Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science 447

Doctoral dissertation at Uppsala University 1993 for the degree of Doctor of Philosophy

The Water Molecule in Gaseous, Liquid and Solid Surroundings

An ab initio and Molecular Dynamics Study

by Lars Ojamäe


Water is the most abundant chemical on the crust of the earth, where it is the only substance which occurs naturally as a gas, a liquid and a solid as well. It is also the prerequisite for the biological processes of life, and provides the milieu for most chemical reactions, be it in nature or in the laboratory. A better understanding of water in its different functions and aggregations is possible only if the water molecule itself and its response to the environment are fully understood at a molecular level. In this thesis, some qualitative and quantitative aspects of the interactions and dynamics of the water molecule have been investigated, and different properties calculated, by quantum-mechanical ab initio calculations and by classical Molecular Dynamics simulations.

The OH stretching vibrational spectrum is experimentally used as a measure of the strength of the hydrogen bond and other intermolecular interactions. Here the OH stretching vibration of the water molecule interacting with other water molecules and ions in clusters and crystals (LiClO4· 3H2O, LiOH·H2O, LiHCOO·H2O, ice II, VIII and IX) has been studied by ab initio methods The relative importance of the nearest-neighbour interactions and the long-range crystal field on the frequency and the electron density was identified. Relationships between the frequency and nearest-neighbour distances, O-H bond lengths and infrared absorption intensity were confirmed.

Molecular Dynamics simulations were used to calculate the OH stretching vibrational spectrum (frequencies and band width) in liquid water. The calculated spectrum was found to be highly sensitive to quantum-corrections and to the dynamics of the neighbouring molecules; both effects are necessary to include in theoretical calculations for the model to be realistic.

Periodic Hartree-Fock ab initio computations were performed for LiOH·H2O and ice VIII. In this method long-range environmental effects are taken properly into account. Electron densities, quadrupole coupling constants, uncoupled OH vibrations, Raman spectra, electronic band structure diagrams and strain-free structures were calculated. For ice VIII, an equation of state was derived and the changes in crystal geometry and vibrational properties brought about by the hydrostatic pressure in the range of 0 to 30 GPa were investigated.

Lars Ojamäe, Institute of Chemistry, Uppsala University, Box 531,
S-751 21 Uppsala, Sweden

(C) Lars Ojamäe 1993

ISBN 91-554-3102-X
ISSN 0282-7468

Thesis Abstract, lars @