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Thursday, September 20, 2007

Signal Intensity (or Missing Signals) in DEPT Spectra

I often get asked by students: Why does my terminal alkyne carbon not show up in my DEPT spectrum? The answer lies in the DEPT pulse sequence which uses a delay based on a multiple of the reciprocal of the average one bond carbon-proton coupling constant. Carbon-proton coupling constants span more than 100 Hz with an average of 145 Hz. The DEPT sequence you use is optimized for this average coupling constant. Carbon signals with couplings significantly different than 145 Hz will be attenuated in the spectrum.

In the lower trace of the above figure is a simulated DEPT 135 spectrum of the indicated fictitious molecule with the one bond carbon-proton couplings set to typical values as follows:
1 250 Hz
3 125 Hz
4 125 Hz
5 125 Hz
6 118 Hz
Note that the terminal alkyne signal is very small. In a real spectrum, this signal could easily be lost in the noise. The upper trace is a similar simulated spectrum where all of the couplings were set to 145 Hz (note the gain in signal strength for the terminal alkyne resonance). Take note that the silyl methyl signals suffer less from this effect than does the terminal alkyne as its coupling constant is much closer to 145 Hz than that of the terminal alkyne.
Take-home message - Expect attenuation in carbon signals in a DEPT spectrum if the one bond carbon-proton coupling constant differs significantly from 145 Hz.

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