Chapter 8 Multiple Choice Questions

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. The shape of an EPR spectrum of a nitrogen atom trapped inside a C60 molecule (Fig. 4.2) is likely to be:

Gaussian correct incorrect

Lorentzian correct incorrect

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. A 0.01 mM solution of benzene radical anion in a 1% benzene-tetrahydrofuran mixture shows a broader EPR spectrum compared to a 0.01% benzene-tetrahydrofuran mixture. Which single sentence describes this phenomenon most accurately?

at higher benzene concentration, the lines are broadened due to the dipole-dipole interaction between the benzene and the benzene radical anion molecules. correct incorrect

at higher benzene concentration, the lines are broadened due to degenerate electron transfer between the benzene and the benzene radical anion molecules. correct incorrect

at higher benzene concentration, the lines are broadened due to the exchange interaction between the benzene and the benzene radical anion molecules correct incorrect

at higher benzene concentration, the lines are broadened due to higher viscosity of solution. correct incorrect

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. Deoxygenation of a TEMPO solution led to a four-fold increase in peak-to-peak intensity in its EPR spectrum. How did the peak-to-peak linewidth change?

decreased two-fold correct incorrect

remained unchanged correct incorrect

increased four-fold correct incorrect

decreased four-fold correct incorrect

increased two-fold correct incorrect

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. Estimate the T₂ relaxation rate for an oxygen-saturated TEMPO solution in acrylic acid (EPR spectrum shown in Fig. 7.21a).

10^-8 s correct incorrect

10^-8 s^-1 correct incorrect

10⁸ s correct incorrect

10⁸ s^-1 correct incorrect

10^-11 s correct incorrect

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. Browse the restricted motion of nitroxides at EPRSimulator.org, and simulate an EPR spectrum with the following parameters: log(D) = 8.6, Frequency = 9.5 GHz, A_zz = 36 G, linewidth = 0.1 G. Hence estimate the rotational correlation time (τ_c) from the simulated spectrum.

5.2 × 10^-13 s correct incorrect

5.2 × 10^-10 s correct incorrect

2.6 × 10^-8 s correct incorrect

8.3 × 10^-3 s correct incorrect

1.2 × 10^-6 s correct incorrect

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. Estimate the diameter of a rigid spherical radical if its EPR spectrum at 40 °C in water shows a rotational correlation time τ_c = 2 x 10^-9 s. The viscosity of water at this temperature is 0.65 mPa s.

0.67 nm correct incorrect

2.99 nm correct incorrect

1.49 nm correct incorrect

5.99 nm correct incorrect

0.30 nm correct incorrect

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. Which interaction leads to the Gaussian lineshape of solution EPR spectra?

conformational exchange correct incorrect

exchange interaction correct incorrect

hyperfine strain correct incorrect

self-exchange correct incorrect

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. Which effect can lead to alternating line widths in solution EPR spectra?

conformation exchange correct incorrect

slow rotational tumbling correct incorrect

self-exchange correct incorrect

hyperfine strain correct incorrect

exchange interaction with paramagnetic quenchers correct incorrect

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. The EPR spectra of Cu(II) ions in solution are dominated by g anisotropy. For a complex producing the EPR spectrum shown in Fig. 6.6a, estimate the rate of rotational tumbling that will be too fast to affect the EPR line shape at W-band.

10¹⁰ s^-1 correct incorrect

10⁶ s^-1 correct incorrect

10⁴ s^-1 correct incorrect

10¹⁴ s^-1 correct incorrect

10⁶ s correct incorrect

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. Which one of the following sentences is true?

EPR spectra of nitroxides do not depend on solvent polarity. correct incorrect

the isotropic g factor of nitroxides increases in less polar solvents correct incorrect

the isotropic a value of nitroxides increases in less polar solvents correct incorrect

the isotropic a value of nitroxides decreases in less polar solvents correct incorrect

the rate of rotational tumbling of nitroxides increases in less polar solvents correct incorrect