AGUA Secret Nature of Hydrogen Bonds

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Secret Nature of Hydrogen Bonds

Hydrogen Bonds Have Covalent Properties

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 hydrogenbonds 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 basicunit 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 thatmake 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 orwavelike 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 "Comptonscattering" that occurred when the x-ray photons ricocheted from ordinary ice. Named

after physicist Arthur Holly Compton, who won theNobel Prize in 1927for 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).
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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 completequantum-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. (Figurecourtesy 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. Tulkin the 18 January 1999 issue ofPhysical Review Letters.

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The Secret Nature of Hydrogen Bonds

Hydrogen bonds are the chemical bonds

that exist between H2O molecules and keep

them together. Recent experiments have

obtained new insights on the hydrogen bond,by shining x-rays of one color (blue arrow)

on an ice crystal and analyzing the color

and direction of the x-rays (red arrow) that

emerge from the ice. For additional

explanation of this figure, see ourdetailed

figure caption at thePhysics News Graphics

website.
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WHAT IS THE NEWS?

Confirming a controversal prediction first made by famous chemist Linus Pauling, a

new experiment has shown that the bizarre rules of the quantum world cause theweak "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 enableresearchers 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.

INTRODUCTION

COLLEGE PARK, MD--January 12,1999--A US-France-Canada physics collaboration hasunambiguously 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 H2Omolecule. 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-LucentTechnologies in the US, the European Synchrotron Radiation Facility in France, and the

National Research Council of Canada, the experiment provides important new details onwater's microscopic properties, which surprisingly remain largely unknown and difficult to

measure. z 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 conditionsfavorable 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
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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 theirmetabolic 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 liquidwater, 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 onthis planet.

TWO TYPES OF BONDS IN WATER

In water, there are two types of bonds. Hydrogen bonds are the bonds between watermolecules, 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 notreally belong to a single atom--it belongs to both simultaneously, and helps to fill each

atom's outer "valence" shell, 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--whichreceives 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 18thcentury 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 LinusPauling 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 even 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 ofmotion (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 phenomenonknown 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 electricallyconducting materials at sufficiently low temperatures.


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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 circumstancesexist 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 thefacility, they studied the "Compton scattering" that occurred when the x-ray photons

ricocheted from ordinary ice.

Crystal structure of ordinary ice. Red balls give the position of oxygen and white balls give

the position of hydrogen. (Figure courtesy Bell Labs/Lucent Technologies.)

THE COMPTON SCATTERING PROCESS

Named after physicist Arthur Holly Compton, who won theNobel Prize in 1927 for itsdiscovery, 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 ofmany 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 experimentaltools 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
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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 studyliquid water, in which the H2O molecules and their hydrogen bonds are pointing in alldifferent 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 thecontributions 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, theresearchers 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.


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Measuring the differences in x-rays' intensity when scattered from various angles in a single crystal of ice,

and plotting this scattering "anisotropy" against the amount of momentum in the electrons scattered in the

ice, the team recorded wavelike interference fringes corresponding to interference between the electrons on

neighboring sigma and hydrogen bonding sites. The red dots show the experimental data points along with

their error bars, the solid black line shows the fit predicted by an accompanying 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. Inset: By performing a mathematical operation known as a Fourier transform on their

data, the researchers are able to obtain information on the distances between bonding sites. (Figure courtesyBell Labs/Lucent Technologies.)

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 recorded 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 distantbonding 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 hadsignificant 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 structuressuch 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 relyheavily 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.

JOURNAL REFERENCE

This research is reported byE.D. Isaacs, A. Shukla, P.M. Platzman, D.R. Hamann, B.Barbiellini, and C.A. Tulkin the 18 January 1999 issue ofPhysical Review Letters.

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