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.