Chemical Shifts in NMR Spectroscopy

The example of B5H9 serves to show how the chemical shift may be used as an aid to determining the stmcture of a molecule and, in particular, in deciding between alternative stmctures. There are many examples in the literature of this kind of application which is reminiscent of the way in which the chemical shift in NMR spectroscopy may be employed. However there is one important difference in using the two kinds of chemical shift. In XPS there are no interactions affecting closely spaced lines in the spectmm, however close they may be. Figure 8.15 illustrates this for the C lx lines of thiophene. In NMR spectroscopy the spectmm becomes more complex, due to spin-spin interactions, when chemical shifts are similar. 

For practical purposes, one usually does not determine absolute chemical shifts in NMR spectroscopy, but rather shifts relative to those of some reference compound. Though absolute shieldings are in principle observable quantities, it turns out to be considerably easier to measure relative shifts in the laboratory. The latter can be determined easily from experimental NMR spectra with high precision (and do not require knowledge of the exact magnetic field strength), while absolute shieldings are typically determined in a rather involved procedure from experimental spin-rotation constants obtained from microwave spectra. In addition, one should note that in... 

Chemical shift (in NMR spectroscopy) referenced to ARNOj) in solution Fraction of the sUanols interacting with probe molecules... 

The same substance is also used as reference for expressing chemical shift in NMR spectroscopy. Chemical shifts in H NMR lie in the range 0-10 ppm, and in NMR the range is 0-250 ppm. 

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