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Thursday, November 8, 2007

Why Does My Quaternary Alkyne Carbon Show Up in My 13C DEPT Spectrum?

The BLOG entry for September 20, 2007 illustrated how some signals from protonated carbons can be either missing or of much reduced intensity in 13C DEPT spectra. This is due to unusual one-bond 13C-1H coupling constants. There are also cases where quaternary carbon signals will show up in 13C DEPT spectra due to unusually large two-bond 13C-1H coupling constants. Such is the case for the alkyne in the figure below. A standard 13C DEPT-135 sequence optomized for a 13C-1H coupling of 145 Hz (the average one-bond 13C-1H coupling constant) was acquired and compared to the standard 13C spectrum with 1H decoupling. One can see that the protonated alkyne carbon has less intensity than expected as its one-bond 13C-1H coupling constant is ~ 250 Hz. Also the quaternary alkyne carbon is present in the spectrum as its 2-bond 13C-1H coupling constant is ~ 50 Hz. Although neither of these couplings is close to 145 Hz, they are both of a magnitude where one would expect to see a signal in the DEPT 135 spectrum.

5 comments:

crash012 said...

can you please provide us with other examples in which a Quaternary carbon apperas on the DEPT135 spectra on the positive phase and if it possible a more detailed explanation on the reason behind it.

Glenn Facey said...

Dear crash012,

DEPT experiments include a delay related to the reciprocal of the 1-bond carbon/proton coupling constant. When a quaternary carbon exhibits a large 2-bond carbon/proton coupling constant, it will appear in a DEPT spectrum. The alkynes have large 2-bond carbon/proton couplings.

Glenn

Anonymous said...

Thanks Glenn - this was very helpful. My group is making compounds derived from 4-pentyn-1-ol and we also see the quaternary carbon show up in our DEPT135 spectra. This clears up a lot of confusion for us.

Anonymous said...

Hi, are there other examples for substructures with large 2-bond carbon/proton coupling constant in addition to the alkynes?

Glenn Facey said...

Anonymous,
I don't know of any off-hand. You should consult tables of 2JCH coupling constants.
Glenn