NMR active nuclei and isotope abundance

Many nuclei possess a property described as spin. The nuclear spin (nuclear angular momentum) is quantized and is described by the spin quantum number / which can have 

When a H NMR spectrum of a hydrogen-containing compound is recorded, virtually all the H atoms in the sample contribute to the observed spectrum in a naturally occurring hydrogen sample, the abundance of h is 99.985%. The fact that only 1% of naturally occurring carbon is means that if a NMR spectrum of a carbon-containing compound is recorded, only 1% of the carbon atoms present are observed. This has important consequences with respect to coupling as we see 

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