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Wednesday, March 25, 2015

13C NMR of Vitamin C - Solids vs. Liquids NMR

In 13C[1H] NMR spectra of liquids, one observes a single resonance for each symmetrically nonequivalent carbon atom.  The same is true of the 13C CPMAS spectra of solids.  The difference is that the molecular symmetry is the determining factor in liquids NMR whereas the crystallographic symmetry is the determining factor in solids NMR.  As a result, solids NMR can give different spectra for different solid polymorphs and multiple resonances due to multiple nonequivalent molecules in the asymmetric unit of the crystal structure.  The figure below compares the solids 13C CPMAS and liquids 13C[1H] NMR spectra of a vitamin C (ascorbic acid) tablet.

The structure is provided in the figure and the assignment for all of the carbon atoms is color coded.  In the liquids spectrum, one observes resonances for each carbon in the molecule.  In the 13C CPMAS spectrum, one observes that three of the six 13C resonances are doubled.  This doubling is consistent with there being two nonequivalent ascorbic acid molecules in the asymmetric unit of the crystal.  Presumably, the other three resonance are doubled as well but there is insufficient resolution in the spectrum for this observation.  The broad features in the spectrum are due to additives in the vitamin C tablet which include, hypromellose, stearic acid and carnauba wax. 

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