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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