Chemistry prof explains vibrational bonds with Winnie the Pooh analogies

Researchers at the TRIUMF institute have confirmed the existence of a new type of chemical bond.

Donald Fleming, a UBC professor emeritus, theorized the existence of vibrational bonds in a paper published in 1989.

“We had an experimental result that really demanded a sophisticated theory calculation,” he said.

The experiment was a reaction between a bromine molecule and an atom of a very specific hydrogen isotope called muonium.

“Normally chemistry doesn’t depend on the isotope,” said Fleming, but this experiment proved that, in the case of bonding, isotopes play a significant role.

As the muonium interacted with the bromine, the energy of the reaction slowed as temperatures rose.

“Normally, chemical reactions, when you heat them up, they speed up,” said Fleming, but because this one slowed down, it meant that the reaction went through an intermediate complex. This is observed when chemical reactants go through a temporary bond phase that eventually leads to the end products.

“I was intrigued,” said Fleming. “We did this experiment again many years later, and we found evidence for this kind of complex.”

This January, Fleming, along with a team of international researchers, observed a vibrational bond in action -- something that could not have been done with the technology of the time when the phenomenon was predicted.

To explain the reaction, Fleming used an interesting analogy, complete with fictional elephants and loveable cartoon characters.

“To trap a Heffalump, what Winnie the Pooh did was he dug a big hole in the ground,” said Fleming. “And then the Heffalump -- this big, lumbering beast -- was supposed to come running along. If the hole was wide enough and deep enough, the Heffalump wouldn’t get to the other side -- he’d run in.”

The Heffalump in Fleming’s analogy represents the hydrogen isotope used in the bond. Either end of the pit can be seen as the spot where the bromine atoms lie. If its energy were high enough -- or it were running fast enough -- the hydrogen could jump across and bond with either atom. However, in the case of muonium, its energy decreases, and the Heffalump gets trapped in the pit, between the bromine atoms, in a vibrational bond.

What happens in this temporary bond is quite simple: the muonium atom reaches a stable point where it is equally attracted to both bromine atoms, and ends up stuck, vibrating between them.

Why this happens is a matter of the muonium’s mass, which is roughly one ninth of a hydrogen atom's mass.

To return to his Heffalump analogy, Fleming suggested that the pit entrapping the creature has a ladder lowered into it, making the capture temporary, just like the vibrational bond. Fleming used the example of the ladder to explain quantum mechanics.

“Most ladders would have equal spacing [between rungs],” said Fleming. “That’s a little bit like the energy levels you can get in a molecule.”

By the laws of quantum mechanics, particles must jump between discrete energy levels -- just as the Heffalump must set its foot down on each rung of the ladder. There is no way to stop between rungs.

"The first rung is not right at the bottom,” said Fleming.

According to Fleming, in a quantum world, there is no such thing as zero energy. Instead, there exists a lowest energy, called the zero-point energy, which is represented by the first rung of the ladder.

“Even if you’re at zero degrees kelvin -- the lowest temperature you can get -- the molecules have to be moving, because otherwise, you’d know where they were … [and] that violates a fundamental principle in quantum mechanics called the uncertainty principle,” said Fleming.

The zero-point energy of an atom happens to depend on its mass. Muonium’s light mass gives it a high zero-point energy.

With this, the first rung of the Heffalump’s ladder is higher, making the climb back up the pit and out of the bond a difficult one -- explaining why it’s stuck the way it is.

The simple detail of the muonium’s mass was "the money figure," said Fleming. “The fact that you can change chemical bonding by changing the mass -- that’s pretty remarkable.”