Nuclear Magnetic Resonance Spectroscopy:

NMR is a physical phenomenon in which nuclei of a given substance under magnetic field absorb and re-emit electromagnetic radiation. This energy is at a particular resonance frequency which depends on the strength of the magnetic field and magnetic properties of the isotope of the atoms.

As nuclei with an odd mass or odd atomic number have nuclear spin (in a similar fashion to the spin of electrons), the spins of nuclei are sufficiently different that NMR experiments can be sensitive for only one particular isotope of one particular element.

The rules for determining the net spin of a nucleus are as follows:

1. If both the number of neutrons and the number of protons are 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)

Note that the overall spin is important. Quantum mechanics shares that a nucleus of spin '/' has 2/+1 possible orientations, i.e., a nucleus with spin 1/2 will have 2 possible orientations. These orientations are of equal energy in the absence of external magnetic field, but the energy levels split once magnetic field is applied.