8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P2.6Tue 611:23-14:00Trapping proton transfer reaction intermediates incryogenic hydrofluoric acid solutionsPatrick Ayotte 11 Université de Sherbrooke, 2500 boulevard université, J1K 2R1, Sherbrooke,CanadaThe adsorption, reaction, and uptake kinetics of impurities by atmospheric particles are importantlimitations in our current understanding of atmospheric chemistry phenomena. Specifically, amolecular-level description of the factors involved in the ionization/dissociation of acids at aqueousinterfaces is needed to understand the surface specificity of acid-base equilibria and their impact onsubsequent heterogeneous atmospheric chemistry processes. Reflection-absorption infrared spectra(RAIRS) of cryogenic HF:H 2 O binary thin films prepared using molecular beam techniqueswill be presented. Optical constants for these cryogenic hydrofluoric acid solutions are obtainedby iteratively solving Fresnel equations for stratified media allowing for a detailed interpretationof the complex interplay between multiple reflections, optical interference and absorption effectsobserved in absorbance spectra of nanoscopic films. The excellent agreement of ab initio moleculardynamics simulations with experimental data provides detailed molecular-level interpretationof the dissociation mechanism. These optical constants are used to interpret spectra for sampleswhere HF was adsorbed and condensed onto amorphous solid water films at various temperaturesrevealing the extent of reaction, intermixing and uptake taking place at these model aqueous interfaces.Finally, a strong intermolecular vibrational coupling is observed between the HF stretchingand HOH bending vibrations of [HF·H 2 O] proton transfer reaction intermediates trapped in binaryamorphous solids. A simple cluster model is used to highlight the covalent character of thisstrong intermolecular H-bond as electrostatic models yield the wrong sign and magnitude for theintermolecular v-v coupling. These observations strongly support the unconventional Mullikendescription of proton transfer reaction providing a rationale for the facile dissociation of acids atthe surface of condensed water substances.6