Jim;
It brings back memories of a dissertation from long ago. Boron that is
3-coordinate has T2’s that make it almost impossible to get a sharp signal.
4-coordinate boron compounds have very sharp, well defined resonances. If
a chemical exchange process is going on then no signal. Cool the sample
down 20-30 C and try to acquire a signal. Try running at 300 MHz to also
avoid Quadruple moment issues.
Too high a field and too small a sample filling factor (use 10 or 15mm
probes).
Alan Kook, PhD
NMR Applications Manager
Austin AI
_____
> From: James.L.Simpson_at_invista.com [mailto:James.L.Simpson_at_invista.com]
> Sent: Friday, March 04, 2005 8:55 AM
> To: ammrl_at_ammrl.org
> Subject: Boron NMR Problems
Greetings all,
I'm having a little trouble with 11B NMR. We have a Varian INOVA 500 system
and I'm currently using a 10mm BB probe. Our sample of interest has low
levels of Boron (1-50ppm per species, using 1ml of Benzene-d6/3ml of
sample). I have been tuning the probe for each sample as it seems the
sample-to-sample variation is quite significant. The Boron species in the
sample are sensitive to the matrix (we do know that benzene does not affect
them). So, we've been using an external reference of 1 v% BF3(Etherate) in
benzene in a 5mm insert. We used the 1 v% BF3(etherate) standard to optimize
the pulse width for a full 90deg pulse (our samples are too dilute to do any
pw90 experiments). We are trying to optimize acquisition time (at) and the
delay (d1). Currently we've been varying the at between 0.05 and 0.2 seconds
with a delay anywhere from 0-0.3 seconds. We are decoupling proton, but
we've been unable to run a gamah2 on these samples.
Our results are hit and miss, resolution pretty low. We've tried running the
sample w/o dilution, but we sacrifice lock. I'm not sure what else to try. I
even tried giving the magnet a Valentine's Day gift, but that didn't seem to
help either. Do our above parameters seem to be in the ballpark for 11B NMR?
Does anyone have any tips for running 11B NMR and/or experience with low
levels of Boron? Any help would be greatly appreciated.
Jim Simpson
Invista
Orange, TX
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Received on Fri Mar 04 2005 - 18:36:48 MST