This is the feedback to the mail I sent out over a week ago about our
difficulty in obtaining 13C spectra in deuterated methanol.
I finally was able to do quite a number of experiments yesterday, and I
think I was able to figure out what was wrong. It seems that our
decoupler irradiation frequency is set by default to the middle of the 1H
frequency range. (The default decoupling is called WAUGH, which is a
multiple pulse decoupling sequence).
I tried a "sample" of regular acetone dissolved in deuterated methanol.
With the default setting, the acetone methyl peak (singlet) is 1/15 the
height of the tallest of the methanol peaks (septet). The carbonyl carbon
of acetone, BTW, is buried in the noise.
Now, when I set the decoupler to irradiate the acetone 1H peak (which is
at around 2.1 ppm), I get a much stronger acetone peak. The methyl is now
almost half the height of the methanol peaks, and the acetone carbonyl is
distinguishable from the noise. I attribute the enhancement to
polarization transfer (and NOE for the carbonyl).
I also did a comparison with CW (selective) decoupling, full 1H power on
the acetone peak, and I did get some enhancement, but I did not get any
better results than the one above.
The one thing I can't explain is why I have to do this for deut. methanol,
and not for the other solvents like deut. benzene, CDCl3, etc.
I'll try to do some more experiments, this time changing pulse widths on
the decoupler, and I'll report any significant results that I get.
Thanks to all those who replied,
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Edward T. Chainani Nuclear Magnetic Resonance Laboratory Manager
National Chemistry Instrumentation Center (NCIC)
Ateneo de Manila University phone: (632) 924-4601 loc. 2309
Loyola Heights, Quezon City e-mail: etch@ginto.chem.admu.edu.ph
Philippines
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