Questions: Electron Paramagnetic Resonance (EPR) Spectroscopy
4 questions to test your understanding
Score: 0 / 4
Question 1 Multiple Choice
What hyperfine pattern does the methyl radical, •CH₃, show in its EPR spectrum?
AA single line
BA 1:1 doublet
CA 1:2:1 triplet
DA 1:3:3:1 quartet
The unpaired electron couples to three equivalent protons, each with I = 1/2. That gives n + 1 = 4 lines with binomial intensities 1:3:3:1.
Question 2 True / False
A compound in which every electron is paired gives no EPR spectrum.
TTrue
FFalse
Answer: True
EPR measures transitions between the spin states of an unpaired electron. With all electrons paired, the net electron spin is zero and there is no EPR transition to observe.
Question 3 Multiple Choice
An X-band spectrometer operates at about 9.5 GHz. At roughly what magnetic field does a radical with g ≈ 2.00 come into resonance?
AAbout 0.0034 T
BAbout 0.34 T
CAbout 3.4 T
DAbout 34 T
From hν = gμ_B B, B = hν/(gμ_B) = (6.63 × 10⁻³⁴ J s × 9.5 × 10⁹ s⁻¹)/(2.00 × 9.27 × 10⁻²⁴ J T⁻¹) ≈ 0.34 T.
Question 4 Short Answer
Why does a nitroxide radical, R₂N–O•, show three EPR lines of equal intensity?
Think about your answer, then reveal below.
Model answer: The unpaired electron is partly on the ¹⁴N nucleus, which has nuclear spin I = 1. A nucleus with spin I splits the line into 2I + 1 = 3 components, one for each orientation m_I = +1, 0, −1, and the three orientations are equally populated.
This is the same counting rule as for protons, generalized to nuclei with I > 1/2. Nitroxides are widely used as spin labels because this simple pattern responds to their surroundings.