Features of Nuclei that Contribute to NMR Activity

NMR-active nuclei are atomic nuclei with a non-zero spin quantum number, such as ¹H, ¹³C, ¹⁹F, and ³¹P, and their usefulness in NMR spectroscopy depends not only on spin but also on their natural isotopic abundance and relative sensitivity, which together determine how strong a signal a given nucleus produces.

Nucleus surrounded by revolving electrons

Most of you would believe that ¹H and ¹³C are the only two nuclei that provide useful NMR spectroscopic information. This is largely true due to the large abundance of these two elements in natural as well as synthetic compounds. However, the contribution of several other nuclei cannot be overlooked. The present article reviews some such nuclei and also the factors that help such nuclei exhibit NMR activity.

The nucleus of an atom comprises of neutral particles called neutrons and positively charged particles called protons. Spinning nuclei have associated spin quantum number I which is the vector sum of the individual spins of constituent protons and neutrons. The spin quantum number is zero for non-spinning nucleus and non-zero for a spinning nucleus.

I= 0 - Examples ¹²C and ¹⁶O

I= 1 - Examples ²H and ¹⁴N

I = 1/2 - Examples ¹H, ¹³C, ¹⁹F, ³¹P

Nuclei having I value of 1/2 or multiples thereof are ideal candidates for NMR activity. However, besides the spin angular momentum there are other factors also that decide the activity and the usefulness of NMR information a nuclei can provide.

Besides 1H the other common nuclei that provide useful NMR information are ¹³C, ¹⁹F and ³¹P. Now we shall see what features contribute to their NMR activity

Isotopic Abundance

All the four nuclei have I=1/2 but there are marked differences in their NMR activity. The isotopic abundance and sensitivity are provided in the table below :

| Isotope | Isotopic Abundance | Relative Sensitivity | | --- | --- | --- | | ¹H | 99.98% | 1.00 | | ¹³C | 1.11% | 0.02 | | ¹⁹F | 100% | 0.83 | | ³¹P | 100% | 0.07 |

Nuclei with high isotopic abundance show strong NMR signals even when sample quantities are low. On the other hand isotopes with low abundance such as ¹³C need a large sample quantity to provide meaningful spectral information. ¹⁷O is another example which has natural abundance of 0.037% and therefore would require significant sample enrichment to get useful spectral data.

Sensitivity

1H has the highest sensitivity of 1.00 and other nuclei have lower values. It is simple logic that for nuclei with low sensitivities and isotopic abundance values both longer times and greater enrichment would be needed to get meaningful spectral information. ¹⁰³Rh is another interesting example. The isotope has 100% abundance but has an extremely low sensitivity value of 0.000031 so the resulting spectral information is almost meaningless. However, ¹⁰³RH can couple with other spin active nuclei such as ¹H and ¹³C to provide useful structural details.

Besides the four common NMR active nuclei there are several others, such as ¹¹B, ²³Na, ¹⁵N, ²⁹Si, ¹⁰⁹Ag, ¹⁹⁹Hg and ²⁰⁷Pb ,which are capable of providing useful NMR information on the basis of abundance and sensitivity. Such features have helped provide useful structural information on organometallic and intermetallic compounds.