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Wednesday, February 27, 2008

Hartman-Hahn Match as a Function of MAS Spinning Speed

Cross polarization with magic angle spinning (CPMAS) is a means of obtaining high resolution, high sensitivity NMR spectra of dilute isotopes in solids. The dipolar coupling between the protons and the dilute isotope to be observed ( typically 13C, 15N, 29Si etc....) is exploited as a means for magnetization transfer from the abundant protons to the dilute isotope. The transfer is only possible when the product of the gyromagnetic ratio and applied power for the dilute isotope equals the product of the gyromagnetic ratio and applied power for the protons. For static or slow spinning samples, one can vary either the X or 1H power level during the contact time and one will observe a maximum signal when the Hartman-Hahn condition is met. The situation is more complicated when a sample is spinning at a rate comparable to or faster than the magnitude of the heteronuclear dipolar coupling used for the magnetization transfer. In such a case the MAS interferes with the dipolar coupling. The effect is that the Harman Hahn matching curve (intensity vs X or 1H power) is split into a series of maxima and minima separated by the spinning speed. This is illustrated in the figure below. When fast MAS CP experiments are to be used, it is important to set up the Hartman-Hahn condition on a maximum for a standard sample at the same spinning speed to be used for the sample of interest.

3 comments:

Kenneth Knott said...

Interesting.. I'd observed this and wondered what was going on...

Piotr Paluch said...

At highht speed it is possible to observe various HH matching condition. At the simplest aproximation HH match is fulfilled when (RF_on_C)-(RF_on_H)=n*ROT (where RF means RF field calculated as RF=1/(4*t90deg), and n is 0,+/-1,+/-2...and ROT is spining speed). At very hight speed it is also possible to obtained signal via so caled DQ-CP and in this case HH match is fulfilled when (RF_on_C)+(RF_on_H)=n*ROT.
Plese see this papper:

http://www.sciencedirect.com/science/article/pii/S1090780713001158

http://www.sciencedirect.com/science/article/pii/S0926204011000774

http://onlinelibrary.wiley.com/doi/10.1002/mrc.3798/abstract;jsessionid=76890C28E7AF79BBA77AE031C58039FF.f04t01

http://www.sciencedirect.com/science/article/pii/S109078070800356X

Situation is even more complicated when we consider quadrupolar nuclei at fast MAS, eg.:

http://www.sciencedirect.com/science/article/pii/S0926204016301126

Glenn Facey said...

Piotr,
Thank you very much for your input and references.
Glenn