[wencai]

Hi,

I am doing a set of DEPT experiments. All the other spectra look fine. But the DEPT90 has both positive and negative peaks with the same relaxation time and processing conditions. Anyone know why?

Thanks!

[Doc Oc]

Someone who knows more about NMR feel free to correct me if I'm wrong, but I believe a negative peak indicates a quaternary carbon.

[Honclbrif]

I thought in 90 only CH's show.

Possibly stupid question, but did you phase the spectrum?

[Dan]

Someone who knows more about NMR feel free to correct me if I'm wrong, but I believe a negative peak indicates a quaternary carbon.

No, a DEPT90 should only have CH peaks, and they should be positive (as Honclbrif says).

I think you're thinking of a DEPT135, which has CH and CH₃ positive, and CH₂ negative (quaternary are absent).

It sounds like a processing issue to me. I have come across it a couple of times in the past, and by fiddling about with the apodisation of the FID in MestreC I got a decent spectrum out of it. It was a very unsophisticated process, basically trial and error, so I can't give you instructions. At that time we got pdfs sent to us, which almost always looked fine, so the processing software attached to the machine was OK - it was my personal processing software that wasn't handling the raw data for some spectra very well.

Do you have dedicated NMR staff you can ask? Contact the manufacturer?

[hailhail]

NMR isn't my main area of study but I find some spectra I get from my compounds have minor peaks in the DEPT90.

Obviously it should only show the CH peaks, but I think concentration of the sample as well as other factors such as number of scans and spectrometer frequency can have an influence and cause the spectrometer to pick them up. For me it's not a huge issue as they're very minor compared to my CH peaks (and are identical chemical shift to CH3s and CH2s in my DEPT135, just way way diminished in intensity whereas DEPT135 shows them as ~1:1:1), but I suppose it depends what you need the spectrum for.