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Dear sirs and madams, I have a natural product whose two carbon resoances are appearing at 46 and 56 ppm but theirs proton resoanances are appearing at 0.80 and 0.83 ppm respectively. I am unable to understand how there could be a downfield carbon (oxygenated carbon) is paird up with a upfield proton. I request your expert answer. Thanks in advance.

asked May 20 '11 at 04:01

Jonnala%20Kotesh%20Kumar's gravatar image

Jonnala Kotesh Kumar
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updated May 23 '11 at 15:05

Evgeny%20Fadeev's gravatar image

Evgeny Fadeev
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Hi Jonnala, I'd like to know if this problem was resolved. - nvgramosa (Oct 08 '11 at 09:08)


4 Answers:
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Hi Jonnala,

Very likely you have spectrum "folding" or maybe incorrect processing of carbon dimension - like a wrong setting of quadrature detection method.

Folding happens when spectral window is narrower than the range of the resonance frequencies.

link

answered May 20 '11 at 20:03

Evgeny%20Fadeev's gravatar image

Evgeny Fadeev
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Thats not so rare situation that chemical shifts of carbons and protons connected to them are uncorrelated and can also "move" into different directions.

link

answered May 23 '11 at 00:54

Vahur's gravatar image

Vahur
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Spatial proximity to aromatic groups or other groups with large anisotropic shifts may give rise to such effects in small but rigid molecules. The H may be positioned in the shielding zone and the attached C may be positioned in the deshielding zone of an aromatic/anisotropic group.

Try to search a database of chemical shifts to see if such a pairing has been observed before. Are you sure that you are observing a H-C pair and not a H-(C-)C pair? Also, check the referencing of the carbon dimension (as suggested by Evgeny above).

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answered May 23 '11 at 05:48

sekhar%20Talluri's gravatar image

sekhar Talluri
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That type of chemical shift pairing is very commonly seen in steroid and similar ring system structures. If you have a look at the spectrum of cholesterol for example (available on the net) you'll see some examples. As this is a natural product there's a good chance it may be some sort of cyclic triterpenoid or similar.

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answered Jun 28 '11 at 00:23

Andy%20Roberts's gravatar image

Andy Roberts
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