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Monday, April 12, 2010

Hahn Echo for 11B Background Suppression in Solids

Solids NMR probes often contain boron rich parts near the coil in which the sample resides. Boron nitride, in particular, is a material very commonly used. This can be very problematic if one wishes to collect 11B NMR data, in that a strong background signal may be observed. Even though these parts are not directly inside the coil with the sample, they do experience a small amount of rf from the coil and the coil does detect a 11B NMR signal from them. One simple way to avoid this problem is to use a Hahn echo to observe the 11B spectrum. Unlike the sample inside the coil, which experiences the 90° and 180° pulses required for the Hahn echo, the boron rich parts outside of the coil experience pulses of much smaller flip angles and therefore the echo signal from them is much reduced. This is illustrated in the figure below, which shows 11B NMR spectra collected in a 4 mm MAS probe without magic angle spinning. In the lower trace a simple one-pulse measurement was made with high power 1H decoupling. The spectrum contains a very large background signal and it is very difficult to extract any useful information from the data. The spectrum in the upper trace was collected with a Hahn echo with high power 1H decoupling during the acquisition. The background signal is completely removed revealing a beautiful, information-rich line shape for the central transition resulting from the second order quadrupolar and chemical shielding anisotropy interactions.
Thank you to Joseph Weiss of David Bryce's laboratory for providing the sample and collecting the data. See more of Joseph's beautiful 11B spectra here:

Joseph W.E. Weiss and David L. Bryce J. Phys. Chem. 114 (2010) ASAP.
http://dx.doi.org/10.1021/jp101416k

8 comments:

Anonymous said...

Hi,

Thanks for your helpful post. What is the difference between solid echo and hahn-echo and how do you know when to use which? I know the difference comes from the second pulse being 180 (hahn-echo) versus 90 (solid echo). What is the advantage of using one over the other, but I don't see the advantage of using one versus the other.

Thanks,
Neil Tanner

Glenn Facey said...

Neil,

The answer to you question is not simple. In the case where the quadrupolar coupling is much weaker than the rf, one should get a maximum in the echo for the central transition when the second pulse in the echo sequence is either 90 or 180 degrees however, the echoes from both the central and satallite transitions will be observed when the second pulse is less than or equal to 90 degrees. The situation is not as simple when the quadrupolar coupling is of the same order as the rf. A good reference is:

p. Man, Phys. Rev B. vol 52, p.9418 (1995).

I hope this helps.

Glenn

Anonymous said...

Hi,

What about non-quadrupolar nuclei (I=1/2) where dipolar coupling is only important? Could you give feedback or provide a reference?

Thanks,

N.T.

Glenn Facey said...

N.T.

Thank you for your question. Using a Hahn echo or solid echo to suppress background signals for spin I = 1/2 nuclei should also work well. I have done this for wideline 1H observation in solids.

Glenn

Anonymous said...

Hi Glenn,

Can you comment on why the echo spectra are not quantitative(I think)? I once had a pure sample which gives proton echo spectra with different aromatic/aliphatic ratio when the delays are varied.

Thanks.
DX

Glenn Facey said...

Anonymous:

Thank you for your comment. The echo spectra would not be quantitative (in comparison to a Bloch decay spectrum) if the T2's are of the same order as the echo delays and different for the two types of protons. Were you using MAS?

Glenn

Anonymous said...

Hi Glenn,

MAS at 15k at RT. A pure amorphous solid containing a para-substituted benzene group. Echo delays varied by multiples of rotor period.

Can you point me to some basic readings of NMR echos?

Thanks a lot, and Happy New Year!

DX

Glenn Facey said...

DX.

I reccomend Melinda Duer's book "Solid-State NMR Spectroscopy" chapter 2 section 6.

Glenn