Nuclear magnetic resonance/Catalogs/Magnetic nuclei

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) spectroscopy both exploit magnetically active atomic nuclei. That is, compounds containing elements with non-zero nuclear spin (I ) can be studied by these techniques. The elements with non-vanishing nuclear spin are said to be magnetic nuclei. We refer to the articles on NMR and MRI for the use of the NMR-type spectroscopies in chemistry and medical diagnostics.

Nuclei with non-vanishing nuclear spin possess a magnetic moment and hence can be studied by magnetic resonance techniques. The prediction whether a nucleus has spin belongs to the realm of nuclear physics. This fact is usuallly taken for granted by the practitioners of NMR and MRI. Below an empirical list of nuclei with non-zero spin is given, together with their sensitivity (which basically is the probability of a spin transition).

Nuclear spin
Although nuclear spin I is listed as a single variable in the table below, it is the vector sum of the individual angular momenta of all protons and neutrons (nucleons). A nucleon has an angular momentum j = l + s, where the strong (in comparison to electromagnetic) nuclear forces couple the nucleon spin s to the nucleon orbit l. Depending on j the magnetic moment of the nucleon may be positive (parallel to j) or negative (antiparallel). The angular momenta j of the individual nucleons are coupled to total nuclear spin I.

In general, all atoms whose numbers of protons and numbers of neutrons are not both even will be magnetically active because the spins of the individual nucleons do not cancel each other out completely, that is total I &ne; 0. When placed in a very strong magnet the degenerate 2I+1   nuclear states will split into 2I+1 energy states whose energy differences are dependent on the strength of the magnetic field. In the simplest case where the nuclear spin I = 1/2, the spin can be aligned with or against the field. By exciting the nuclei with energy, typically in the radio frequency range of the electromagnetic spectrum, the lowest energy state is excited to a higher energy state. A signal, called the free induction decay (FID) is then measured as the excited state relaxes back to the lower energy state.

The table below lists the isotope(s) of elements that have non-zero nuclear spin and therefore may be used in NMR or MRI spectroscopy. Only three chemical elements with atomic numbers less than bismuth (Z=81) do not have magnetically active isotopes: argon, cesium and promethium.

NOTE: All nuclear spins I are positive. Negative spin values shown below indicate that the gyromagnetic ratio has a negative value, which means that the atomic magnetic moment and the nuclear spin are aligned antiparallel to each other. For nuclear magnetic resonance spectroscopists designing complex excitation schemes, it is convenient to think of such atoms as having negative nuclear spins because, compared to atoms with positive gyromagnetic ratios, the nuclei align in the opposite direction and therefore also precess (rotate) about the magnetic field in the opposite direction. (No matter what sign the spin value has they respond to a strong magnetic field one in the direction Z and the other in the direction &minus;Z. Important is to note they respond likewise only the direction differs by 180˚in the direction of the applied magnetic field.)