Magnetic Resonance

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Magnetic Resonance

• MSN 506 Notes

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Overview

• Essential magnetic resonance
• Measurement of magnetic resonance
• Spectroscopic information obtained by magnetic
  resonance
• 1D
• 2D Correlation and spin transfer related MR

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Magnetic Resonance

• Nuclei and electrons carry spin (magnetic dipole
  moment)

g is called the gyromagnetic ratio or g-factor
Nuclear magneton (unit for nuclear magnetic spins)
µN 5.050 783 10-27 JT-1
Bohr magneton (unit for electron magnetic spins)
µB 9.274 009 10-24 JT-1
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Energy of dipole in an external magnetic field
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NMR spectrometer
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Free Induction Decay

• The spins that are in a non-equilibrium initial
  position lose energy by radiating and return to
  equilibrium state

The released electromagnetic energy is picked up
by coils
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Free Induction Decay

• Field generated per spin is very small
• Large number of atoms/molecules (1023) can
  produce measurable signal if they add up
  coherently

Bolztmann Factor
Small portion of spins have net effect at room
temperature
Field Uniformity If B field is not uniform, spins
at different locations oscillate at different
frequencies
Solvents should be chosen so as not to confuse
the experiment
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Couplings

• Local magnetic field of a nuclei is affected by
  its surroundings

Nuclei produce fields at each others location
Electrons in bonds produce fields at nuclei
locations
Atom 2
Atom 1
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Couplings are used to identify bond structure of
molecules
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Dephasing
There are characteristic times for energy loss
and phase loss, which can also be measured by MR
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2D NMR

• Measures correlations between NMR lines by pulsed
  excitation
• Correlations indicate closely placed nuclei

The complicated 1D NMR spectrum of a protein
molecule does not say much about the molecular
structure
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Correlation measurement
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2D correlations help you estimate structure