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 |
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. answered Jun 28 '11 at 00:23 Andy Roberts |
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). answered May 23 '11 at 05:48 sekhar Talluri |
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. answered May 20 '11 at 20:03 Evgeny Fadeev |
Hi Jonnala, I'd like to know if this problem was resolved. - nvgramosa (Oct 08 '11 at 09:08)