Wednesday, January 21, 2009
The Effect of Magic Angle Spinning and High Power 1H Decoupling on 13C Cross Polarization NMR Experiments
Cross polarization (CP), magic angle spinning (MAS) and high power 1H decoupling are all routine methods used in solid state NMR experiments. It is useful to see the effect of each of these techniques on a solid sample. The figure below shows 13C cross polarization NMR spectra of glycine at 4.7 Tesla collected with various combinations of magic angle spinning and high power 1H decoupling.The bottom spectrum was collected with neither MAS nor high power 1H decoupling. One can see two very broad overlapping lines due to the carbonyl and methylene carbons. The broadening is due to chemical shielding anisotropy and heteronuclear dipolar coupling between the 13C and both 1H and 14N. The second trace from the bottom was collected with high power 1H decoupling but no magic angle spinning. The spectrum contains two broad resonances with very informative line shapes. The high power 1H decoupling effectively removes the 13C - 1H heteronuclear dipolar interaction. The line shapes are determined from the chemical shielding anisotropy and 13C - 14N dipolar coupling interactions. The second trace from the top was collected with magic angle spinning at 4.5 kHz but no high power 1H decoupling. The spectrum apparently contains only one broad resonance with spinning sidebands. The magic angle spinning effectively removes the 13C chemical shielding anisotropy interaction. Although MAS does help average the 13C - 1H heteronuclear dipolar interaction, the averaging is not very effective at a speed of 4.5 kHz. Also, MAS only partially averages the 13C - 14N heteronuclear dipolar interaction. The resonances are therefore broadened out by residual heteronuclear dipolar coupling. The methylene resonance is broadened to such an extent that it does not show up in the spectrum at all. The top spectrum was collected with both MAS and high power 1H decoupling. One can see two very sharp resonances due to the carbonyl and methylene carbons. The 13C chemical shielding anisotropy and 13C - 1H heteronuclear dipolar coupling interactions are effectively removed by the MAS and high power 1H decoupling, respectively. Since MAS does not average J coupling and only partially averages dipolar coupling between a spin I = 1/2 and quadrupolar nucleus, the methylene carbon shows fine structure due to both J coupling and residual 13C - 14N dipolar coupling (see inset in yellow).