2. Nuclear magnetic resonance, or NMR as it is
abbreviated by scientists, is a phenomenon which
occurs when the nuclei of certain atoms are
immersed in a static magnetic field and exposed to
an oscillating electromagnetic field. Some nuclei
experience this phenomenon, and others do not,
dependent upon whether they possess a property
called spin.
The versatility of NMR makes it pervasive in
the sciences.
NMR Spectroscopy
3. Nuclear magnetic resonance spectroscopy is the use of the
NMR phenomenon to study physical, chemical, and
biological properties of matter. As a consequence, NMR
spectroscopy finds applications in several areas of science.
NMR spectroscopy is routinely used by chemists to study
chemical structure using simple one-dimensional techniques.
Two-dimensional techniques are used to determine the
structure of more complicated molecules.
NMR is the most valuable spectroscopic technique used
for structure determination
More advanced NMR techniques are used in biological
chemistry to study protein structure and folding
NMR Spectroscopy
4. 1H or 13C nucleus spins and the internal
magnetic field aligns parallel to or against an
aligned external magnetic field
Parallel orientation is lower in energy making
this spin state more populated
Radio energy of exactly correct frequency
(resonance) causes nuclei to flip into anti-parallel
state
Energy needed is related to molecular
environment (proportional to field strength)
NMR Spectroscopy
5. Used to determine relative location of atoms
within a molecule
Most helpful spectroscopic technique in
organic chemistry
Maps carbon-hydrogen framework of
molecules
Depends on very strong magnetic fields
(imagine the strongest electromagnet you can
and the imagine it on steroids)
Use of NMR Spectroscopy
6. The spin state of a nucleus is affected by
an applied magnetic field
7. The energy difference between the two spin
states depends on the strength of the magnetic
field (that the atom “feels”)
9. Nature of NMR Absorptions
Electrons in bonds shield nuclei from magnetic field
Different signals appear for nuclei in different
environments
10. The NMR Measurement
The sample is dissolved in a solvent that does
not have a signal itself* and placed in a long thin
tube
The tube is placed within the gap of a magnet
and spun
Radiofrequency energy is transmitted and
absorption is detected
Species that interconvert give an averaged
signal that can be analyzed to find the rate of
conversion
Can be used to measure rates and activation
energies of very fast processes
12. 13C NMR Spectroscopy: Signal
Averaging and FT-NMR
Carbon-13: only carbon isotope with a nuclear
spin
Natural abundance 1.1% of C’s in molecules
Sample is thus very dilute in this isotope
Sample is measured using repeated
accumulation of data and averaging of signals,
incorporating pulse and the operation of Fourier
transform (FT-NMR)
All signals are obtained simultaneously using a
broad pulse of energy and resonance recorded
Frequent repeated pulses give many sets of
data that are averaged to eliminate noise
Fourier-transform of averaged pulsed data
gives spectrum
13. Applications of NMR
The microstucture of polymer chains
Resonance assignement
regioisomerism
Stereochemical configuration
Geometric isomerism
Characterization in the Solid State
Chain conformation in the solid state
Solid-solid transitions
Organization in the solid state
In multi-phase polymers
Orientation
Imaging
14. Applications of NMR
Dynamics of Polymers in the Solid State
Semicristalline polymers
Amorphous polymers
Polymer Systems
Polymer blends and miscibility
Multiphase systems