Research Confirms Pauling's Theory On Hydrogen Bonds In Water
A US-France-Canada physics collaboration has unambiguously confirmed for the first time the controversial notion—first advanced in the 1930s by chemist and Nobel Laureate Linus Pauling—that the weak hydrogen bonds in water partially get their identity from stronger covalent bonds in the H2O molecule. As Pauling correctly surmised, this property is a manifestation of the fact that electrons in water obey the laws of quantum mechanics.
Performed by researchers at Bell Labs-Lucent Technologies in the US, the European Synchrotron Radiation Facility (ESRF) in France, and the National Research Council of Canada, the experiment provides important new details on water's microscopic properties. These new details will not only allow researchers to improve predictions involving water and hydrogen bonds, but may also advance areas such as nanotechnology and superconductors.
While working at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, the research team took advantage of the facility's ultra-intense X-rays to study the Compton scattering of ice. 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. By studying the properties of many Compton-scattered photons, one can learn a great deal about the properties of the electrons in a material.
Compton scattering is a very powerful technique because it 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. From this information, one can reconstruct the electron's ground-state wavefunction.
The effect that the experimenters were looking for—the overlapping of the electron waves in the sigma and hydrogen bonding sites—was a very subtle one to detect. Rather than study liquid water, in which the H2O molecules and their hydrogen bonds are pointing in all different directions at any given instant, the researchers decided to study solid ice. In ice, hydrogen bonds are pointing in only four different directions because the H2O molecules are frozen in a regularly repeating pattern.
The effect was still expected to be fairly small—only a tenth of all the electrons in ice are associated with the hydrogen bond or sigma bond. The rest are electrons that do not form bonds. What also complicates matters is that Compton scattering records information on the contributions from all the electrons in ice, not just the ones in which the researchers were interested.
However, the experimenters had a couple of advantages. First, the ESRF is a latest-generation facility that can produce very intense beams of X-ray photons—allowing the experimenters to obtain enough Compton-scattering events to perform a meaningful statistical analysis that would allow them to uncover the effect in the data. Second, the researchers shined the X-rays from several different angles. Measuring the differences in the scattering intensity from these different angles allowed them to remove uninteresting contributions from nonparticipating electrons.
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.
The presence of these fringes demonstrates that electrons in the hydrogen bond are covalent—just as Pauling had predicted. The experiment was so sensitive that the team even saw contributions from more distant bonding sites. From theoretical analysis and experiment the team estimates that the hydrogen bond gets about 10% of its behavior from a covalent sigma bond.
The research may allow nanotechnologists to design more advanced self-assembling materials, many of which rely heavily on hydrogen bonds to put themselves together properly. Researchers are also hoping to apply their experimental technique to study numerous hydrogen-bond-free materials such as superconductors and switchable metal-insulator devices, in which one can control the amount of quantum overlap between electrons in neighboring atomic sites.
Research was conducted by Eric Isaacs, Phil Platzman and Donald Hamann of Bell Labs/Lucent Technologies; Bernardo Barbiellini, now at Northeastern University; Abhay Shukla, European Synchrotron Radiation Facility; and Chris Tulk, National Research Council of Canada.
The research will be published in the Jan. 18, 1999, issue of the journal Physical Review Letters.
For more information, call 301-209-3091, or e-mail bstein@aip.acp.org.