Which nuclei are suitable for NMR spectroscopic studies

A wide range of nuclei may be observed by NMR spectroscopy, but the inherent properties of some nuclei (e.g. a large quadrupole moment) may make their observation difficult. The main criterion is that the nucleus possesses a value of the nuclear spin quantum number (Table 2.3). Secondly, it is advantageous (but not essential) for the nucleus to occur in signihcant abundance. Carbon-13 is an example of a low abundant isotope which is, nevertheless, extensively used for NMR spectroscopy isotopic enrichment may be used to improve signal noise ratios. A third requirement is that the nucleus possesses a relatively short spin-relaxation time (tj) this property depends not only on the nucleus itself but also on its molecular environment. Some elements exhibit more than one NMR active nucleus and the choice for experimental observation may depend upon the relative inherent values of Tj. For example, Li and Li are NMR active, but whereas t values for Li are typically 3 s, those for Li lie in the range 10-80s Li is thus more appropriate 

The range of chemical shifts over which NMR spectroscopic signals appear is dependent on the nucleus. The most commonly observed nucleus is H and, in organic compounds, a spectral window from 5+15 to 0 usually encompasses most signals. In inorganic compounds, the window may have to be widened if, for example, H nuclei attached to metal centres are to be observed, or if 

Nucleus Natural abundance / % / Frequency of observation / MHz Chemical shift reference 

An example of a paramagnetic centre is a Co ion which, in an octahedral complex, has one or three unpaired electrons (see Chapter 20). The figure below shows the NMR spectrum of the Co complex [Co(phen)3] (phen= 1,10-phenanthroline), the structure of which is shown below. There are four different aromatic proton environments in the complex, and the chemical shifts of the signals assigned to these nuclei fall in the range + 110 to +15. 

Bertini and C. Luchinat (1996) Coordination Chemistry Reviews, vol. 150 - NMR of paramagnetic substances.  

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