Ice crystal IN LIQUID WATER, the molecules are disordered. But in ice (left) the weak hydrogen bonds between molecules all point in only four different directions because the molecules are frozen in a regularly repeating pattern. By shining x rays on ice from several different angles, an international team of researchers was able to confirm the controversial notion advanced in the 1930's by Nobel Laureate Linus Pauling that the weak hydrogen bonds in water partially get their identity from stronger covalent bonds in the H2O molecule.

If Pauling was correct, the electrons in the hydrogen bonds would also exhibit the wavelike properties typical of quantum states, just like the covalent bonds. The electron waves on the covalent bonding sites--which hold water molecules together--and those on hydrogen bonding sites--which attract individual water molecules to each other--would overlap, making the individual electrons somewhat indistinguishable.

To test Pauling's conjecture, the researchers turned to a phenonmenon known as Compton scattering. Named after physicist Arthur Holly Compton, who won the Nobel Prize in 1927 for his discovery, Compton scattering occurs when a photon impinges upon a material containing electrons. The photon transfers some of its kinetic energy to the electrons, and emerges from the material with a different direction and lower energy .

Compton scattering is one of the few experimental tools that can obtain direct information on the low-energy state of an electron in an atom or molecule. By measuring the energy lost by a photon and its direction as it scatters from a solid, scientists can determine the momentum it transfers to the electrons in a molecule--and learn about the original momentum state of the electron itself. From this information, they can reconstruct the electron's "ground-state wavefunction"--the complete quantum-mechanical description of an electron in a hydrogen bond in its lowest-energy state.

The Compton effect, however, is very subtle. Only a tenth of all the electrons in ice are associated with either the hydrogen bond or the sigma bond. The rest are electrons which do not form bonds--yet their data would be collected as well.

But the researchers used the most powerful tool available--the European Synchrotron Radiation Facility in Grenoble, France, which produces ultra-intense beams of x-ray photons, allowing the experimenters to obtain enough Compton-scattering events to perform a meaningful statistical analysis. Also, by shining the x-rays from several different angles, the researchers were able to subtract out contributions from nonparticipating electrons.

By taking the differences in scattering intensity into account and plotting the intensity of the scattered x rays against their momentum, the team observed wavelike fringes corresponding to interference between the electrons on neighboring sigma and hydrogen bonding sites. These fringes demonstrate that electrons in the hydrogen bond are quantum mechanically shared. From theoretical analysis and experiment the team estimates that the hydrogen bond gets about 10 percent of its behavior from a covalent sigma bond.

In the graph below, the red dots show the experimental data points along with their error bars, the solid black line shows the fit predicted by theory, and the black dots indicate what the data would look like if the electrons on hydrogen bonds were unaffected by the strongly covalent sigma bonds. The conclusion: the ground-state wavefunction in ice indicates that there is a quantum-mechanical overlap of the electrons on neighboring H2O molecules. Thus, the hydrogen bond is partly covalent as Pauling predicted.

data plot

Images: Bell Labs

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