EPR Spectroscopy Worksheet

How do you improve resolution? What causes the improved resolution?

• Resolution is improved by using high magnetic fields. This reduces the interactions between paramagnetic centers, which sharpens and simplifies the spectra. Also, using a pulsed EPR spectrometer provides time resolution, making it possible to measure dynamic properties. This is analogous to using Fourier transform to improve NMR.

• Resolution is improved by lowering the temperature, which increases the interactions between paramagnetic centers and broadens the spectra.

• Resolution is improved by using low magnetic fields, which increases noise and decreases spectral clarity.

• Resolution is improved by increasing sample concentration, which leads to overlapping signals and reduces spectral resolution.

Which of the following is the approximate g factor value for a free electron?

• 2.0023

• 1.0000

• 9.1094

• 0.0005

What perturbs the g value in compounds?

• Spin orbit coupling alters the g value in complexes.

• Temperature changes the g value in complexes.

• Pressure increases the g value in complexes.

• Magnetic field has no effect on the g value in complexes.

Define isotropic, axial, and rhombic.

• Isotropic: All 3 g values are the same (along 3 perpendicular axes). Axial: 2 are the same. Rhombic: All are different. G values are often graphed as derivative plots. For each g value in a plot, there will be a change in the sign of the slope. 1 change in sign is observed for isotropic, 2 for axial, and 3 for rhombic.

• Isotropic: All 3 g values are different. Axial: All are the same. Rhombic: 2 are the same. G values are always constant in plots.

• Isotropic: 2 g values are the same. Axial: All are different. Rhombic: All are the same. G values are never graphed as derivative plots.

• Isotropic: All 3 g values are the same. Axial: All are different. Rhombic: 2 are the same. G values are graphed as integral plots only.

Why is resolution improved in frozen samples versus liquid?

• In a liquid sample, the molecules are not stationary. The g values are anisotropic (directionally dependent), so molecular motion makes them very difficult to measure.

• Frozen samples have higher concentrations of solute, which increases resolution.

• Liquid samples absorb more light, reducing resolution.

• Frozen samples allow for better color contrast, improving resolution.

Describe hyperfine structure.

• It is a multiplet structure due to coupling of electron spin to magnetic nuclei. A nucleus with spin I will split the EPR line into 2I+1 lines of the same intensity.

• It is the splitting of spectral lines due to the interaction between electron orbital motion and external electric fields.

• It is a phenomenon where electron spin couples only with external magnetic fields, not with nuclei.

• It refers to the broadening of spectral lines due to thermal motion of atoms.

What is CV, and what does it tell us?

• CV stands for cyclic voltammetry, and it tells us the redox potential of a complex, and from that, we can determine its stability. It also gives us information on thermodynamic and kinetic properties. In CV, the current between two electrodes is measured as the potential difference, and it is changed cyclically, to produce a triangular waveform.

• CV stands for constant voltage, and it tells us the melting point of a compound. It is used to measure the temperature change during a reaction.

• CV stands for chemical valency, and it tells us the number of bonds an atom can form. It is used to predict molecular geometry.

• CV stands for current variation, and it tells us the rate of electron flow in a wire. It is used to measure electrical resistance.

What metals are typical of the working electrode?

• The working electrode is usually made out of Pt, Ag, Au, or graphite.

• The working electrode is usually made out of Na, K, or Ca.

• The working electrode is usually made out of Fe, Cu, or Zn.

• The working electrode is usually made out of Al, Mg, or Ti.

What metals are typical of the reference electrode?

• The reference electrode is usually made of Ag/AgCl.

• The reference electrode is usually made of Fe/Cu.

• The reference electrode is usually made of Zn/Pb.

• The reference electrode is usually made of Ni/Cr.

What two critical species are contained in the solution? How concentrated are they?

• CV is performed with a redox couple. The solution is very dilute, on the micro-molar scale or lower.

• CV is performed with a strong acid and base. The solution is highly concentrated, on the molar scale.

• CV is performed with a single inert gas. The solution is saturated with the gas.

• CV is performed with a buffer and indicator. The solution is moderately concentrated, on the milli-molar scale.