H₃O⁺ ions in aqueous acid solutions. The infrared spectra revisited

Abstract

We have re-examined the infrared absorption between 4000 and 800 cm−1 of aqueous solutions of the four hydrogen halides at concentrations up to saturation, and under better experimental conditions than heretofore. The new spectra confirm definitely our previous assignment of the three broad bands around 2900, 1730, and 1200 cm−1 to fundamental vibrations of the H3O+ ion. Other proposed interpretations are shown to be untenable; in particular that of Ackermann, based on alleged similarities in the spectra of strong acids and bases. In hydrofluoric acid, hydrogen-bonded ion pairs are responsible for the shifts of the H3O+ frequencies. There is no evidence in the spectra for higher species, such as H5O2+. We conclude that the H3O+ ion has an appreciably longer lifetime in concentrated aqueous acids than in water. It also forms much stronger hydrogen bonds than H2O because of its ionic charge.

The Structure of the Hydrogen Ion (Haq+) in Water

“…1 red) gives the often noted but poorly understood continuous broad absorption ( cba , blue) overlaid with Guassian bands (green) with frequency variances for the four different acids of 3134 ± 12, 2816 ± 40, 1746 ± 11, 1202 ± 4 and 654 ± 12 cm -1 . This spectrum for these 0.2-0.75 M acids refines those reported earlier for more concentrated acids3,4,26 where only bands at ∼1740 and ∼1200 cm -1 could be identified with H aq + . The small variation with anion is ascribed to weak outer-sphere ion-pairing effects on the H(H 2 O) 6 + ion.…”

“…The value of n in H(H 2 O) n + is poorly defined and there has been a long debate over the relative importance of Eigen-type2 H 3 O + ·3H 2 O ( I ) versus Zundel-type3 H 5 O 2 + ( II ) structures:4-19…”

H3O+and OH−in Water

“…The frequency for H 3 0 + is broadly distributed and shifted to a lower frequency direction compared to the result for H 2 0 (z.e., by 12% red shift). This trend of the red shift is in agreement with the result by Tuckerman et al (by 17% red shift) and with results by experiments (e.g., shift to 2900 cm-' [21]). These frequency shifts are related to the fact that the oxygen-oxygen distance becomes shorter for H 3 0 t in water than for pure water [22] as shown in Figure 1.…”

Easily polarizable hydrogen bonds and solvate structure in aqueous solutions of acids with pKa < 1

“…The bending vibration is observed, however, with presence of structure *I: however: favours the n > 2 (see following section). ~h u s , in aqueous soltransfer of the proton into the network of the utions, in contrast to other statements (35)(36)(37)(38). the hydrogen-bonded water molecules.…”