalcone (3) from an aldehyde (1) and a ketone (2) (Scheme 1).1
<img alt="Scheme 1" src="/files/ftp_upload/5680/5680scheme1.jpg" /> 
Scheme 1. Synthesis of (2E)-3-(4-methoxyphenyl)-1-(4-methylphenyl)-2-propen-1-one.

NMR Spectroscopy: Principle, NMR Active Nuclei, Applications

Source: Laboratory of Dr. Henrik Sund&#233;n &#8211; Chalmers University of Technology

Nuclear magnetic resonance (NMR) spectroscopy is a vital analysis ...

Organic Chemistry

Cyclic Voltammetry (CV)

Source: Laboratory of Dr. Kayla Green &#8212; Texas Christian University
A Cyclic Voltammetry (CV) experiment involves the scan of a range of potential voltages while measuring current. In the CV experiment, the potential of an immersed, stationary electrode is scanned from a predetermined starting potential to a final value (called the switching potential) and then the reverse scan is obtained. This gives a &#39;cyclic&#39; sweep of potentials and the current vs. potential curve derived from the data is called a cyclic voltammogram. The first sweep is called the &#39;forward scan&#39; and the return wave is called the &#39;reverse scan&#39;. The potential extremes are termed the &#39;scan window&#39;. The magnitude of reduction and oxidation currents and the shape of the voltammograms are highly dependent on analyte concentration, scan rates, and experimental conditions. By varying these factors, cyclic voltammetry can yield information regarding the stability of transition metal oxidation state in the complexed form, reversibility of electron transfer reactions, and information regarding reactivity. This video will explain the basic setup for a cyclic voltammetry experiment including analyte preparation and setting up the electrochemical cell. A simple cyclic voltammetry experiment will be presented.

Cyclic Voltammetry CV: Measuring Redox Potentials and Currents

Source: Laboratory of Dr. Kayla Green &#8212; Texas Christian University
A Cyclic Voltammetry (CV) experiment involves the scan of a range of potential ...

Electron Paramagnetic Resonance (EPR) Spectroscopy

Source: David C. Powers, Tamara M. Powers, Texas A&#38;M
In this video, we will learn the basic principles behind Electron Paramagnetic Resonance (EPR). We will use EPR spectroscopy to study how dibutylhydroxy toluene (BHT) behaves as an antioxidant in the autoxidation of aliphatic aldehydes.

EPR Spectroscopy, Paramagnetic Molecule and Zeeman Effect

Source: David C. Powers, Tamara M. Powers, Texas A&#38;M
In this video, we will learn the basic principles behind Electron Paramagnetic Resonance (EPR). ...

Inorganic Chemistry

Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals

Ideally, an unpaired electron shows a single peak in the EPR spectrum due to the transition between the two spin energy states. However, coupling interactions can occur between the spins of the unpaired electron and any neighboring spin-active nuclei. This hyperfine coupling results in hyperfine splitting, where the EPR signal is split into multiplets. The signals split into 2nI + 1 peaks, where n is the number of equivalent nuclei and I is the nuclear spin. These splitting patterns provide valuable information about the radical. In a methyl radical, for example, the electron spin is coupled with the three spin-active hydrogen nuclei. The EPR spectrum shows four peaks (quartet) with relative peak intensities in the ratio of 1:3:3:1.
In the case of the 1,4-benzosemiquinone radical, the unpaired electron is delocalized over the ring and oxygen atoms making all protons equivalent. The electron is coupled with the four equivalent protons, splitting the signal into five peaks with relative peak intensities in the ratio of 1:4:6:4:1. The distance between the peaks is the hyperfine coupling constant, which is measured in gauss or millitesla units. The magnitude of the coupling constant indicates the geometry of various radicals.&#160;

Electron Paramagnetic Resonance EPR Spectroscopy: Organic Radicals

Ideally, an unpaired electron shows a single peak in the EPR spectrum due to the transition between the two spin energy states. However, coupling ...

Unit 1

Radical Chemistry

KEY TERMS AND CONCEPTS

IR and UV&ndash;Vis Spectroscopy of Carboxylic Acids

In IR spectroscopy of carboxylic acids, the C=O bond shows a characteristic band between 1710 and 1760 cm&#8315;&sup1;, and the O&#8211;H bond exhibits a broad band between 2500 and 3300 cm&#8315;&sup1;.
However, the stretching absorptions for the C=O bond vary depending on the structure of carboxylic acids. The C=O bond of the free carboxylic acids shows a higher stretching frequency, 1760 cm&#8722;1, while H-bonded carboxylic acids (dimers) exhibit stretching absorptions at a lower frequency, 1710 cm&#8722;1. The C=O bond of the conjugated aliphatic and aromatic carboxylic acids shows an even lower stretching frequency at 1690 cm&#8722;1.
In IR spectra of carboxylic acids, the stretching absorptions for the C&#8211;H and O&#8211;H bonds overlap and exhibit absorption around 3000 cm&#8722;1, having sharp C&#8211;H peaks and a broad O&#8211;H peak. Many carboxylic acids also exhibit small spikes or shoulder bands between 2500 and&#160; 2700 cm&#8722;1.
UV&ndash;vis spectroscopy of the carboxylic acids shows a weak band at a lower wavelength of 200&#8211;215 nm due to an n&#8594;&#960;&#8727; transition. However, in highly conjugated carboxylic acids, the n&#8594;&#960;&#8727; transition shifts to a higher wavelength with higher molar absorptivity.

IR and UV&amp;#8211;Vis Spectroscopy of Carboxylic Acids

In IR spectroscopy of carboxylic acids, the C=O bond shows a characteristic band between 1710 and 1760 cm&#8315;&sup1;, and the O&#8211;H bond exhibits a ...

Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds

Summary

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Chapters in this video

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Electrochemical Impedance Spectroscopy as a Tool for Electrochemical Rate Constant Estimation

Raman and IR Spectroelectrochemical Methods as Tools to Analyze Conjugated Organic Compounds

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Ultraviolet-Visible (UV-Vis) Spectroscopy

Nuclear Magnetic Resonance (NMR) Spectroscopy

Cyclic Voltammetry (CV)

Electron Paramagnetic Resonance (EPR) Spectroscopy

Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals

IR and UV–Vis Spectroscopy of Carboxylic Acids