|Number of protons||Number of Neutrons||Spin Quantum Number (I)||Examples|
|Even||Even||0||12C, 16O, 32S|
|Odd||Even||1/2||1H, 19F, 31P|
|"||"||3/2||11B,35Cl, 79Br, 127I|
When a nucleus with a non-zero spin is placed in a magnetic field, the nuclear spin can align in either the same direction or in the opposite direction as the external magnetic field. A nucleus that has its spin aligned with the external field will have a lower energy than when it has its spin aligned in the opposite direction to the field. Thus, these two nuclear spin alignments have different energies and application of a magnetic field results in an energy level diagram like:
Nuclear magnetic resonance (NMR) spectroscopy uses the transition between these levels to detect and quantify nuclei. The magnitude of the energy splitting between these levels for nuclei in a strong magnetic field is in the range of radiofrequency (RF) radiation. Absorption of the RF radiation causes nuclear spins to realign or flip in the higher-energy direction. After absorbing energy, nuclei will reemit RF radiation and return to the lower-energy state.
The energy (and thus frequency) of an NMR transition depends on the magnetic-field strength and a proportionality factor for each nucleus called the magnetogyric ratio, γ. The frequency of a transition is given by:
ν = γ/2π H
where ν is the frequency of the resonant
radiation and H is the strength of the magnetic field.
The resonance frequencies of different nuclei in an atom are described by a relative shift compared to the frequency of a standard. This relative shift is called the chemical shift, δ, and is given by:
δ = [(νsample- nref)/νref)] 1·106
where δ has units of ppm. For 1H
NMR spectroscopy the reference compound is tetramethylsilane, Si(CH3)4,
For more extensive information and resources on NMR spectroscopy please
van Bramer's NMR pages.
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