I have run a C13 dept 90 and dept 135 NMR on my sample (which is a modified water soluble polymer). The dept 90 shows a group of positive peaks which, as it is a dept 90, should indicate CH groups. However the dept 135 spectrum shows the same peaks as negative, meaning they are CH3 groups. Which is correct
Does anyone have any idea what is happening. Is this a regular occurrence in C13 NMR? The spectra have been repeated on a different spectrometer and the same pattern is seen.
The sample is about 8 wt% in D20 and the spectrometer is at 500 MHz. Any suggestions would be great.