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Hello all!
I'm sorry to not have been able to respond to individual responses-
there were a lot of really good ones! Briefly, I was able to resolve
this issue by ensuring that the 1H channel was tuned and matched. When
done, I saw the same loss in relative signal-to-noise (~1/10th) in
both 1H and 13C NMR experiments in d6-DMSO compared to CDCl3. The
logical explanation being that the 1H decoupling was not done
efficiently (or at all in this case due to the 1H channel being out of
tune). I don't think this quite well explains the fact that we saw the
carbonyl carbons in ibuprofen, but none of the aliphatic ones. Perhaps
the relaxation effect from the solvent is only seen on the latter
carbons ...? When both channels are properly tuned though, both the
routine 13C and DEPT-135 are perfect- albeit with about the 1/10 hit
in sensitivity- the solvent effect makes sense here. I haven't had the
chance yet to (re)calibrate the 13C 90s in DMSO, so that may bring the
sensitivity back to something more respectable, but current values do
work reasonably well.
Thanks to the following in no particular order: Raju, Bob, Charlie,
Chris, Tim, John T, Russ, Craig, Greg, Dave, John R and WuuYung!
Here are some of the responses- since many expressed a desire to see
what I received. My responses if appropriate are in bold.
~*~*~*~*~*~*~*~*~*~*~*~*
C13 relaxation times in DMSO can be pretty long.
~*~*~*~*~*~*~*~*~*~*~*~*
Dare I ask? Is ibuprofen readily soluable in DMSO? My guess would be
"probably not". Yes, it has a carboxylic acid group; but there is an
awful lot hydrocarbon around. You did note a drop in 1H sensitivity.
I would suggest looking carefully at the NMR tube.
At 25 mg/mL it apparently/fortunately is.
~*~*~*~*~*~*~*~*~*~*~*~*
There is no reason why this should be happening. We use DMSO-d6
regularly, and as expected, it works as other NMR solvents do
(although DMSO-d6 can be considerably better when trying to observe
protons such as -OH). You are not dropping the temp a bit, to the
point where the solvent is freezing (happens close to room temp, 19C,
for DMSO)? We've done that when not thinking clearly a couple times.
Or perhaps the analyte is not soluble (I haven't tried ibuprofen)?
The experimental temperature is at 300 K- should be enough the MP of
d6-DMSO to not be a problem. This assumes the MP for d6-DMSO is close
to that of H6-DMSO!
~*~*~*~*~*~*~*~*~*~*~*~*
Is the proton channel tuned as well? You said the 13C channel was
tuned and matched. In one case, assuming composite pulse decouple,
the 1H decoupling would be poor and in the other (dept) the proton
pulse widths would not be integral (and 1H decoupling would be poor).
never had a problem when proton channel was tuned.
This is the one that made me check the 1H channel tuning- thanks!
It may also be that the sample is nervous in dmso which is flammable
after all and so may burn in the wild fires more readily than either
cdcl3 or d2o. just a thought.
We did have problems with the fires here, but not that bad! ;-{)
~*~*~*~*~*~*~*~*~*~*~*~*
This sounds very odd to me and I have never obeserved anything as
dramatic as this with DMSO.
Are you using automation?- could the instrument be loading in
incorrect pulse calibrations (durations, power levels) for 13C/DMSO
from a look-up table (eg prosol for Bruker- you don't mention
instrumentation used)? Can you check these in the finished experiment
to be sure the pulses & powers for 13C are correct as it sounds like
they are missing to me...
No automation and all pulse parameters were fine- at least for CDCl3
since it's used most commonly.
My instinct is that this is an instrument effect and not a direct
solvent effect.
~*~*~*~*~*~*~*~*~*~*~*~*
Sorry if you considered this too obvious to mention but did you
recalibrate the pw90s? Even with tuning and matching I think the 90s
could be substantially different in d6-DMSO. I would think the
viscosity would increase s/n by increasing T1. Anyway, I'd be
interested to hear what you find out.
No please do ask such questions as it is many times the obvious we
fail to see!
