Thank you to everyone who responded to my question about the NMR of
boron containing compounds. I have read a lot of great information and
have learned quite a bit. Following is a summary are the responses I
received.
Thank you again for your assistance, it is truly appreciated.
Adrienne
****
Boron 11 (spin 3/2) is ~ 80% abundant, and boron10 is ~ 20% (spin 3).
If I remember correctly (it's been a few years),
the B11 attached protons can give sharp lines, but the B10 attached
protons show up as broad shoulders on the B11attached proton
resonances.
****
The answer is, it depends. But in borohydride 1H shifts upfield
(towards TMS), and since there are two boron nuclides, you do see an
isotope shift. If the boron is at a site of high symmetry, you will see
nice narrow 1H lines with biggish couplings to 11B and 10B. If the
boron is low symmetry, you see very broad 1H lines, because of
quadrupolar relaxation to the boron that decouples the boron from the 1H
but not too efficiently. See JACS 115, 11084 (1993).
****
Greg Fu at MIT has done a lot of boron chemistry, so just about any
search of the literature with "Fu" and "boron" should generate a number
of useful hits.
****
Boron will tend to shift things to high field - BH4 is up around -.5 ppm
I believe.
Also, Boron can give some very distinctive coupling patterns. Between
the 20/80 ratio of the two isotopes, the spins of 3 and 3/2, and
generally a significant isotope shift, you can get patterns that are
very indicative of boron coupling.
BF4 in particular has a very pretty F-19 spectrum.
****
Have your colleague look up "Steve Strauss" from Colorado State
University. It may be "Stephen" and it may include the middle initial
"H", etc. but you get the idea. He has published many articles in this
area over perhaps a twenty year period and that should provide leading
references.
****
Boron can cause rather large splittings due to J-couplings. There are
two major isotopes of boron, both of which are NMR active. 11B is a
spin 3/2 so it will split every 1H into 4 lines. It makes up about 80%
of the boron. 10B is a spin 3 and makes up the other 20%. It will split
the 1H signal into 7 lines. So many times you will see a strong signal
split into 4 lines with a weaker set of signals split into 7 lines.
****
Have you search archives concerning borohydrides? There should be a
wealth of 1H NMR info there.
****
Let me call your attention to a review on B-11 NMR published in
Chem.Rev. 1992, 92, 325-362 by S.Hermanek.
****
My original e-mail:
________________________________
From: Hoeglund, Adrienne [mailto:AHoeglund_at_aoc-resins.com]
Sent: Friday, October 21, 2005 8:18 AM
To: AMMRL
Subject: NMR of Boron Containing Compounds
Hello to everyone. I tried searching the archives and did not find what
I was looking for, so I was wondering if you all might help me out.
I have a colleague who wants to run proton NMR of boron containing
compounds. He came to me and asked me how the boron would effect the
proton shifts. I was not able to give him an answer that I was happy
with, so I thought I would turn to your expertise for a better answer.
So here is the question: How does Boron effect the proton shifts? Are
their any good journal articles with spectra containing boron containing
compounds?
Thanks much,
Adrienne
Adrienne B. Hoeglund
NMR Specialist
AOC, LLC.
950 Highway 57 East
Collierville, TN 38017
(901) 854-2800 ext. 532 - work
(901) 853-1582 - fax
www.aoc-resins.com
Received on Mon Oct 31 2005 - 11:55:49 MST