The reactivity of water ice and its characterization by XAS

Abstract

Water seems at first sight to be a very simple substance. However, it possesses numerous anomalous properties, and despite the fact that a huge number of models have been developed, none can properly account for all those properties. Hydrogen bonded systems are of major interest in many different fields and water is perhaps the simplest model of a hydrogen bonded system. The first section of this thesis focuses on the reactivity of hydrogen bonded systems. First, the reaction of formaldehyde with ammonia to generate NH2CH2OH, an amino acid precursor, is studied at various levels of theory. The reaction mechanism consists of a nucleophilic attack of the carbon by the lone pair on the nitrogen, accompanied by the simultaneous transfer of a proton to the formaldehyde oxygen. In the presence of water, the proton transfer occurs via a chain of proton transfers and the activation energy is lowered with the addition of two water molecules. This first study having proven that water molecules can catalyze a reaction, another question arises: how do extended hydrogen bond networks modify the properties of water molecules. Water is well known to present cooperative effects. The present work tries to evaluate the extent of such an effect in water ice structures using both cluster and periodic models. The cooperative effect is shown to be very important. It is proven here that even if the charge transfer plays a major role for the shorter chains, electrostatic interactions become the most important factor as the chains lengthen. The extent of the cooperativity depends on the method and basis set employed, but whatever the methodology, a small cluster seems to be a very poor model to mimic the cooperativity present in ice structures. Finally, the characterization of various ice and water structures are studied by means of calculated X-ray absorption spectra. DFT calculations have been performed on spherical clusters cut out from the experimental structures of the four proton ordered ices, i.e. ices II, VIII, IX and XI as well as on water and various amorphous ice structures ranging between the HDA and LDA structures. The influence of the periodic crystalline environment is examined by computing spectra calculated for the naked clusters and for the same clusters surrounded by point charge distributions designed to reproduce the Madelung potential of the crystal. A systematic study of the influence of the size of the quantum cluster is also performed. Small clusters (70 water molecules, radius ≈7A) can be used, as long as the cluster is subjected to a converged Madelung potential and the total dipole moment of the cluster is small. In the case of water and amorphous ices, sampling a large number of oxygen centers is required to obtain a converged spectrum. The directly bound nearest neighbors of the active oxygen center have been reported to exert considerable influence on the spectral lineshape. Therefore, a study of the influence of the basis set on those nearest neighbors is performed. The outer coordination shells and the long-range electrostatic potential must be taken into account in a quantitative simulation. A reasonable agreement is obtained with experimental results. However, while the method used here seems to perform well in the pre-edge region, it fails to reproduce the post-edge region exactly as a localized basis set, even one including diffuse functions, cannot account for a delocalized band structure. In the case of amorphous ices, for which calculated spectra do not provide good results, multiple electron transitions might have to be considered in the calculation of X-ray absorption spectra.