~*~*~*~*~*~*~*~*~*~*~*~*
Perhaps the dissolved oxygen concentration is dramatically higher in
CDCl3 than DMSO-d6? Most people only use DMSO-d6 if they cannot
dissolve their compounds in a more volatile solvent or possibly to
afford less signal overlap, but in the former case, a comparison with
a CDCl3 would not be possible.
~*~*~*~*~*~*~*~*~*~*~*~*
We had exactly the same problem yesterday (I'd never seen it before
either) - i.e. acceptable 1H spectr in CDCl3 and DMSO, but very weak
13C when run in DMSO compared to CDCl3, however in our case it is
complicated by dynamic broadening in both solvents (13C line widths
are twice as broad in DMSO, so it may exarcerbated by that) - and it
was a sample prepared and run by a student, so I am always a little
dubious of sample composition!
~*~*~*~*~*~*~*~*~*~*~*~*
In general, you should be able to obtain lovely 13C spectra also in
DMSO. I strongly suspect that your 13C and/or proton decoupling pulses
are wrong. They could be significantly different in DMSO.
Recalibration will be necessary.
~*~*~*~*~*~*~*~*~*~*~*~*
I haven't noticed disappearing 13C signals, but a customer/company
always requested their 13C/1H spectra in DMSO. Mostly because they
didn't want to check solubility in various solvents. They've since had
me rerun a dozen of the proton in CDCl3 because the spectra were
dramatically different.
~*~*~*~*~*~*~*~*~*~*~*~*
I can't imagine what is going on here unless your compound is
insoluble in DMSO (very unlikely!). We run in DMSO all the time and it
is a wonderful solvent -- its viscosity helps with reducing the
relaxation delay required so it is usually EASIER to get better S/N
carbons. Let us know if you figure out what is going on!!
~*~*~*~*~*~*~*~*~*~*~*~*
Is it possible that micro hard-core micelles (which might be
transparent) were formed and the signals got flattened, i.e. not
visible? My point is that you were basically running a solid sample
using liquid-state NMR techniques.
I think/hope this is not the case!
~*~*~*~*~*~*~*~*~*~*~*~*
Paul Shin, Ph.D.
Department of Chemistry and Biochemistry, CSUN
California State University
18111 Nordhoff St
Northridge, CA 91330
818-677-6887 phone
818-677-4068 fax
www.csun.edu/chemistry/Facilities.html
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Hello all!
I'm sorry to not have been =
able to respond to individual responses- there were a lot of really good =
ones! Briefly, I was able to resolve this issue by ensuring that the 1H =
channel was tuned and matched. When done, I saw the same loss in =
relative signal-to-noise (~1/10th) in both 1H and 13C NMR experiments in =
d6-DMSO compared to CDCl3. The logical explanation being that the 1H =
decoupling was not done efficiently (or at all in this case due to the =
1H channel being out of tune). I don't think this quite well explains =
the fact that we saw the carbonyl carbons in ibuprofen, but none of the =
aliphatic ones. Perhaps the relaxation effect from the solvent is only =
seen on the latter carbons ...? When both channels are properly tuned =
though, both the routine 13C and DEPT-135 are perfect- albeit with about =
the 1/10 hit in sensitivity- the solvent effect makes sense here. I =
haven't had the chance yet to (re)calibrate the 13C 90s in DMSO, so that =
may bring the sensitivity back to something more respectable, but =
current values do work reasonably well.
Thanks to the following in =
no particular order: Raju, Bob, Charlie, Chris, Tim, John T, Russ, =
Craig, Greg, Dave, John R and WuuYung!
Here are some of the =
responses- since many expressed a desire to see what I received. My =
responses if appropriate are in bold.
~*~*~*~*~*~*~*~*~*~*~*~*C13 relaxation times in DMSO =
can be pretty =
long.~*~*~*~*~*~*~*~*~*~*~*~*Dare I =
ask? Is ibuprofen readily soluable in DMSO? My guess would =
be "probably not". Yes, it has a carboxylic acid group; but there =
is an awful lot hydrocarbon around. You did note a drop in 1H =
sensitivity. I would suggest looking carefully at the NMR =
tube.At 25 mg/mL it apparently/fortunately =
is.~*~*~*~*~*~*~*~*~*~*~*~*There is no reason why this should be happening. We use =
DMSO-d6 regularly, and as expected, it works as other NMR solvents do =
(although DMSO-d6 can be considerably better when trying to observe =
protons such as -OH). You are not dropping the temp a bit, to the =
point where the solvent is freezing (happens close to room temp, 19C, =
for DMSO)? We've done that when not thinking clearly a couple =
times. Or perhaps the analyte is not soluble (I haven't tried =
ibuprofen)?
