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

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.