Dear NMRists
In my very new function as an NMR manager, I was confronted with a strange
sample behavior that I cannot completely explain, but perhaps has been
observed elsewhere in the NMR community. The observation is described in the
paper
Title: Total
<
http://apps.isiknowledge.com/full_record.do?product=UA&search_mode=GeneralS
earch&qid=1&SID=X1eg6Df9c_at_BD1aACbOo&page=1&doc=2&colname=MEDLINE> synthesis
of spirastrellolide f methyl ester-part 2: macrocyclization and completion
of the synthesis.
Benson, Stefan; Collin, Marie-Pierre; O'Neil, Gregory W, et al.
Angew Chem Int Ed Engl Volume: 48 Issue: 52 Pages: 9946-50
Published: 2009.
In brief, this retro-synthesized natural compound, dissolved in benzene,
shows slow shifts and broadenings of all the -OH and -OMe peaks in the 1H
spectrum over the course of 1-2 days. See figure 3
(
http://www3.interscience.wiley.com/cgi-bin/fulltext/123206963/mfig003 ).
The authors explain this phenomenon as "the compound reorganizes an
intricate hydrogen bonding network", and also involves trace amounts of
water. They believe that a slow structural reorganization of the compound is
taking place and may be functionally relevant.
I tend to believe that this is simply some form of instability in the
conditions of the sample. All the measurements were done before I was hired
here, but as far as I know, the sample was dissolved in benzene, with no
particular effort to dry it. Water may also have been dragged in by the
compound itself. The sample was in a closed but unsealed 3-mm tube and
measurements were at room temperature. The phenomenon is very reproducible.
The sample concentration is very low (1H spectrum took 20min each on a
cryoprobe).
So my questions for all the experts out there are:
-is it possible that the bezene is evaporating out of/water is condensing
into the tube? what could cause something like a pH changes in benzene?
-what would explain the strange broadening?
-what simple experiments could I do to test your hypothesis? (The amount of
sample is extremely low).
thanks for your help
Christophe Fares
NMR Director
Max-PIank-Institut für Kohlenforshung
Received on Thu Jan 14 2010 - 07:17:23 MST