The experimental temperature is at 300 =
K- should be enough the MP of d6-DMSO to not be a problem. This assumes =
the MP for d6-DMSO is close to that of =
H6-DMSO!~*~*~*~*~*~*~*~*~*~*~*~*Is the proton =
channel tuned as well? You said the 13C channel was tuned and =
matched. In one case, assuming composite pulse decouple, the 1H =
decoupling would be poor and in the other (dept) the proton pulse widths =
would not be integral (and 1H decoupling would be poor). never had =
a problem when proton channel was tuned.This is the one that =
made me check the 1H channel tuning- thanks!It may =
also be that the sample is nervous in dmso which is flammable after all =
and so may burn in the wild fires more readily than either cdcl3 or =
d2o. just a thought.We did have problems with =
the fires here, but not that bad! =
;-{)~*~*~*~*~*~*~*~*~*~*~*~*This sounds very odd to me and I have never obeserved =
anything as dramatic as this with DMSO. Are you using automation?- could the instrument be =
loading in incorrect pulse calibrations (durations, power levels) for =
13C/DMSO from a look-up table (eg prosol for Bruker- you don't mention =
instrumentation used)? Can you check these in the finished experiment to =
be sure the pulses & powers for 13C are correct as it sounds =
like they are missing to me...No automation and =
all pulse parameters were fine- at least for CDCl3 since it's used most =
commonly.My instinct =
is that this is an instrument effect and not a direct solvent =
effect.~*~*~*~*~*~*~*~*~*~*~*~* =
Sorry if you considered this too obvious to mention but did you =
recalibrate the pw90s? Even with tuning and matching I think the =
90s could be substantially different in d6-DMSO. I would think the =
viscosity would increase s/n by increasing T1. Anyway, I'd be =
interested to hear what you find out.
No please do ask =
such questions as it is many times the obvious we fail to =
see!~*~*~*~*~*~*~*~*~*~*~*~*Perhaps the dissolved oxygen concentration is =
dramatically higher in CDCl3 than DMSO-d6? Most people only use =
DMSO-d6 if they cannot dissolve their compounds in a more volatile =
solvent or possibly to afford less signal overlap, but in the former =
case, a comparison with a CDCl3 would not be =
possible.~*~*~*~*~*~*~*~*~*~*~*~*We had exactly the same problem yesterday (I'd never seen it =
before either) - i.e. acceptable 1H spectr in CDCl3 and DMSO, but very =
weak 13C when run in DMSO compared to CDCl3, however in our case it is =
complicated by dynamic broadening in both solvents (13C line widths are =
twice as broad in DMSO, so it may exarcerbated by that) - and it was a =
sample prepared and run by a student, so I am always a little dubious of =
sample =
composition!
~*~*~*~*~*~*~*~*~*~*~*~*In =
general, you should be able to obtain lovely 13C spectra also in DMSO. I =
strongly suspect that your 13C and/or proton decoupling pulses are =
wrong. They could be significantly different in DMSO. Recalibration will =
be =
necessary.
~*~*~*~*~*~*~*~*~*~*~*~*=
I can't imagine what is going on here unless your compound is =
insoluble in DMSO (very unlikely!). We run in DMSO all the time and it =
is a wonderful solvent -- its viscosity helps with reducing the =
relaxation delay required so it is usually EASIER to get better S/N =
carbons. Let us know if you figure out what is going =
on!!~*~*~*~*~*~*~*~*~*~*~*~* Paul Shin, =
Ph.D.Department of Chemistry and Biochemistry, =
CSUNCalifornia State =
University18111 Nordhoff StNorthridge, CA 91330818-677-6887 =
phone818-677-4068 faxwww.csun.edu/chemis=
try/Facilities.html
=
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