NMR Spectroscopy

1. Nuclear Magnetic Resonance
Spectroscopy

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”)

8. The electrons surrounding a nucleus affect the
effective magnetic field sensed by the nucleus

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

11. The NMR Measurement

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