As we have previously discussed in the Basics of Magnetism - spinning nuclei of atoms produce magnetic fields. However, not all nuclei of atoms spin. Nuclei of elements have a characteristic spin I. Values of I can be
- integral e.g. 1,2,3. – These are complicated to detect and therefore not useful for NMR.
- fractional e.g. 1/2 , 3/2, 5/2 – These are simple to detect and useful for NMR.
- they can be zero. When I=0 these atoms do not produce any magnetic field as they do not spin.
What decides these values of I?
Spin Quantum Number are deduced from complicated mathematics. However to simplify the rule the I value depends upon the number of protons and the number of neutrons present in the nucleus. The rules state that:
- If the number of neutrons and the number of protons are both even, then the nucleus has NO spin. Therefore the value of I = 0. e.g. 12C6.
- 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) e.g. 13C6 and proton 1H.
- 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) e.g. deuterium 2H1.
Quantum mechanics tells us that a nucleus of spin I will have 2I+1 possible orientations for angular momentum. Therefore a nucleus of spin 1/2 will have only two possible orientations and it is therefore easy to detect these nuclei with NMR. Higher I value nuclei have more angular momentum possible and therefore more difficult to detect using NMR techniques.
Thereofore in theory any nuclei with non-zero value of I can be theoretically detected by NMR as they have different angular momentums and thus produce magnetic fields. However, practically only half-integral spin nuclei are detected using NMR as they are simpler to detect.
Atoms such as H1, C13, F19 and P31 have half-integer spins which is easy to analyze and hence most commonly detected by nuclear magnetic resonance. Even with just these few atoms a person can get enough structural information out of the compound to identify it.
So how is this information useful in NMR? Read ahead in the next chapter on the Principle of Nuclear Magnetic Resonance.
Further Reading and References: