The top spectrum is a simple 1D 13C spectrum recorded with decoupling throughout the inter-scan relaxation delay to increase signal intensity via the nuclear Overhauser effect. A 30o pulse and reduced relaxation delay (0.5s) was used to speed acquisition.
The middle three spectra are the three types of DEPT (Distortionless Enhancement by Polarization Transfer) experiment. The DEPT 45 shows positive signals from all protonated carbons, the DEPT 90 shows only positive signals from CH carbons, and the DEPT 135 shows positive signals from CH and CH3 carbons and negative signals from CH2 carbons.
The bottom spectrum is an APT (Attached Proton Test) experiment which, like the DEPT 135, shows positive signals from CH and CH3 carbons and negative signals from CH2 carbons, but also includes negative signals from quaternary carbons.
The 13C spectrum has the lowest signal to noise and the worst baseline of all the spectra. No attempt was made to correct the baseline in any of these spectra, and in fact the 13C baseline can be easily corrected during processing, but presenting it this way shows the difference between the experiments.
The DEPT spectra have the greatest sensitivity but took the longest to acquire, as a 5 second relaxation delay was used. One reason for doing this is so that the signal intensities are not reduced by incomplete relaxation between scans. Reduced signal intensity makes cancellation of signals less effective when adding and subtracting different combinations of the three DEPT spectra to obtain individual spectra of just the CH, CH2 and CH3 carbons.
The APT spectrum is not as sensitive as the DEPT spectra but it shows the quaternary carbons (170,140 and 43 ppm). It is more sensitive than the 13C experiment, gives a flatter baseline, and adds discrimination of the different types of carbons, but it takes longer. The durations of all the experiments are listed in the table below.
| Experiment | Relaxation delay | Duration |
|---|---|---|
| C13 | 0.5 sec | 3 min 14 sec |
| DEPT 45 | 5.0 sec | 13 min 0 sec |
| DEPT 90 | 5.0 sec | 13 min 0 sec |
| DEPT 135 | 5.0 sec | 13 min 0 sec |
| APT | 2.0 sec | 6 min 36 sec |
In cases where you have a lot of sample and only need a quick and dirty spectrum, to simply count carbons for example, the 13C experiment will give you what you need in the least amount of time. In general, however, I recommend using the APT experiment.
Again, Brendan, always informative, always professionally done. I've always preferred the DEPT experiment (devised, btw, by fellow Australians Doderell and Pegg) for a couple of reasons. a) S/N - you can't get enough of this when recording 13C NMR. If you know you have no Cq's why not just record DEPT? (rhetorical question) b) Solvent signal elimination. Notice both 13C NMR and APT experiments show the residual 1:1:1 CDCl3 solvent signal, but DEPT does not (no protonated C in deuteriochloroform!). This can be very useful if you have a small sample (nanomole, anyone?) and suspect a C signal is hiding' under your solvent peaks. DEPT will reveal it (as long as it's not Cq). Finally, I'll always cut corners to get as many scans as possible into my time. Reducing D1 (the interscan delay) to 1s instead of 5T1's will still get you pretty good DEPT spectra but in much less time. QUESTION. Does introducing some dummy scans (4-8) in DEPT with short D1 help to avoid some of the 'bleed through'?
ReplyDeleteThanks for your comments Ted. As always they're very helpful!
ReplyDeleteAs you said DEPT is more sensitive than APT (for 13C about 4x versus 3x, compared to direct detection without NOE) but the lack of quaternary signals means its a non-starter for most of our users running 1D 13C spectra.
Reducing the interscan delay to increase sensitivity per unit time is certainly worth trying. I used the Bruker recommended 5 second value here but normally would use something much less. Your question about dummy scans compensating for reduced D1 may be the subject of my next post!