Debus Study Questions – NMR
Debus 70% Dunn 30% ~10 questions
Debus will most likely pull his questions straight from here
1. Write an expression for the Boltzmann distribution and explain what it tells you about
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Debus Study Questions – NMR
Debus 70% Dunn 30% ~10 questions
Debus will most likely pull his questions straight from here
1. Write an expression for the Boltzmann distribution and explain what it tells you about
energy level spacings in NMR versus those in optical absorption or vibrational
spectroscopy.
nB/na = e^(-ΔE/RT)
In NMR there is a very small energy gap (1/25,000) between the ground state and the
excited state and most atoms are in the ground (up spin) state. In optical absorption only
the ground state is populated at equilibrium and there is a large energy gap.
2. What is a transition dipole moment and why is it important in spectroscopy?
A transition dipole moment is a measure of charge redistribution that accompanies a
transition. A transition is only active if significant dipolar charge redistribution occurs.
3. What causes 1H chemical shifts in NMR?
The environment: either J coupling, electron shielding, or the presence of 13C or 15N.
4. What is “J coupling” (also known as “spin-spin coupling” or “indirect dipole dipole
coupling”) and what causes it?
J-coupling occurs when magnetic nuclei interact through intervening electrons in
covalent bonds. It is a weak interaction which can be sensed through at most 2-3 bonds.
5. What is the primary advantage of Fourier-Transform NMR over continuous-wave
NMR?
FT (Pulse) NMR improves signal averaging and allows the recording of 100 free
induction decays in the time it takes one continuous wave spectrum to record. There is
also a 10x increase in signal/noise ratio.
6. Why is it possible to excite 13C or 15N nuclei independently of 1H nuclei?
13C and 15N nuclei transitions appear on different regions of the electromagnetic
spectrum; therefo
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