The Role of the Lone Pairs in Hydrogen Bonding

Monday, February 5 16:30-17:30
WHAT COMPTON SCATTERING TELLS ABOUT THE INTRA-
AND INTERMOLECULAR STRUCTURE OF WATER?
M. Hakala
Division of X-Ray Physics, Department of Physical Sciences, P.O.B. 64, FI-00014 University of Helsinki,
Finland Email: mikko.o.hakala@helsinki.fi
Besides being the most familiar and ubiquitous liquid, water is most interesting from the microscopic
point of view. The structure and properties of this molecular liquid are determined by the hydrogen bond
(H bond) network. The H-bond network as well as the intramolecular geometry (e.g. the length of the
covalent O-H bond) depend on the thermodynamic state. The fact that even the structure of the first
coordination shell continues to be debated [1] gives an ample motivation to study water with new and
improved methods. In this talk I will review our recent Compton scattering experiments on water using
synchrotron radiation and the findings we have obtained with model calculations. The experiments we
have carried out include: (i) temperature effect of the liquid [2], (ii) ice-liquid transition [3], (iii) isotope
effect, and (iv) temperature effect of ice. From each set one can extract difference Compton profiles (CPs)
J(q), which can be interpreted by the changes in the intramolecular (O-H length, H-O-H angle) and/or
intermolecular (H-bond length and angle) parameters. I will discuss two computational approaches used
in this work. The first one is a nonselfconsistent model [4], in which additive nearest-neighbor interactions
are assumed between the molecules. The model incorporates the possible changes in the intramolecular
geometry and in the local nearest-neighbor interaction, but excludes the cooperative and anticooperative
properties of the H-bonds. I show that when applied to difference CPs, this model gives a good physical
insight into the changes in the H-bond network, and can be adjusted to reasonably well capture the
main features observed in the experiments. The second approach is an ab initio method, where the CPs
are directly calculated from snapshot structures of a Car-Parrinello molecular dynamics simulation. The
resulting difference CP suffices to qualitatively describe the experimental temperature effect of water.
[1] See, for example, M. Odelius, M. Cavalleri, A. Nilsson, and L. G. M. Pettersson, Phys. Rev. B 73
(2006) 024205; D. Prendergast and G. Galli, Phys. Rev. Lett. 96 (2006) 215502. [2] M. Hakala, K.
Nyg˚ard, S. Manninen, S. Huotari, T. Buslaps, A. Nilsson, L. G. M. Pettersson, and K. Hämäläinen, J.
Chem. Phys 125 (2006) 084504. [3] K. Nyg˚ard, M. Hakala, S. Manninen, A. Andrejczuk, M. Itou, Y.
Sakurai, L. G. M. Pettersson and K. Hämäläinen, Phys. Rev. E 74 (2006) 031503. [4] M. Hakala, K.
Nyg˚ard, S. Manninen, L. G. M. Pettersson and K. Hämäläinen, Phys. Rev. B 73 (2006) 035432.
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