WHAT IS THE NEWS?
--Confirming a controversial prediction first made by famous chemist Linus Pauling, a new experiment has shown that the bizarre rules of the quantum world cause the weak "hydrogen bonds" in water molecules to get part of their identity from stronger "covalent" bonds within H2O.--Because hydrogen bonds play a significant role in determining water's properties, such as its unusual ability to shrink when heated, this experiment is likely to shed light on the numerous mysteries associated with water, which brought about many of the conditions favorable for life on this planet.
--The information from this experiment may help improve the understanding of biological structures that contain hydrogen bonds, such as DNA. It may enable researchers to design better self-assembling materials, which rely heavily on hydrogen bonds. And researchers hope to apply the techniques used in this experiment to learn new things about certain non-hydrogen-bond-containing materials, such as superconductors.
COLLEGE PARK, MD--January 12,1999--A US-France-Canada physics collaboration has unambiguously confirmed for the first time the controversial notion--first advanced in the 1930s by famous 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 bizarre laws of quantum mechanics, the modern theory of matter and energy at the atomic scale. Performed by researchers at Bell Labs-Lucent Technologies in the US, the European Synchrotron Radiation Facility in France, and the National Research Council of Canada, the experiment provides important new details on water's microscopic properties, which surprisingly remain largely unknown and difficult to measure. To be published in the January 18 issue of the journal Physical Review Letters, these new details will not only allow researchers to improve predictions involving water and hydrogen bonds, but may also advance seemingly unrelated areas such as nanotechnology and superconductors.
THE UNUSUAL PROPERTIES OF WATER
One of the most important components of life as we know it is the hydrogen bond.
It occurs in many biological structures, such as DNA. But perhaps the simplest
system in which to learn about the hydrogen bond is water. In liquid water and
solid ice, the hydrogen bond is simply the chemical bond that exists between H2O
molecules and keeps them together. Although relatively feeble, hydrogen bonds
are so plentiful in water that they play a large role in determining their
properties.
Arising from the nature of the hydrogen bond and other factors, such as the disordered arrangement of hydrogen in water, the unusual properties of H2O have made conditions favorable for life on Earth. For instance, it takes a relatively large amount of heat to raise water temperature one degree. This enables the world's oceans to store enormous amounts of heat, producing a moderating effect on the world's climate, and it makes it more difficult for marine organisms to destabilize the temperature of the ocean environment even as their metabolic processes produce copious amounts of waste heat.
In addition, liquid water expands when cooled below 4 degrees Celsius. This is unlike most liquids, which expand only when heated. This explains how ice can sculpt geological features over eons through the process of erosion. It also makes ice less dense than liquid water, and enables ice to float on top of the liquid. This property allows ponds to freeze on the top and has offered a hospitable underwater location for many life forms to develop on this planet.
TWO TYPES OF BONDS IN WATER
In water, there are two types of bonds. Hydrogen bonds are the bonds between
water molecules, while the much stronger "sigma" bonds are the bonds within a
single water molecule. Sigma bonds are strongly "covalent," meaning that a pair
of electrons is shared between atoms. Covalent bonds can only be described by
quantum mechanics, the modern theory of matter and energy at the atomic scale.
In a covalent bond, each electron does not really belong to a single atom--it
belongs to both simultaneously, and helps to fill each atom's outer "valence"
shell of electrons, a situation which makes the bond very stable.
THE ELECTROSTATIC NATURE OF THE HYDROGEN BOND
On the other hand, the much weaker hydrogen bonds that exist between H2O
molecules are principally the electrical attractions between a positively
charged hydrogen atom--which readily gives up its electron in water--and a
negatively charged oxygen atom--which receives these electrons--in a neighboring
molecule. These "electrostatic interactions" can be explained perfectly by
classical, pre-20th century physics--specifically by Coulomb's law, named after
the French engineer Charles Coulomb, who formulated the law in the 18th century
to describe the attraction and repulsion between charged particles separated
from each other by a distance.
ANOTHER SIDE OF THE H-BOND'S PERSONALITY
After the advent of quantum mechanics in the early 20th century, it became clear
that this simple picture of the hydrogen bond had to change. In the 1930s, the
famous chemist Linus Pauling first suggested that the hydrogen bonds between
water molecules would also be affected by the sigma bonds within the water
molecules. In a sense, the hydrogen bonds would partially assume the identity of
these bonds!
