We have a person on this site (Professor H. C. Brown) with an interest in
11-B NMR spectroscopy |-). We saw exactly the same effect as noted below
and in earlier posts to AMMRL on Brown's first dedicated NMR spectrometer
and in its later replacement. Strangely enough, we did not see the problem
in the department's first (partly home-built) FT system on which we did our
initial 11-B work. The first system used probes with quartz receiver coil
forms while the later commercial systems used Pyrex glass instead of quartz.
Pyrex is about 15% B203. This produces an FID with a high initial
intensity and a rapid decay compared to organo-boron compounds in solution
at reasonable concentrations in the NMR tube. The result in the spectrum
-- when all other parameters are correct -- is much like that described by
Jiejun Wu. Essentially, there is a very broad component in the spectrum
which causes receiver overload and is itself folded at spectral windows of
less than about 100 kHz.
We purchased "boron free" probes for these systems and the problem
disappeared. Note: the user must also supply "boron free" NMR tubes or
some of the value of the probe is lost. Our current boron-free probe was
obtained from Nalorac. We have a quad-probe that is set up to run 1H or
19-F, 31-P, 11-B and 13-C. In order to have a low boron background, it was
necessary to use insert materials that have a strong signature at aluminium
frequencies. This was an acceptable trade-off in this application.
--Bob Santini
Robert E. Santini, Director of Instrumentation
Department of Chemistry, BRWN-1393
Purdue University
W. Lafayette, IN 47907-1393
E-mail santini@chem.purdue.edu
Voice: (765) 494-5227
Facsimile: (765) 494-9388
At 01:40 PM 5/22/98 -0800, you wrote:
>Dear All,
>
>I guess the B11 spectrum needed more first order phase correction than I
>expected. After I rephased the spectrum, it looks fine. The two main peaks
>and sidebands are all standing up, with a curly baseline plus a little bit
>"heavy-peak" syndrome. Thanks to Alan Olson, Steven Silber, Bob Sato, Tom
>Pratum, and Alan M Kenwright who replied to my question and pointed out two
>possibilities: too narrow a sweep width and wrong phase correction.
>
>I guess I was afraid of some sort of strange things shown up by the
>qudrapolar B11 nucleus in the solid state.
>
>Best Regards,
>
>Jiejun
>
>
>----------
>Jiejun Wu NMR Facility Manager
>Dept. of Chemistry (949)824-6010 (office)
>516 Physical Sciences 1 (949)824-5649 (lab)
>Univ. of California, Irvine (949)824-8571 (fax)
>Irvine, CA 92697-2025 jwu@uci.edu (internet)
Robert E. Santini, Director of Instrumentation
Department of Chemistry, BRWN-1393
Purdue University
W. Lafayette, IN 47907-1393
E-mail santini@chem.purdue.edu
Voice: (765) 494-5227
Facsimile: (765) 494-9388