Interpretation of the Raman spectra of aqueous acid solutions in terms of the polarizability of hydrogen bonds. Part 1.—Aqueous HCl solutions

Abstract

In the Raman spectra of pure water and aqueous HCl solutions a continuous scattering is found, similar to the continuous absorption observed with the i.r. spectra of acid solutions. The continuum of energy level differences probably arises from the extremely large proton polarisability of the hydrogen bond in the H₅O⁺₂ grouping, since the large polarisability causes interaction effects of these hydrogen bonds with their environment, so shifting the energy levels. Below 300 cm^–1 an extremely strong intensity increase for the acid solution is caused by the O ⋯ O stretching vibration. One reason for this Raman intensity may be the large change of the hydrogen bond polarisability caused by the proton as a function of the O ⋯ O distance in the hydrogen bond (OH ⋯ O)⁺. The intensity of the H₂O torsional vibrations at 780 and 450 cm^–1 rises, too, with increasing acid concentration, but not as strongly. This, also, may be explained by the large proton polarisability of the H-bond.