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Monday, November 24, 2008

90 Degree Pulses for I = n/2 Quadrupolar Nuclei in the Solid State

The 90 degree pulse for an I = n/2 quadrupolar nucleus in the solid state depends on the strength of the rf pulse with respect to the quadrupolar frequency. If the strength of the pulse is much greater than the quadrupolar frequency, the pulse is non-selective and excites all transitions equally. If however it is much less than the quadrupolar frequency, then the pulse is selective to the central (m = 1/2 - m = -1/2) transition. The duration of the pulse producing a maximum signal is shorter for selective vs. non-selective pulses at a similar power level. In solution, where the quadrupolar interactions is averaged by random isotropic molecular motion or in the solid state, if the symmetry around the I = n/2 nucleus is cubic, the quadrupolar frequency is small with respect to the strength of the rf pulses and the pulses are non-selective. When the symmetry around the I = n/2 nucleus in the solid state is non-cubic, the quadrupolar frequency is significant and the pulses are very often selective to the central transition. This is illustrated in the figures below for the 23Na MAS spectrum of a mixture of NaCl (cubic) and Na2SO4 (non-cubic). The first figure shows the 23Na MAS spectrum labelling each component of the mixture. The second figure shows the effect of increasing the pulse duration. One can clearly see that the 90 degree pulse for NaCl is close to twice that of Na2SO4.

2 comments:

Thomas said...

Hi,

I would like to understand how a quadrupolar frequency effects the intensity evolution. Does the quadrupolar interaction prevent inefficient flipping of spins when pulses are applied? I guess I don't understand how the coupling between the quadrupole moment and EFG would effect the 90 or 180 degree time from a qualitative perspective for say two systems that have different quadrupolar frequencies.

Any advice?

Thomas

Glenn Facey said...

Dear Thomas,

Thank you for your question. The answer has to do with the fact that for small quadrupolar coupling constants an rf pulse is nonselective. It excites all of the transitions. For large quadrupolar couplings a typical rf pulse is selective only towards the central transition, the satellite transitions being too far off resonance to be excited. A detailed answer to your question is given in Melinda Duer's book, "Solid-State NMR Spectroscopy" chapter 5 section 5.5.2, page 249. I hope this helps.

Glenn