Hydrogen bonding

The properties of for example liquid water are commonly considered to be anomalous as compared to those of compounds of similar size and composition. In this project we study the molecular interactions in systems containing water molecules, and in particular the hydrogen bond (H-bond). Some examples of current calculations are given below.

Ice

In a true crystal the atoms are ordered in a regular and repeating pattern. In ordinary ice, ice Ih, the oxygen atoms are ordered, but the hydrogens of each water molecule may point more or less randomly in different directions to the neighbouring molecules. The crystal is said to be proton-disordered. According to some experiments the proton-ordered crystal, termed ice XI, will be formed at a temperature below -200^oC, whereas others have indicated a phase transition at a much higher temperature.

By analysing the different H-bond topologies to identify structural characteristics, and using the computed lattice energies, we derive parametric relations between topology and lattice energy. Finally, by utilizing these in Monte Carlo simulations the thermodynamic properties are computed.

The calculations correctly predict the ferroelectric ice XI structure to be the most stable of practically all H-bond topologies. Most important, from the variation of entropy with temperature, an order-disorder phase transition is demonstrated to occur and the transition temperature established to be close to -200^oC. The entropy is found to be slightly lower than the Pauling estimate even near the melting temperature, which implies ordinary ice to be more ordered than expected.

It follows that ice on Earth should be proton-disordered. But on other celestial bodies, like Jupiter's moon Europa and in comets, phase transitions to the ordered form will take place.

Water clusters

In the atmosphere water (H₂O)_n and protonated water H⁺(H₂O)_n clusters participate in environmentally important chemical reactions, such as the ozone depletion mechanism. Quantum-chemical ab initio calculations and MC/MD simulations (sporting our polarizable and dissociative water potential model) are used to describe the structure and dynamics of such clusters.

A central idea of this study is to find out how the stabilities of the clusters are related to their H-bond topologies. From quantum-chemical computations reliable cluster structures and formation energies are obtained, and it is found that the topology indeed significantly governs the stabilities. By a proper inclusion of the H-bond statistics, dynamical effects and thermodynamic equilibria, it is possible to reproduce qualitatively the "magic-numbers" observed in mass spectra of ionized clusters. This in turn is an indication that the computed structures are the correct ones.

Liquid water

MD simulations of for example liquid water are being performed.

• The structure of the first coordination shell in liquid water
  Ph. Wernet, D. Nordlund, U. Bergmann, M. Cavalleri, M. Odelius, H. Ogasawara, L. Å. Näslund, T. K. Hirsch, L. Ojamäe, P. Glatzel, L. G. M. Pettersson and A. Nilsson
  Science 304, 995 (2004)
• Hydrogen-bond topology and the ice VII/VIII and ice Ih/XI proton-ordering phase transitions
  S. J. Singer, J-.L. Kuo, T. K. Hirsch, C. Knight, L. Ojamäe and M. L. Klein
  Phys. Rev. Letters 94, 135701 (2005)
• On the stability of dense versus cage-shaped water clusters: quantum-chemical investigations of zero-point energies, free energies, basis-set effects and IR spectra of (H₂O)₁₂ and (H₂O)₂₀
  A. Lenz and L. Ojamäe
  Chem. Phys. Letters 418, 361-367 (2006)
• Theoretical IR spectra for water clusters (H₂O)_n (n=6-22, 28, 30) and identification of spectral contributions from different H-Bond conformations in gaseous and liquid water
  A. Lenz and L. Ojamäe
  J. Phys. Chem. A 110, 13388-13393 (2006)

This work was supported by the Swedish Research Council.