Re: Relaxing fluorine
G. Pearson (gpearson@blue.weeg.uiowa.edu)
Thu, 12 Oct 1995 08:00:11 -0500
> To: ammrl@bloch.cchem.berkeley.edu (ammrl nmr)
> Date: Tue, 10 Oct 1995 11:03:41 +0100 (BST)
> From: "J.W.Emsley" <J.W.Emsley@soton.ac.uk>
>
> I want to shorten the relaxation times for fluorine in C6F6. I have tried
> Chromium acac, but it is not very soluble and does not reduce T1
> sufficiently for me. Any suggestions for a more soluble relaxing agent for
> this sample? Jim Emsley
Chromium dpm should do the job very nicely. The dmp ligand is the same as the
acac ligand, except that the methyl groups have been replaced by t-butyl
groups. The chelate is essentially a magnetized BB inside of a ball of wax.
Extremely soluble in most organic solvents -- you can't even crystallize it
from most solvents, because you just get a tar!
BTW, as I pointed out in a letter to TAMU several years ago, Cr(dpm)3 is
closer to the ideal relaxation reagent than is Cr(acac)3. Although the
"shift-reagent" effects are small for both, the effects are smaller (I
measured a factor of 3 or 4) in the case of Cr(dpm)3. So why doesn't everyone
use Cr(dpm)3 instead of Cr(acac)? Tradition! "Everyone else has been using
Cr(acac)3, and everyone else knows more than I do, so Cr(acac)3 must be the
best thing to use."
The OTHER problem is that Cr(dpm)3 is NOT commercially available. No one
makes & sells it, because no one uses it. No one uses it because no one makes
& sells it. Catch 22. Both George Levy and I have tried unsuccessfully to
persuade companies to offer it for sale. At work, I've got a reference to an
article in INORGANIC SYNTHESIS several years ago, in which Jack Doyle
describes the straightforward goof-proof synthesis he worked out.
I'm tied up right now with jury duty (hopefully for less time than with the
O.J.Simpson trial!), but I'll try to dig out the reference when I get back to
work. I don't know all the details, but my memory says this:
1. Starting materials are commercially available.
2. The recipe for making Cr(acac)3 does not work without modification,
mainly because the dpm ligand is totally immiscible with water, and because it
is EXTREMELY soluble in lipophylic solvents.
3. I believe that Jack used methanol instead of water in the synthesis
steps, but my recollation could be faulty.
4. Add water to the mix after reaction, & the Cr(dpm)3 drops out of
solution. Vacuum filter, Buchner funnel.
5. First purification step is vacuum sublimation.
6. Final step is recrystallization from methanol. Just about any other
organic solvent will give you a tar, and Cr(dpm)3 is totally insoluble in
water.
Hope this helps.
-- Gerry
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Gerald A. Pearson INTERNET: gerald-pearson@uiowa.edu
Chem. Dept., Univ. of Iowa VOICE: 319-335-1336
Iowa City, IA 52242-1219, USA FAX: 319-335-1270
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