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Tuesday, March 25, 2008

Solution vs Solid State MAS NMR

The number of peaks present in a standard 13C solution state NMR spectrum with proton decoupling depends on the number of symmetry independant carbon atoms present in the molecule. If the molecule has no symmetry then the number of peaks equals the number of different types of carbon in the molecule. When the molecule has symmetry, the number of 13C resonances is reduced by the symmetry. The molecular symmetry determines the number of resonances. In the solid state, on the other hand, the molecules are part of a crystal lattice. In solids it is the crystallographic symmetry rather than the molecular symmetry that determines the number of peaks in the spectrum. There will be a resonance for each carbon in the asymmetric unit of the crystal. This is illustrated in the figure below. In this case, for the solution state spectrum, there is half as many 13C resonances as there are carbon atoms due to the molecular symmetry. In the solid, there is one entire molecule in the asymmetric unit of the crystal therefore the 13C CPMAS spectrum has one peak for each carbon in the molecule.

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