Stellar effort: New chemical bond revealed

Cosmic force

 
Sirius A and the white dwarf star
Sirius B (pinpoint lower left)

Bond is back. But, this time it's different, none of the positives and negatives, none of the sharing. In this stellar incarnation, bond is magnetic. A technical diversion from their regular studies has allowed scientists using quantum mechanics calculations to discover that the intense magnetic field around a type of fast-spinning, high-density star known as a white dwarf might be strong enough to distil a new type of chemistry.

The 100,000 tesla field around these objects, which are the size of the Earth but way as much as half a Sun, could lead to bonding types other than the well known covalent and ionic bonds that form in terrestrial molecules. Such is the magnetic charms of the white dwarf that the calculations, carried out by Trygve Helgaker at the University of Oslo in Norway, suggest that even the usually inert helium atom might be persuaded to react to form exotic, diatomic helium molecules.

The team used full configuration-interaction (FCI) to investigate the behaviour of hydrogen and helium atoms in a magnetic field that is almost 10,000 times the strength of an MRI magnet. They suggest that the field strength forces the molecule into a perpendicular orientation relative to the field. What further happens is that the triplet state (which is normally an excited dissociative state) becomes the ground state (and bounded). However, the magnetic field prevents the molecule from splitting so that although the electrons would normally not be in an energetically favourable state the two atoms nevertheless hold together through this novel bonding mode, which is neither covalent nor ionic.

Spectroscopically, magnetically bonded dihydrogen or the putative dihelium, will be very different from the terrestrial counterparts. This therefore raises the intriguing possibility of detecting magnetically bonded molecules in the atmospheres of white dwarves. Separate studies will be required to determine what effects the much more intense fields around neutron stars might have. "Around neutron stars, the magnetic fields are much stronger, so here the chemistry will be different again, and this we have not studied," Helgaker told SpectroscopyNOW. "Essentially, in this regime magnetism dominates and electrostatics become a perturbation - unlike around white dwarves, where magnetism and electrostatics are equally important."

The stellar magnetic field strengths being discussed are way beyond terrestrial magnets at the moment, but there is no reason to imagine that they will remain inaccessible to science in perpetuity. One day it might be possible to generate suitably strong fields on a more localised scale and so generate magnetic bonds in the laboratory, such molecules might be useful in a far-future quantum computer

"Our original aim was to develop more reliable density-functional (DFT) methods for calculations of molecular magnetic properties," Helgaker told us, "This project was a diversion from that project (which we are still pursuing)." He adds that, "For this project, we needed to be able to calculate molecular electronic structure in finite magnetic fields, which then gave us the tools to study chemistry in strong magnetic fields - which turned out to be very interesting and fascinating in itself, as a contrast to 'usual' chemistry." The next important step will be to study molecular dynamics in strong fields, including nuclear motion.

Helgaker works with Kai K. Lange, E. I. Tellgren and M. R. Hoffmann, Hoffmann is also based in the Chemistry Department, at the University of North Dakota, in Grand Forks.