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Friday, November 23, 2007

31P Decoupling

All of you routinely use proton decoupling in your 31P NMR spectra, many of you without giving it a thought. Few people realize that it is a simple matter to decouple 31P in your 1H NMR spectra. This will allow you to determine which protons are coupled to phosphorus. An example of this is shown below. The lower trace is a standard 1H spectrum and the upper trace is a 1H spectrum with 31P decoupling.


7 comments:

Jim Lin said...

Hi! Glenn: I did a 31P NMR of a small molecule that contained two 31P, the two 31p are separated by CH2. I observed both 31p peaks as doublet, with J= ~5 Hz. Since I have decoupled proton, I suspected this is caused by 31P-31p coupling, how to prove it?

Glenn Facey said...

Jim Lin.

If the two 31P resonances are of the same chemical shift then you would observe a singlet and not see the coupling between them. If the two 31P resonances are of different chemical shift and coupled to one another, you would expect to see two doublets separated by the chemical shift difference each with splitting 2JP-P. If 2JP-P is very small, your spectrum may appear to be two lines separated by the the chemical shift difference. Run your sample on a spectrometer with a different field strength. If the separation between the peaks (in Hz) increases or decreases with the field then the separation between the resonances is due to a chemical shift difference. If the separation between the peaks (in Hz) remains constant at a different field strength, then the separation between the resonances is due to coupling.

Glenn

Jim Lin said...

Hi! Glenn:

Thanks. The two 31P have different chemical shift (~20 ppm). Unfortunately, I have 500, and 600 MHz instrument available but, only 500 MHz instrument is equipped with BBO probe. 600 MHz is eupipped with TCI probe only. Therefore, I can not try this method. Someone suggested that I try 31P-31P COSY to see if there is coupling between two 31P. What do you think. Thanks in advance.

Glenn Facey said...

Jim Lin,

a 31P-31P COSY should give you the answer.

Glenn

Jim Lin said...

Hi! Glenn:

It works, the 31P-31P COSY shows the two 31P do coupled. Thanks.

Jim

Tiago said...

Hi Glenn,

In the coupled spectrum, should I expect to see a doublet in the lower ppm signal? I don't understand why is a triplet.

Thank you,
Tiago

Glenn Facey said...

Tiago,
I believe one would expect a second order spectrum which in this case fortuitously appears as a triplet.
http://u-of-o-nmr-facility.blogspot.ca/search?q=Virtual

Glenn