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Experimenters have confirmed the controversial idea first proposed by Nobel Laureate Linus Pauling in the 1930s that the rules of quantum mechanics cause the weak hydrogen bonds between H2O molecules in ice get part of their identity from stronger covalent bonds within the H2O molecule. The figure depicts the quantum-mechanical nature, or covalency, of the hydrogen bond between neighboring H2O molecules in the ice structure. The basic unit of ice is the H2O molecule which is depicted here using red balls for the oxygen atoms and white balls for the hydrogen atoms. The two relatively strong electronic bonds that make up the H2O molecule itself are represented in the figure by the darker yellow clouds. While the intermolecular bonds, or hydrogen bonds, are primarily electrostatic in nature, in which the molecules are attracted by means of separated electric charges, the experimenters found that the bond is in part quantum mechanical, or covalent in nature, in which electrons are spread out and shared between atoms. The quantum-mechanical or wavelike aspect of this bond is depicted by the lighter yellow clouds. In water and ice the intermolecular interaction is due primarily to the hydrogen bond. In ice, the hydrogen-bonded molecules are ordered in a regular array to form a molecular crystal.
Working at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, the US-France-Canada research team designed an experiment which utilized the ultra-intense x-rays that could be produced at the facility. With these x-rays, they studied the "Compton scattering" that occurred when the x-ray photons ricocheted from ordinary ice. Named after physicist Arthur Holly Compton, who won the Nobel Prize in 1927 for its discovery, Compton scattering occurs when a photon impinges upon a material containing electrons. When an incoming photon (blue arrow), produced by the synchrotron, strikes the ice sample, it transfers some of its energy of motion (kinetic energy) to the electrons, and emerges from the material with a different direction and lower energy (red arrow).
By studying the properties of many Compton-scattered photons, one can learn a great deal about the properties of the electrons in a material. In particular, Compton scattering is uniquely able to measure a solid's "ground-state electronic wavefunction," the complete quantum-mechanical description of an electron in its lowest energy state. The ground-state wavefunction in ice indicates that there is a quantum-mechanical overlap of the electrons on neighboring H2O molecules, i.e., that the hydrogen bond is partly covalent. (Figure courtesy of Bell Labs/Lucent Technologies. Thanks to Eric Isaacs of Bell Labs/Lucent Technologies for supplying much of the caption.)
This research is reported by E.D. Isaacs, A. Shukla, P.M. Platzman, D.R. Hamann, B. Barbiellini, and C.A. Tulk in the 18 January 1999 issue of Physical Review Letters.