Friday, August 8, 2008
Setting the Magic Angle with Glycine
One of the most precise ways of setting the magic angle is to maximize the number of rotational echos in the FID of a suitable spin I = n/2 quadrupolar nucleus (n =3, 5, 7 ....). When setting up for 13C CPMAS, one usually uses the 79Br resonance of KBr as the resonance frequency of 79Br is very close to that of 13C. An alternative method of setting the magic angle is to use the 13C carbonyl resonance of glycine. This has the advantage in that the glycine can also be used to set the Hartman Hahn matching condition and to check the decoupling power. The width of the carbonyl resonance is very sensitive to the setting of the magic angle. The angle can easily be adjusted and set properly while maximizing the duration of the signal in the FID interactively. The figure below shows the 13C CPMAS FID and spectrum for glycine on- and off-angle with digital filtering such that the methylene resonance is outside of the spectral width. The spectra were collected at 11.7 Tesla using a spinning speed of 12 kHz. When the angle is mis-set, one can see that the line shape for the resonance is a miniature version of the powder pattern observed in the absence of magic angle spinning.
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5 comments:
Glenn,
My experience is that the Na23 of NaNO3 is better than glycine or KBr. It's much more sensitive to the angle and the signal is huge. The T1 is also very short.
Glenn.
Hi Glenn,
Why some nuclei (or types of carbon) are more sensitive to mis-settings of the magic angle than others? Is it completely due to chemical shift anisotropy?
Many thanks.
Anonymous,
Thank you for the question. For spin I=1/2 nuclei, the CSA indeed determines the sensitivity of the magic angle setting. The larger the CSA the more sensitive the signal is to the precision of the magic angle. The satellite transitions of spin I=n/2 quadrupolar nuclei are very sensitive to the precision of the magic angle due to the very large breadth of the satellite transitions in the absence of MAS. This is why the 79Br resonance of KBr is often used to set the magic angle.
Glenn
Hi Glenn,
On the topic of glycine and the HH match, would you know why the carbonyl with no attached protons of glycine is so sensitive to the HH matching?
Thank you
Anonymous,
Thank you for the question. The Haetman-Hahn match in CP measurements is very sensitive when spinning samples at the magic angle at rates comparable to the heteronuclear dipolar coupling interaction. See this post:
http://u-of-o-nmr-facility.blogspot.ca/2008/02/hartman-hahn-match-as-function-of-mas.html
and this post:
http://u-of-o-nmr-facility.blogspot.ca/2008/03/importance-of-proper-hartman-hahn-match.html
Samples like adamantane have reduced dipolar coupling due to molecular motion. Samples with non-protonated carbon atoms like glycine have reduced dipolar couplings due to the distance between the carbon and the protons. In such samples, the heteronuclear dipolar coupling is comparable to moderate MAS spinning rates (5 -10 kHz) and the Hartman-Hahn matching curve is split into a series of matching sidebands spaced at the spinning frequency. In such cases the Hartman Hahn match is very sensitive. The sensitivity can be reduced by using ramped CP pulses. See this link.
http://u-of-o-nmr-facility.blogspot.ca/2008/05/cross-polarization-using-ramped-pulses.html
I hope this helps.
Glenn
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