From time to time I get in the 1H-NMR spectrum three peaks around 7 ppm (see attachements). It happens for different compounds. Do You know what they are from? Is it some DMSO impurity?
Thanks in advance.
Really I don't know why it is happening. But I'm sure it is not impurity of DMSO. Because DMSO solvent peaks appears at nearly 39.5 ppm about 4- 6 peaks.
I've done all measurements in DMSO. I wonder if that peaks come from impurities present in the solvent...
Indeed, this is due to 1H spin coupling to 14N (S = 1) in NH4+. The spin coupling constant should be about 53 Hz. If NH4 is not a part of your compound, look for the source of some kind of hydrolysis to give ammonium ion (May be some moisture in your solvent is responsible)
The peak labels in your spectra show the lines to be 0.1 ppm apart. You don't give the field strength of your spectrometer. If the impurity is due to NH4+, your B0 must be 11.7 T, i. e. 500 MHz spectrometer (for 1H). Is this the case??
I agree with the previous answers. One usually does not see coupling to 14N except where the quadrupolar nitrogen nucleus is in a symmetrical electrical environment. I saw an amusing case many years ago in the proton spectrum (at 1.4T!) of Al(BH4)3 where the spectrum was a single line 600 Hz wide; the apparently more symmetrical complex with Et3N showed proton coupling to both boron and aluminum nuclei!
Thanks for the additional education Roy! That is an interesting clarification.
Like other people said, this is typical for the signal of NH4+ group due to coupling with 14N nucleus (similar to 13C signal in CDCl3). In DMSO it is positioned around 7.0-7.3 ppm.
Was wondering if this unusual coupling is has a temperature dependent component? Some have called a high field NMR a very very expensive pH meter. Could the appearance / disappearance of such a spectral line be a highly precise temperature marker for a phase or gel transition? Am most interested only secondarily in properly characterizing individual phase solid/liquid phase, but as their spectral signature is not identical, collecting data DURING a phase or gel transition to document the change. The steady state condition is where an equal amount of two phases are present and at a temperature in which NMR peaks typically broaden. Much easier to do and/or interpret when only a single vibrational mode is the marker of such a phase or gel transition.
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