Dissociation of Trifluoroacetic Acid in Amorphous Solid Water: Charge-Delocalized Hydroniums and Zundel Continuum Absorption

Abstract

We studied the ionic dissociation of trifluoroacetic acid (TFA) in D2O amorphous solid water (ASW) using reflection absorption infrared spectroscopy, low-energy sputtering, and H/D isotopic exchange experiment. TFA readily dissociated to hydronium and counterions in ASW at 60 K, which indicates a significant increase in the acidity as compared with that in aqueous solution at room temperature. The acid dissociation produced a Zundel continuum band in the 1000–3000 cm–1 region and an accompanying lowered intensity of the O–D stretching band of D2O. The reduced intensity of D2O was several times larger than that expected for 1:1 stoichiometric proton transfer from TFA to water. Excess protons released from the acid migrated through as many as 20 water layers in ASW. These observations indicate that excess protons are highly mobile and dynamically delocalized in the hydrogen-bonded water chain. Such characteristics of excess protons may be responsible for the appearance of the Zundel continuum absorption.

Entropy-Driven Spontaneous Reaction in Cryogenic Ice: Dissociation of Fluoroacetic Acids

“…At 140 K, the spectrum showed other minor changes (Supporting Information Figure S1), including the change of the bulk ν(O−D) band shape due to the crystallization of ice 6 and the increase in the ν(O−H) band of HDO due to H/D exchange reactions. 10,11 These observations showed that the spontaneity of acid dissociation was unaffected by temperature changes between 8 and 140 K.…”

“…Acetic acid (AA; pK aq = 4.76) did not dissociate in ice either. 11 These observations located the boundary of spontaneous dissociation of carboxylic acids in ice to be between pK aq = 2.82 and 3.75 in terms of the acidity scale in aqueous phase.…”

“…This is because proton passage reverses the H-bond polarization of the chain, which facilitates backward proton transfer along the same path. 11 In any case, the proton hopping process alone does not induce macroscopic polarization in the sample. This is a different situation from charge conduction through ice that is driven by external bias voltage at high temperature and causes macroscopic polarization.…”

“…For amorphous ice films prepared at 70−80 K (similar to those shown in Figure 1), excess protons migrate across a distance of 5−20 water layers in one direction. 11,22 For the present samples we simply assume that the proton migration distance is ∼10 water molecules in one direction, which means that a 3Dl volume of ∼10 3 water molecules is accessible by a mobile proton. Each water molecule in the ice lattice has two possible positions to accept a proton, and protons are indistinguishable.…”

“…One was the formation of HDO by H/D exchange reaction of acid and D 2 O in the sample. This HDO species forms when proton transfer occurs in D 2 O–ice in conjunction with water reorientation, which requires thermal activation. , Therefore, the intensity of this band varied with sample temperature. The other source was the deposition of residual HDO gas in the chamber during sample preparation.…”