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Topic Name: Some Fundamental Interactions of Matter May Turn Out to be Fundamentally Different than Thought

Category: Chemical

Research persons: Richard Zare, Stuart Greaves, Eckart Wrede

Location: Stanford University, United States

Details

Collisions have consequences. Everyone knows that. Whether it's between trains, planes, automobiles or atoms, there are always repercussions. But while macroscale collisions may have the most obvious effects—mangled steel, bruised flesh—sometimes it is the tiniest collisions that have the most resounding repercussions.

Such may be the case with the results of new experimental research on collisions between a single hydrogen atom and a lone molecule of deuterium—the smallest atom and one of the smallest molecules, respectively—conducted by a team led by Richard Zare, a professor of chemistry at Stanford University.

When an atom collides with a molecule, traditional wisdom said the atom had to strike one end of the molecule hard to deliver energy to it. People thought a glancing blow from an atom would be useless in terms of energy transfer, but that turns out not to be the case, according to the researchers.

"We have a new understanding of how energy can be transferred in collisions at the molecular scale," said Zare, senior author of a paper presenting the results in the July 3 issue of Nature.

Every atom or molecule, even if it has no charge, has electrostatic forces around it—sort of like the magnetic field of the Earth. Those chemical forces exert a pull on any other atom or molecule within range, trying to form a chemical bond.

What Zare and his team found is that a speeding hydrogen atom does not have to score a direct hit on a deuterium molecule, a form of molecular hydrogen made up of two heavy isotopes of hydrogen, to set the molecule vibrating. It only needs to pass closely enough to exert its tiny chemical force on the molecule. Vibrating molecules matter because they are more energized, making them more reactive. Thus, energy transfer effectively softens them up for future reactions.

"This has changed a very simple idea that we cherished—that to make a molecule highly vibrationally excited, you basically had to crush it, squeeze it, hit it over the head. Compress some bond and the molecule would snap back," Zare said. "We found quite the opposite."

One could compare it to the difference between a punch in the stomach and a caress on the cheek. Both can set the senses tingling, but in very different fashions.

Zare's team discovered that as a hydrogen atom passed close to a deuterium molecule, the chemical forces tugged on the nearest of the deuterium atoms in the molecule, pulling it away from the other deuterium atom. But if the tug was not strong enough to break the two deuterium atoms apart, as the hydrogen atom moved farther away its hold on the deuterium atom would weaken. The deuterium atom would eventually slip from its grip and snap back toward the other deuterium atom, initiating an oscillation, or vibration.

"What we are really seeing is the result of a frustrated chemical reaction," Zare said. "The molecule wants to react. It just didn't get into the right position with the right conditions so that it could react."

Zare went on to picture this process as follows: "The deuterium molecule is in a happily married state until the hydrogen atom flies by and attracts the nearest deuterium atom. This deuterium atom in the middle is in a giant tug of war. It is being fought over by two lovers, two highly similar atoms that are both attracted to the middle deuterium atom. This affair is a love triangle. In energy transfer, the original spouse wins out. The middle deuterium atom decides not to stray and rebounds to the other deuterium atom—its first love—setting both to vibrate rapidly."

The new findings may have ramifications for understanding what happens in any chemical reaction, in addition to interactions between chemicals that do not result in a reaction but instead result in energy transfer. So far, one instance has been discovered, but Zare believes that this behavior is likely to be found in many other collision systems.

"This is very fundamental stuff as to what happens in transformations of matter from one state to another," Zare said. "It's very fundamental chemistry."

Comparing the ramifications of the new findings to a ripple spreading out from a pebble dropped into a pond, Zare said, "Maybe this will be the sound of one hand clapping, if the ripple doesn't go anywhere. Taken together, the only way we advance is making these ripples and following them as they spread outward."

Zare's group did the experiments that revealed the energy transfer occurring during "soft" collisions between the hydrogen atom and the deuterium molecule by using techniques and equipment for measuring the molecular interactions that had previously been developed in his laboratory. The experimental work is a major portion of the doctoral thesis of his graduate student Noah T. Goldberg, who was assisted in these measurements by Jianyang Zhang, a postdoctoral researcher, and graduate student Daniel J. Miller. The theoretical calculations that provided the model used to explain the observations is the result of work done by co-authors Stuart Greaves of the University of Bristol and Eckart Wrede of the University of Durham, both in Britain.

About Deuterium
Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in 6500 of hydrogen (~154 PPM). Deuterium thus accounts for approximately 0.015% (on a weight basis, 0.030%) of all naturally occurring hydrogen in the oceans on Earth. Deuterium abundance on Jupiter is about 6 atoms in 10,000 (0.06% atom basis); these ratios presumably reflect the early solar nebula ratios, and those after the Big Bang. There is little deuterium in the interior of the Sun, since thermonuclear reactions destroy it. However, it continues to persist in the outer solar atmosphere at roughly the same concentration as in Jupiter.

The nucleus of deuterium, called a deuteron, contains one proton and one neutron, whereas the far more common hydrogen nucleus contains no neutrons. The isotope name is formed from the Greek deuteros meaning "second", to denote the two particles composing the nucleus.

Deuterium occurs in trace amounts naturally as deuterium gas, written ²H2 or D2, but most natural occurrence in the universe is bonded with a typical ¹H atom, a gas called hydrogen deuteride (HD or ¹H²H).

The existence of deuterium on Earth, elsewhere in the solar system (as confirmed by planetary probes), and in the spectra of stars, is an important datum in cosmology. Stellar fusion destroys deuterium, and there are no known natural processes, other than the Big Bang nucleosynthesis, which might have produced deuterium at anything close to the observed natural abundance of deuterium. This abundance seems to be a very similar fraction of hydrogen, wherever hydrogen is found. Thus, the existence of deuterium is one of the arguments in favor of the Big Bang theory over the steady state theory of the universe. It is estimated that the abundances of deuterium have not evolved significantly since their production more than 14 billion years ago.

The world's leading "producer" of deuterium (technically, merely enricher or concentrator of deuterium) was Canada, until 1997 when the last plant was shut down. Canada uses heavy water as a neutron moderator for the operation of the CANDU reactor design. India is now probably the world's largest concentrator of heavy water, also used in nuclear power reactors.

The physical properties of deuterium compounds can be different from the hydrogen analogs; for example, D2O is more viscous than H2O. Deuterium behaves chemically similarly to ordinary hydrogen, but there are differences in bond energy and length for compounds of heavy hydrogen isotopes which are larger than the isotopic differences in any other element. Bonds involving deuterium and tritium are somewhat stronger than the corresponding bonds in light hydrogen, and these differences are enough to make significant changes in biological reactions.

Deuterium can replace the normal hydrogen in water molecules to form heavy water (D2O), which is about 10.6% more dense than normal water (enough that ice made from it sinks in ordinary water). Heavy water is slightly toxic in eukaryotic animals, with 25% substitution of the body water causing cell division problems and sterility, and 50% substitution causing death by cytotoxic syndrome (bone marrow failure and gastrointestinal lining failure). Prokaryotic organisms, however, can survive and grow in pure heavy water (though they grow more slowly). Consumption of heavy water would not pose a health threat to humans unless very large quantities (in excess of 10 liters) were consumed over many days. Small doses of heavy water (a few grams in humans, containing an amount of deuterium comparable to that normally present in the body) are routinely used as harmless metabolic tracers in humans and animals.

The research done at Stanford was funded by the National Science Foundation. The research done in Britain was funded by the Engineering and Physical Sciences Research Council.