13C CP/MAS and direct single pulse 13C MAS NMR with high power decoupling can give very different results for a wide variety of materials. 13C CP/MAS NMR relies on the transfer of magnetization from protons to 13C via the dipolar coupling mechanism whereas the direct single pulse method does not. An interesting material to demonstrate this principle is the shell of a chicken egg. Chicken eggshell is a complex bio-mineral consisting largely (~ 95%) of the calcite polymorph of calcium carbonate as well as proteins and lipids. The 13C CP/MAS and 13C single pulse MAS NMR spectra of a sample of dry ground eggshell, from which the membranes had been removed, are shown in the lower and upper traces in the figure below, respectively.
The 13C CP/MAS spectrum in the lower trace has very broad resonances in the aliphatic region of the spectrum due to the proteins and lipids. The carbonyl region of the spectrum consists of two resonances; one at ~173 ppm due to the carbonyl carbons of the amino acid residues of the proteins and a resonance at ~169 ppm which has been shown1,2 to originate from bicarbonate ions (HCO3)-. Although ~95% the eggshell consists of CaCO3, the carbonate resonance is not present in the 13C CP/MAS spectrum as there are no proximate protons for cross polarization. In contrast, the single pulse 13C MAS spectrum in the top trace shows only the 13C resonance from the carbonate ions which has a coincident chemical shift with that of the bicarbonate ions. All of the other 13C resonances are buried in the noise as they are in much lower concentration. These spectra are an excellent example of how one can obtain different information from 13C CP/MAS and single pulse 13C MAS spectra.
1. D.M. Pisklak, L. Szcleszczuk, I. Wawer, Journal of Agricultural and Food Chemistry, 60, 12254 (2012).
2. J. Feng, Y.J. Lee, R.J. Reeder, B.L. Phillips, American Mineralogist, 91, 957 (2006).