Nuclei active in NMR

Some atomic nuclei behave like microscopic bar magnets. The general condition for a nucleus to behave in this way is to have an odd number of protons and/or neutrons (i.e., to have the spin quantum number, I different from zero). Most common nuclei in NMR (iR, C, igp) have the spin number (I) equal 1/2 and are named spin 1/2 nuclei. The following discussion will be particularized for the spin 1/2 nuclei, but the general rules and conclusions are valid for all cases. 

Fortunately, although not all the nuclei in the periodic table are active in NMR, almost every element has one or more than one isotope active in NMR. Some difficulties arise when the natural abundance of the active isotopes is very low. But with present instrumentation, spectra of practically any active nucleus can be recorded. 

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