Water is one of the most important chemical compounds on the earth, where its properties are essential to many physical, chemical, biological, and geological processes. No life can exist without water.

The structure and dynamics of the hydrogen bonding network in water are what makes water unique. The understanding of this network is one of the most important scientific questions in the last 100 years.

Although the structure of water has never been clearly experimentally determined, it has been regarded as being understood, based on various experimental observations and theoretical simulations.

Applying, for the first time, high resolution x-ray spectroscopic techniques to water gave results which contradict the textbook picture and led us to propose a radically different structural model. Consequently, our study has generated an intense debate and a renewed interest regarding the structure of water.

The new discovery was that the spectrum of water was very different from that of ice. This was very much unexpected since the traditional picture assumes a tetrahedral hydrogen-bonding network in the liquid, similar to ice.

The result implies that the local structure of water must be different and from experiments on model systems and theoretical simulations of the experimental probe we could conclude that, on the average, a dominating fraction of the water molecules are not in a tetrahedral symmetry but asymmetrically bonded, with two strong and two weak hydrogen bonds.

Water is a unique chemical substance on earth with a number of unusual properties such as: higher density in the liquid compared to the solid, density maximum at 4°C, high heat capacity, high surface tension, ¹etc.

In fact, no other substance even closely approaches so many strange properties. Water is the basis of our existence and it is essential that we can understand the origin of these properties in terms of the molecular structure of the liquid. In this aspect, our paper has opened up a new direction.

We became involved with water research through a coincidence. We were developing x-ray spectroscopic methods for studies of the chemical bonding of molecules in aqueous solutions and needed to measure water, since it provided a background in our experiment. When we saw the water spectrum we became very puzzled, since it looked so different from the ice spectrum. This led us to switch our focus to concentrate on a better understanding of liquid water. Since then, water research has been one of our main activities.

Anders Nilsson, Professor
Stanford Synchrotron Radiation Laboratory
Surface Science and X-ray spectroscopy group 
Stanford University
Stanford, CA, USA 
and
Department of Physics
Stockholm University
Stockholm, Sweden

Lars G. M. Pettersson, Professor
Lars G. M. Pettersson Group Quantum Chemistry 
Department of Physics
Stockholm University
Stockholm, Sweden

Related Links:
1	http://www.lsbu.ac.uk/water/anmlies.html
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