Raman Spectra of Amino Acids and Related Substances III. Ionization and Methylation of the Amino Group

Raman spectra have been determined for methylated amines, and the corresponding ammonium chlorides in water; also for ethylamine and its hydrochloride, and the hydrochlorides of hydroxylamine, hydrazine and trimethylamine oxide. The spectra of the hydrochlorides arise only from the positively charged ammonium ion, since the chloride ion is not attached by any covalent bond, and, therefore, is not directly involved in any of the observed vibrations. Thus the spectrum of a methylated ammonium ion is found to be exactly similar in type to that of the analogous hydrocarbon in which a C atom is substituted for the positively charged N atom. If the —CH3 and —NH3 groups are treated schematically like single atoms of the same mass, all these compounds may be represented by simple models. The characteristic frequencies of such models may be correlated with lines in the observed spectra. Qualitative polarization measurements have been made on several of the substances studied and have proved of value in assigning observed frequencies to modes of molecular vibration. Bond force constants and valence angles have been evaluated for many of the substances studied, making use of certain approximate potential functions frequently employed in describing systems of this sort. In the solution of hydrazine dihydrochloride, certain Raman lines arise from the ion +H3N–NH3+; others from +H3N–NH2. Both ions are present in appreciable concentrations, and their relative amounts are roughly what would be expected from known data on dissociation constants. The Raman spectra of the amines here studied in aqueous solution are nearly identical with those reported by other observers for the same amines as anhydrous liquids. The characteristic valence vibrations of the uncharged amino group, between 3300 and 3400 cm—1, are absent in the charged —NH3+ or NH2+ group, being apparently replaced by much weaker lines of lower frequency. The sodium salts of glycine and alanine show strong Raman lines above 3300 cm—1, which are absent in the isoelectric amino acids. This fact supplies further evidence that the amino acids exist as electrically charged dipolar ions, +H3N–R–COO—.