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Monday, January 21, 2008

Finding "Invisible" Protons with 27Al Decoupling

Sometimes when protons are coupled to quadrupolar nuclei, Q, the 1H signals can range from being sharp singlets, to very broad or "invisible" signals, to sharp multiplets. The appearance of the proton signal depends on the relaxation properties of the quadrupolar isotope coupled to the proton and can also be broadened by chemical exchange. When the relaxation rate between the energy states of the quadrupolar isotope is fast with respect to the coupling constant, JH-Q, the 1H signal will be a sharp singlet. When the relaxation rate between the energy states of the quadrupolar isotope is slow with respect to JH-Q, the 1H signal will be a sharp multiplet governed by the spin of Q. If the relaxation rate is of the same order as JH-Q, the 1H signal will be broad and may even be "invisible". Such is the case for the complex below where a hydride proton is coupled to 27Al. In this case the hydride signal can be "found" by applying 27Al decoupling while observing protons.

2 comments:

nmrfreak said...

Glenn, I think Al-27 has a 300 or 400 ppm shift range, doesn't it?Did you run the aluminum spectrum first to find an optimum position for O2P? Or is the aluminum shift in this type of system already well characterized?

Mike

Glenn Facey said...

Mike,

Thanks for the comment. Yes, I ran the 27Al spectrum and set the decoupler offset to the 27Al shift. I used GARP decoupling.

Glenn