Tetraoxygen
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- 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.<ref>Lewis, Gilbert N. (September 1924). "The Magnetism of Oxygen and the Molecule O2". Journal of the American Chemical Society 46 (9): 2027–2032. DOI:10.1021/ja01674a008.</ref> 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.<ref>Oda, Tatsuki, Alfredo Pasquarello (October 2004). "Noncollinear magnetism in liquid oxygen: A first-principles molecular dynamics study". Physical Review B 70 (134402): 1–19. DOI:10.1103/PhysRevB.70.134402.</ref> True O4 does exist, however, as a stable red solid at pressures above 10 GPa.<ref name="Gorelli">Gorelli, Federico A., Lorenzo Ulivi, Mario Santoro, and Roberto Bini (November 1999). "The ε Phase of Solid Oxygen: Evidence of an O4 Molecule Lattice". Physical Review Letters 83 (20): 4093–4096. DOI:10.1103/PhysRevLett.83.4093.</ref> 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.<ref name="Cacace">Cacace, Fulvio, Giulia de Petris, and Anna Troiani (October 2001). "Experimental Detection of Tetraoxygen". Angewandte Chemie International Edition 40 (21): 4062–4065. DOI:10.1002/1521-3773(20011105)40:21%3c4062::AID-ANIE4062%3e3.0.CO;2-X.</ref>
[edit] 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,<ref name="Akahama">Akahama, Yuichi, Haruki Kawamura, Daniel Häusermann, Michael Hanfland, and Osamu Shimomura (June 1995). "New High-Pressure Structural Transition of Oxygen at 96 GPa Associated with Metallization in a Molecular Solid". Physical Review Letters 74 (23): 4690–4694. DOI:10.1103/PhysRevLett.74.4690.</ref> and it changes color from blue to deep red.<ref>Nicol, Malcolm, K. R. Hirsch, and Wilfried B. Holzapfel (December 1979). "Oxygen Phase Equilibria near 298 K". Chemical Physics Letters 68 (1): 49–52.</ref> Based on its infrared absorption spectrum, this phase is believed to consist of O4 molecules in a crystal lattice.<ref name="Gorelli" />
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.<ref>Ball, Phillip. "New form of oxygen found", Nature News, 16 November 2001. Retrieved on 13 July 2006.</ref>
At 96 GPa, oxygen undergoes another phase transition and becomes metallic.<ref name="Akahama" />
[edit] Free molecule
Theoretical calculations have predicted the existence of metastable O4 molecules with two different shapes: a "puckered" square like cyclobutane,<ref>Hernández-Lamoneda, R., A. Ramírez-Solís (September 2000). "Reactivity and electronic states of O4 along minimum energy paths". Journal of Chemical Physics 113 (10): 4139–4145. DOI:10.1063/1.1288370.</ref> and a "pinwheel" with three oxygen atoms surrounding a central one.<ref>Røeggen, I., E. Wisløff Nilssen (May 1989). "Prediction of a metastable D3h form of tetra oxygen". Chemical Physics Letters 157 (5): 409–414.</ref>
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.<ref name="Cacace" /> 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.
[edit] See also
- Ozone (O3)
