Amine salts, group frequencies

The CH2 or CH3 groups next to the nitrogen atom in amines are shifted somewhat. The in-phase stretch in secondary and tertiary amines at 2830-2760 cm is lowered in frequency and intensified, and so stands out among other aliphatic bands. This is because the CH bond which is trans to the non-bonding electron pair on the attached nitrogen has a smaller force constant than the other CH bonds (see Table 5.2). This only applies for amines and not for amides or amine salts (see Table 11.1). 

Sheppard then made the methiodide salt of the tertiary amine, trimethylaiulinium iodide, in which the lone pair is bound to a methyl group and found that the low-frequency C H stretch disappeared. A further example from this early work of Sheppard was a comparison between the spectra of trimethylamine and tetrametylammonium iodide... 

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