Tetraoxygen

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.^[1] Today it seems Lewis was off, but not by much: 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] In 1999, researchers thought that solid oxygen existed in its ε-phase (at pressures above 10 GPa) as O₄.^[3] However, in 2006, it was shown by X-ray crystallography that this stable phase known as ε oxygen or red oxygen is in fact O₈.^[4] Nevertheless, tetraoxygen has been detected as a short-lived chemical species in mass spectrometry experiments.^[5]

Absorption bands of the O₄ molecule e.g. at 360, 477 and 577 nm are frequently used to do aerosol inversions in atmospheric optical absorption spectroscopy. Due to the known distribution of O₂ and therefore also O₄, O₄ slant column densities can be used to retrieve aerosol profiles which can then be used again in radiative transfer models to model light paths.^[6]

Free molecule

Theoretical calculations have predicted the existence of metastable O₄ molecules with two different shapes: a "puckered" square like cyclobutane,^[7] and a "pinwheel" with three oxygen atoms surrounding a central one in a trigonal planar formation similar to boron trifluoride.^[8][9] It was previously pointed out that the "pinwheel" O₄ molecule should be the natural continuation of the isoelectronic series BO₃^3-, CO₃^2-, NO₃^-,^[10] and analogous to SO₃; that observation served as the basis for the mentioned theoretical calculations.

Theoretical structures of metastable O4.
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.^[5] 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.

References

1. ^ Lewis, Gilbert N. (1924). "The Magnetism of Oxygen and the Molecule O₂". Journal of the American Chemical Society 46 (9): 2027–2032. doi:10.1021/ja01674a008. 
2. ^ Oda, Tatsuki; Alfredo Pasquarello (2004). "Noncollinear magnetism in liquid oxygen: A first-principles molecular dynamics study". Physical Review B 70 (134402): 1–19. Bibcode 2004PhRvB..70m4402O. doi:10.1103/PhysRevB.70.134402. 
3. ^ Gorelli, Federico A.; Lorenzo Ulivi, Mario Santoro, and Roberto Bini (1999). "The ε Phase of Solid Oxygen: Evidence of an O₄ Molecule Lattice". Physical Review Letters 83 (20): 4093–4096. Bibcode 1999PhRvL..83.4093G. doi:10.1103/PhysRevLett.83.4093. 
4. ^ Lars F. Lundegaard, Gunnar Weck, Malcolm I. McMahon, Serge Desgreniers and Paul Loubeyre (2006). "Observation of an O8 molecular lattice in the phase of solid oxygen". Nature 443 (7108): 201–204. doi:10.1038/nature05174. PMID 16971946. 
5. ^ ^{a b} Cacace, Fulvio; Giulia de Petris, and Anna Troiani (2001). "Experimental Detection of Tetraoxygen". Angewandte Chemie International Edition 40 (21): 4062–4065. doi:10.1002/1521-3773(20011105)40:21<4062::AID-ANIE4062>3.0.CO;2-X. PMID 12404493. 
6. ^ Friess, U. and Monks, P. S. and Remedios, J. J. and Wagner, T. and Platt, U. (2005). "MAX-DOAS O4 measurements: A new technique to derive information on atmospheric aerosols - Retrieval of aerosol properties". Journal of Geosphysical Research. 
7. ^ Hernández-Lamoneda, R.; A. Ramírez-Solís (2000). "Reactivity and electronic states of O₄ along minimum energy paths". Journal of Chemical Physics 113 (10): 4139–4145. doi:10.1063/1.1288370. 
8. ^ Røeggen, I.; E. Wisløff Nilssen (1989). "Prediction of a metastable D_3h form of tetra oxygen". Chemical Physics Letters 157 (5): 409–414. doi:10.1016/0009-2614(89)87272-0. 
9. ^ Hotokka,M.; P.Pyykkö (May 1989). "Ab initio study of bonding trends in the series BO_3^3-, CO_3^2-, NO_3^- and O₄(D_3h)". Chemical Physics Letters 157(5):415-418.
10. ^ Jubert,A.H.; E.L.Varetti (1986). "On the possible existence of the O₄ molecule with D_3h symmetry". Anales de Química (Spain)82:227-230.

See also

• Tetranitrogen (N₄)
• Solid oxygen
• Liquid oxygen
• Oxygen