THE PHYSICS OF NUCLEAR SPINS AND NMR INSTRUMENTS

Type 1 Nuclei with 1 = 0. These nuclei do not interact with the applied magnetic field and are not NMR chromophores. Nuclei with 1 = 0 have an even number of protons and even number of neutrons and have no net spin. 

This means that nuclear spin is a property characteristic of certain isotopes rather than of certain elements. The most prominent examples of nuclei with I = 0 are and the dominant isotopes of carbon and oxygen. Both oxygen and carbon also have isotopes that can be observed by NMR spectroscopy. 

Type 2 Nuclei with I = V2. These nuclei have a non-zero magnetie moment and are NMR visible and have no nuclear eleetrie quadrupole (Q). The two most important nuclei for NMR speetroseopy belong to this eategory H (ordinary hydrogen) and (a non-radioaetive isotope of earbon occurring to the extent of 1.06% at natural abundanee). Also, two other commonly observed nuclei and P have I = V2. Together, NMR data for H and C account for well over 90% of all NMR observations in the literature and the discussion and examples in this book all refer to these two nuelei. However, the spectra of all nuclei with I = V2 can be understood easily on the basis of common theory. 

Type 3 Nuelei with I V2. These nuclei have both a magnetie moment and an eleetrie quadrupole. This group ineludes some eommon isotopes e.g. and N) but they are more difficult to observe and speetra are generally very broad. This group of nuclei will not be diseussed further. 

In the case of NMR, the energy required to induce the nuclear spin flip also depends on the strength of the applied field, H. It is found that 

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