Slides in this set
NMR relies on nuclei aligning when placed in an external magnetic
Nuclei must possess 'spin' to be studied.
Measured in Nuclear magnetons (N).
Examples of magnetic isotopes are 13C, 1H, 19F,14N, 17O, 32P,
The rules for determining the net spin of a nucleus (Quantum
1. If the number of neutrons and the number of protons are both
even, then the nucleus has NO spin.
2. If the number of neutrons plus the number of protons is odd, then
the nucleus has a half-integer spin (i.e. 1/2, 3/2, 5/2)
3. If the number of neutrons and the number of protons are both
odd, then the nucleus has an integer spin (i.e. 1, 2, 3)
A nucleus of spin I will have 2I + 1 possible orientations. A nucleus
with spin 1/2 will have 2 possible orientations.…read more
For 1H there are two orientations as I=1/2.
The alignment with the field is also called the "alpha" orientation.
The alignment against the field is called the "beta" orientation.
No magnetic field Presence of external magnetic field
The lower energy
level will contain
slightly more nuclei
than the higher level.…read more
Electromagnetic Radiation with the same energy as the difference between
the spin states according to E=hf, causes nuclei to precess.
Energy difference between spin states:
There are two major relaxation processes:
1. Spin - lattice (longitudinal) relaxation 2. Spin - spin (transverse) relaxation
·The sample in which the nuclei are ·Between neighbouring nuclei
held is called the lattice. with identical precessional
frequencies but differing
·Magnetic field caused by motion of magnetic quantum states.
nuclei within the lattice is called
the lattice field. ·nuclei can exchange quantum
·Components of the lattice field can
interact with nuclei in the higher · There is no net change in the
energy state, and cause them to lose populations of the energy states,
energy. but the average lifetime of a
nucleus in the excited state will
Why do we see peaks?
When the excited nuclei in the beta orientation start to relax back down to
the alpha orientation, a fluctuating magnetic field is created. This fluctuating
field generates a current in a receiver coil that is around the sample. The
current is electronically converted into a peak. It is the relaxation that
actually gives the peak not the excitation.
Why do we see peaks at different positions?
A peak will be observed for every magnetically distinct nucleus in a
molecule. This happens because nuclei that are not in identical structural
situations do not experience the external magnetic field to the same extent.
The nuclei are shielded or deshielded due to small local fields generated by
orbiting electrons.…read more