Chapter 3 Multiple Choice Questions

. Calculate the g values for the paramagnetic centre shown in Fig. 3.6.

g₁= 2.080; g₂ = 2.035; g₃ = 2.025

g₁= 2.070; g₂ = 2.025; g₃ = 2.015

g₁= 2.040; g₂ = 2.030; g₃ = 2.028

g₁= 2.050; g₂ = 2.010; g₃ = 2.000

g₁= 2.060; g₂ = 2.030; g₃ = 2.010

. The EPR spectrum of an ·RH₂ radical centre is shown in Fig. 3.9. Estimate the g and a values of the radical.

g= 2.00; a = 0.55 mT

g= 2.04; a = 1.05 mT

g= 2.03; a = 0.85 mT

g= 2.02; a = 0.25 mT

g= 2.01; a = 0.50 mT

. Calculate the relative occupancies of the α and β spin energy levels for a radical species with g = 2.05, at L- and W-band frequencies (take T_S = 300 K).

Nα/Nβ = 0.9800 at L-band; Nα/Nβ = 0.9509 at W-band

Nα/Nβ = 0.9950 at L-band; Nα/Nβ = 0.9609 at W-band

Nα/Nβ = 0.9910 at L-band; Nα/Nβ = 0.9709 at W-band

Nα/Nβ = 0.9998 at L-band; Nα/Nβ = 0.9849 at W-band

Nα/Nβ = 0.9850 at L-band; Nα/Nβ = 0.9809 at W-band

. Calculate the Q-factor for an X-band EPR cavity with a resonator bandwidth of 1.58 MHz.

Q = 1012

Q = 2012

Q = 3012

Q = 5012

Q = 6012

. The room temperature X-band EPR spectrum of a Cu(II) ion is shown in Fig. 1.6. Which of the following would be a suitable choice of experimental conditions for recording the spectrum (assuming no solubility issues).

Solvent = water;
MW power ≈ 0.001 mW;
Concentration = 2 mM;
Sweep width = 1.0 mT.

Solvent = dichloromethane;
MW power ≈ 0.1 mW;
Concentration = 10 nM;
Sweep width = 100.0 mT.

Solvent = methanol;
MW power ≈ 1 mW;
Concentration = 1 M;
Sweep width = 25.0 mT.

Solvent = toluene;
MW power ≈ 10 mW;
Concentration = 0.5 mM;
Sweep width = 70.0 mT.

. The field modulation amplitude for the ·RH₂ radical centre shown in Fig. 3.9 should ideally be:

1 mT

0.05 mT

0.1 mT

5 mT

10 mT

. Higher microwave powers can typically be employed when recording signals from metal ions compared to organic radicals, due to:

Inherently broader line-widths in metal ions are generated at higher powers.

Compared to organic radicals, metal ions generally have faster relaxation times.
Incorrect

Organic radicals are often measured using solvent of high dielectric constants, and therefore lower microwave powers must be used.

Lower microwave powers are required for organic radicals due to the lower modulation amplitude used when recording the spectra.

. A square planar Co^II ion in the solid state is characterized by g values of 2.3 and 2.0. Estimate the magnitude of the sweep width required to record the spectrum at X- and Q-band frequencies.

~ 150.0 mT at X-band; ~ 360.0 mT at Q-band

~ 25.0 mT at X-band; ~ 160.0 mT at Q-band

~ 50.0 mT at X-band; ~ 160.0 mT at Q-band

~ 2.0 mT at X-band; ~ 500.0 mT at Q-band

~ 5.0 mT at X-band; ~ 250.0 mT at Q-band

. Calculate the resonance frequency of a rectangular EPR cavity (of dimensions d > a > b) operating in the TE₁₀₂mode (i.e., TE_mnp) where a = 2.5 cm and d = 4 cm, given .

ν_res = 1.53 GHz

ν_res = 3.53 GHz

ν_res = 9.59 GHz

ν_res = 34.53 GHz

ν_res = 94.53 GHz

. A microwave cavity operating in the TE₁₀₁ mode is not suitable for EPR measurement because:

The cavity dimensions would not be compatible with the microwave frequency.

Both E₁ and B₁ fields would be co-aligned along the sample axis.

The E₁ electric field would be maximum along the sample axis, with the B₁ field at a minimum.

The orthogonal E₁ and B₁ fields would prevent resonance absorption by the sample.

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