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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. 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. True O4 does exist, however, as a stable red solid at pressures above 10 GPa. 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.
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, and it changes color from blue to deep red. Based on its infrared absorption spectrum, this phase is believed to consist of O4 molecules in a crystal lattice.
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 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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Tetraoxygen". A list of authors is available in Wikipedia.|