Chemical bond discovered that only exists in space
There's a new bond in town, and this secret agent works best in extreme situations.
The bond, of the chemical variety, occurs in the presence of very strong magnetic fields, such as those found around ultra-dense white dwarf stars. Its discovery not only demonstrates the existence of an unfamiliar and exotic type of chemistry, it may also give insight into the behaviours of these mysterious stellar bodies.
White dwarfs are the remnant cores of low-mass stars that have exhausted all their fuel. They are thought to be the final state for most of the stars in our galaxy. Though they have masses comparable to that of our sun, white dwarfs only occupy the same amount of space as a small planet like Earth, making them incredibly dense.
They also exhibit super-strong magnetic fields on the order of 100,000 tesla – 10 billion times greater than Earth's magnetic field, and 10 million times greater than that of an average refrigerator magnet. This intense field can affect the behaviour of the electrons that make up chemical bonds.
On Earth, atoms usually bond either covalently, by sharing electrons with neighbouring atoms, or ionically, via electrostatic attractions created by the transferral of electrons.
The electrons that give rise to these bonds are governed by the Pauli exclusion principle: two cannot occupy the same quantum state simultaneously. To avoid this scenario, electrons in bonds normally pair up in couples of opposing spin. But under the intense magnetic field of a white dwarf, "this spin interacts with the external field, acting like a little magnet," says lead author Kai Lange at the University of Oslo in Norway.
As a result, the spins of both electrons align with the external field, forcing one of the electrons to move into a different position known as an anti-bonding orbital. Normally, this would spell the end of any chemical bonds. "In a normal molecule these anti-bonding orbitals are not occupied by electrons," says Lange. "If they are occupied, the atoms are no longer bound together and the molecule breaks apart."
Lange and his colleagues wondered if things might be different around white dwarfs. "Chemistry and molecular physics become very different in the presence of a strong magnetic field," says Erik Tellgren, Lange's colleague. "Even very simple systems behave in exotic and unfamiliar ways compared to what we are used to under normal conditions."
With this in mind, the researchers used quantum chemical simulations to model chemical bonding in hydrogen and helium atoms in the magnetic field of a white dwarf. In both cases, the atoms were drawn into strongly bonded pairs.
Because the electrons in these bonded atoms occupied anti-bonding orbitals – which is forbidden in both types of known chemical bond – the researchers say this is a new type of bond. They have dubbed it "perpendicular paramagnetic bonding".
The work shows that "molecules that don't exist under normal conditions can exist in a sufficiently large magnetic field," says Lange.
David Clary of the University of Oxford, who was not involved in the study, called the research "excellent", adding that "the results show that a magnetic field can stabilise molecules".
Reading the stars
Although the authors say that replicating the new bonds on Earth isn't feasible, the finding highlights how molecular chemistry may change in the presence of extreme conditions.
"I think there are probably other weird or unfamiliar types of bonding to be discovered," says Tellgren.
Such work will also help to further our knowledge of astrophysical objects like white dwarfs. By understanding how matter behaves around these objects, it may be possible to interpret their observed spectra more easily and accurately, and to better unravel what is happening in their atmospheres.
Journal reference: Science, DOI: 10.1126/science.1219703