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OXYGEN is an incredibly stable molecule, and will withstand pressures of up to 1.9 terapascal – some 19m times atmospheric pressure – before breaking up and polymerising.

The value, calculated by a collaborative team of researchers which used computer simulations to model how the molecule would cope with rising pressure, is almost a full order of magnitude higher than previous estimates which suggested that oxygen molecules would be stable to at least 250 GPa.

At 1.9 TPa, the models predict that the oxygen molecules will break up to form a spiral chain-shaped polymer, which changes first into a zig-zag chain and finally a planar phase if the pressure is raised even further.

“Our study allows us to gain a much better understanding of the polymerisation of molecules under pressure,” lead researcher, Jian Sun of Germany’s Ruhr Universitaet Bochum’s department of theoretical chemistry, tells tce. He adds that this pressure-resistance is very unusual, as other simple molecules such as nitrogen or hydrogen break up at much lower pressures of 110 GPa and 400 GPa respectively. It’s also counter-intuitive, considering that nitrogen molecules have triple covalent bonds, which one would expect to be stronger than oxygen’s double bonds, he adds.

The team notes that the behaviour of oxygen with increasing pressure is very complicated. Its electrical conductivity first increases, then decreases, and finally increases again. However, in the condensed phase when pressure increases, the molecules become closer to each other. They suggest that, under these conditions, the electron lone pairs on different molecules repel one another strongly, thus hindering the molecules from approaching each other. Since oxygen has more lone pairs than nitrogen, the repulsive force between these molecules is stronger, which makes polymerisation more difficult. However, the number of lone pairs cannot be the only determinant of the polymerisation pressure. “We believe that it is a combination of the number of lone pairs and the strength of the bonds between the atoms,” says Sun.

The team also included researchers from University College London and Cambridge University in the UK, and the National Research Council of Canada.

Physical Review Letters, doi: 10.1103/PhysRevLett.108.045503