Tetraoxygen

Additional recommended knowledge Essential Laboratory Skills Guide What is the Correct Way to Check Repeatability in Balances? How to quickly check pipettes? Contents 1 Red oxygen 2 Free molecule 3 See also 4 References

O4 is also a subclass of O-class stars.

The tetraoxygen molecule (O₄) was first predicted in 1924 by Gilbert N. Lewis, who proposed it as an explanation for the failure of liquid oxygen to obey Curie's law.^[1] Today it seems Lewis was off, but not by far: computer simulations indicate that although there are no stable O₄ molecules in liquid oxygen, O₂ molecules do tend to associate in pairs with antiparallel spins, forming transient O₄ units.^[2] True O₄ does exist, however, as a stable red solid at pressures above 10 GPa.^[3] This phase is known as ε oxygen or red oxygen. Tetraoxygen has also been detected as a short-lived chemical species in mass spectrometry experiments.^[4]

Red oxygen

As the pressure of oxygen at room temperature is increased through 10 GPa, it undergoes a dramatic phase transition to a different allotrope. Its volume decreases significantly,^[5] and it changes color from blue to deep red.^[6] Based on its infrared absorption spectrum, this phase is believed to consist of O₄ molecules in a crystal lattice.^[3]

Liquid oxygen is already used as a rocket fuel, and it has been speculated that red oxygen could make an even better fuel, because of its higher energy density.^[7]

At 96 GPa, oxygen undergoes another phase transition and becomes metallic.^[5]

Free molecule

Theoretical calculations have predicted the existence of metastable O₄ molecules with two different shapes: a "puckered" square like cyclobutane,^[8] and a "pinwheel" with three oxygen atoms surrounding a central one.^[9]

In 2001, a team at the University of Rome La Sapienza conducted a neutralization-reionization mass spectrometry experiment to investigate the structure of free O₄ molecules.^[4] Their results did not agree with either of the two proposed molecular structures, but they did agree with a complex between two O₂ molecules, one in the ground state and the other in a specific excited state.

See also

• Ozone (O₃)
• Tetranitrogen (N₄)

References