O4 is also a subclass of O-class stars.
The tetraoxygen molecule (O4), also called oxozone 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. 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. In 1999, researchers thought that solid oxygen existed in its ε-phase (at pressures above 10 GPa) as O4. However, in 2006, it was shown by X-ray crystallography that this stable phase known as ε oxygen or red oxygen is in fact . Nevertheless, tetraoxygen has been detected as a short-lived chemical species in mass spectrometry experiments.

Free molecule

Theoretical calculations have predicted the existence of metastable O4 molecules with two different shapes: a "puckered" square like cyclobutane, and a "pinwheel" with three oxygen atoms surrounding a central one in a trigonal planar formation similar to Boron Trifloride.

D2d structure
D3h structure

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. 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.


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