How do hydrogen bonds obtain their double identity? The answer lies with the electrons in the hydrogen bonds. Electrons, like all other objects in nature, naturally seek their lowest-energy state. To do this, they minimize their total energy, which includes their energy of motion (kinetic energy). Lowering an electron's kinetic energy means reducing its velocity. A reduced velocity also means a reduced momentum. And whenever an object reduces its momentum, it must spread out in space, according to a quantum mechanical phenomenon known as the Heisenberg Uncertainty Principle. In fact, this "delocalization" effect occurs for electrons in many other situations, not just in hydrogen bonds. Delocalization plays an important role in determining the behavior of superconductors and other electrically conducting materials at sufficiently low temperatures.
Implicit in this quantum mechanical picture is that all objects--even the most solid particles--can act like rippling waves under the right circumstances. These circumstances exist in the water molecule, and the electron waves on the sigma and hydrogen bonding sites overlap somewhat. Therefore, these electrons become somewhat indistinguishable and the hydrogen bonds cannot be completely be described as electrostatic bonds. Instead, they take on some of the properties of the highly covalent sigma bonds--and vice versa. However, the extent to which hydrogen bonds were being affected by the sigma bonds has remained controversial and has never been directly tested by experiment--until now.
A NEW EXPERIMENT PROVIDES UNAMBIGUOUS EVIDENCE
Working at the European Synchrotron Radiation Facility (ESRF) in Grenoble,
France, a US-France-Canada research team designed an experiment that would
settle this issue once and for all. Taking advantage of the ultra-intense x-rays
that could be produced at the facility, they studied the "Compton scattering"
that occurred when the x-ray photons ricocheted from ordinary ice.
THE COMPTON SCATTERING PROCESS
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. 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. By measuring the energy lost by a photon and its direction as it scatters from a solid, one 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, one 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.
A SUBTLE EFFECT TO CAPTURE
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 which the hydrogen bonds are
pointing in only four different directions because the H2O molecules are frozen
in a regularly repeating pattern.
Still, the effect was 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 which 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 subtract out uninteresting contributions from nonparticipating electrons.
WAVELIKE INTERFERENCE BETWEEN ELECTRONS IN WATER
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 quantum mechanically shared--covalent--just as Linus 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.
IMPLICATIONS OF THIS EXPERIMENT
For many years, many scientists dismissed the possibility that hydrogen bonds in
water had significant covalent properties This fact can no longer be dismissed.
The experiment provides highly coveted details on water's microscopic
properties. Not only will it allow researchers in many areas to improve theories
of water and the many biological structures such as DNA which possess hydrogen
bonds. Improved information on the h-bond may also help us to assume better
control of our material world. For example, it may allow nanotechnologists to
design more advanced self-assembling materials, many of which rely heavily on
hydrogen bonds to put themselves together properly. Meanwhile, researchers are
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.
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. For a copy of the article please contact Ben Stein at 301-209-3091 or bstein@aip.acp.org.
RESEARCHERS INVOLVED IN THE EXPERIMENT
Bell Labs/Lucent Technologies: Eric Isaacs, Phil Platzman, Donald Hamann (To
contact these researchers, please call Steve Eisenberg at 908-582-7474)
Bernardo Barbiellini, now at Northeastern University; Abhay Shukla, European
Synchrotron Radiation Facility; Chris Tulk, National Research Council of Canada
(For contact info on these researchers, call Ben Stein, 301-209-3091, or send an
email at bstein@aip.acp.org)
OTHER EXPERTS
Alenka Luzar, UC Berkeley, 510-643-3172, alenka@wilma.cchem.berkeley.edu
Gene Stanley, Boston University, 617-353-2617, hes@bu.edu
David Clary, University College, London, 011-44-171-391-1488,
Jose Teixeira, CEA-Saclay, France, 011-33-1-69-08-66-50,
teix@llb.saclay.cea.fr
For more information, please contact:
Ben Stein, American Institute of Physics, 301-209-3091, bstein@aip.acp.org
Steve Eisenberg, Media Relations Manager, Bell Labs/Lucent Technologies, 908
582-7474, seisenberg@lucent.com
Recommended Background Reading on the Mysteries of Water
"Water, a Wonder Molecule," by R. Sanjeevi and B. Viswanathan, published in the
February 12, 1998 issue of the Indian national newspaper The Hindu. Available
online at:
http://www.webpage.com/hindu/daily/980212/08/08120001.htm
Journal
Physical Review Letters