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Sticking out like a Trimethyl Ammonio Group

When dealing with a structure elucidation problem where little or no previous fragment information exists, the number of possibilities can seem endless. Although typically one envisions a neutral compound for the unknown, ionic compounds, such as salts and zwitterions, are also just as likely and thus can add to the complex nature of an elucidation.

The trimethyl ammonio group shown below exhibits an intense 1H signal generally between 2.8 – 3.2 ppm and a 13C signal generally between 39 – 62 ppm.

RNCH33+_Str_Apr

The 1H NMR spectrum below exhibits an obvious singlet circa 3.1 ppm belonging to an R-N(CH3)3+ group. Compare the difference in chemical shifts to a t-butyl group listed here.

RNCH33+_Spec_Apr

3 Replies to “Sticking out like a Trimethyl Ammonio Group”

  1. I am getting a singlet around 8.4 ppm for my compound. This compound was purified by Prep HPLC using Amm. Carboanate+CH3CN+H2O. Please tell me whether this peak could correspond to ammonium ions..Please tell me how to check this peak corresponds to ammonium ion? Thanks..

  2. Hello Marto,
    If a 1H signal is attributed to an exchangeable proton, then adding some D2O to the sample will cause the signal to drop in intensity when collecting a second 1H NMR spectrum.
    Furthermore, if you have some extra time on the NMR instrument, then a 1H-15N HSQC might be worth a try.
    Below are some references that discuss NMR and ammonium ions:
    Ref1 J. Am. Chem. Soc.,1961,v.83,p.4726
    Ref2 Chem. Phys. Lett.,1988,v.143,p.471
    Ref3 Magn. Reson. Chem.,2008,v.46,p.356
    Ref4 J. Chem. Soc. (A),1968,p.1535
    Ref5 S. Berger, S. Braun, H.-O. Kalinowski. NMR Spectroscopy of the Nonmetallic Elements (John Wiley & Sons, Chichester, 1997).,p.245
    Ref6 Magn. Reson. Chem.,2008,v.46,p.356

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