Raman Spectra of Amino Acids and Related Compounds IV. Ionization of Di- and Tricarboxylic Acids

Abstract

A systematic study of Raman spectra has been made for a large number of carboxylic acids and their salts dissolved in water. In particular, malonic acid and its two sodium salts, and malonic-d2 acid-d2 and its disodium salt, have been studied in detail. The substitution of deuterium in this molecule permits the vibrations arising primarily from the — CH2 group to be distinguished from those arising primarily from the carboxyl groups. The Raman spectrum of deuterium oxide has been redetermined in connection with these studies. The following general conclusions have been reached: (1) The powerful ``carbonyl'' frequency near 1700 cm—1 is always found when an un-ionized carboxyl group is present, and always vanishes on ionization of the carboxyl. (2) One or more intense Raman lines near 1400 cm—1 can always be found in a substance containing an ionized carboxyl group. Such lines are definitely polarized, and presumably correspond to a symmetrical valence oscillation of the COO— group. Like the ``C=O'' frequency near 1700, these lines are almost unaffected by deuterium substitution. Deformation frequencies arising from —CH2 or —CH3 groups commonly lie in the same range; the marked effect of deuterium substitution, however, clearly differentiates these from the vibrations of the —COO— group. (3) Other Raman lines, near 1330 and 1580 cm—1, are frequently but not invariably found in substances containing ionized carboxyl groups. (4) Most of the substances studied show strong Raman lines between 700 and 950 cm—1, which increase in frequency by 30 to 50 cm—1 on ionization. These lines are strongly polarized, and in malonic acid are noticeably depressed in frequency by deuterium substitution. (5) Certain frequencies below 600 cm—1 are virtually unaffected either by ionization or by deuterium substitution. These presumably represent bending or twisting vibrations of the heavy molecular framework. The ``C=C'' frequency near 1650 in crotonic and maleic acids is unchanged by the ionization of a neighboring carboxyl group. The Raman spectra of d-tartaric acid and of mesotartaric acid, although very similar, show definite differences which are well beyond the experimental error. The structure of the ionized carboxyl group is undoubtedly closely related to that of the nitro group. Both show intense polarized Raman lines near 1400, probably corresponding to the same type of oscillation in both cases; and the bond strength in the two groups is probably nearly the same.