I synthesized some 1,2,4-triazoles and the two quaternary carbons of the 1,2,4-triazole ring were appeared as very low intensity (even broad) peaks in 13C NMR spectra. This observation is with the 50 mg of sample (completely soluble in DMSO-D6) with 10000 scans on 400 MHz NMR instrument. Noted that no peak was observed properly upto 1024-2048 scans. Please suggest how to defend the synthesis with the help of this 13C NMR data. Please also share some references of the same if you encounter with any from the literature.
I don’t think solubility is a factor.
It turns out in some circumstances the N-14 relaxation rate can be long compared to the to the C-N J coupling constant. Since the N-14‘s spin is 1, the C-13 in a C-N bond could be a 1:1:1 amplitude triplet (just like the C-D coupling observed for CDCL3).
When the N-14 relaxation rate is >>> than the C-N J coupling constant, the J-coupling averages to zero and the C-13 peak line-width is normal (narrow).
As the N-14 relaxation rate become faster, there is a narrow range of N-14 relaxation raters where the C-N J-coupling is unresolved because the C-13 line width becomes similar to the C-N J-coupling constant. In this case, the C-13 in a C-N bond appears to be broad.
The physics is nearly identical to the effects of chemical exchange of on resonance line with.
Decades ago Richard Ernst published a paper describing the effects of relaxation on J-Coupling. Unfortunately I no longer have that paper in my data base.
If the nitrogen attached carbon is a quaternary carbon, the intensity of its peak will be lower than expected. I suggest you review the nuclear overhauser effect (NOE).
In order to observe these peaks, you need to extend the analysis time.
- Badges
- Science topic
- Similar topics
- Methodology
- Laboratory