The infrared spectra of aqueous solutions of glycine, N-methylglycine (sarcosine), N,N-dimethylglycine, and N,N,N,-trimethylglycine (betaine) in different states of ionization are reported. The spectra of the dipolar forms of the first three compounds are shown to consist of bands represented in the spectra of both the positively and negatively charged forms, together with certain bands peculiar only to the dipolar form. In the spectrum of dipolar betaine, which exhibits no negatively charged structure, certain of the bands are also demonstrable in the spectrum of the positively charged form. Aqueous solutions of the hydrochlorides of the four compounds exhibit a strong band at 7.94 or at 8 00 microns, which disappears and is replaced by a band appearing at 7.5 to 7.6 microns when aqueous solutions of the sodium salts are examined. Methyl substitution on the nitrogen is revealed by a band between 67 and 68 microns, which is absent in the unsubstituted compound, glycine.
Donald M. Kirschenbaum, "The Infrared Spectra of Acidic, Neutral, and Basic Aqueous Solutions of Glycine, N-Methylglycine, N,N-Dimethylglycine, and N,N,N-Trimethylglycine," Appl. Spectrosc. 17, 149-153 (1963)