Allotropes of oxygen
There are several allotropic forms of oxygen.
Oxygen is a typical non-metallic. It is in the gaseous state from individual molecules. The element forms several allotropes, which are to be distinguished according to the number of oxygen atoms. The most common form of oxygen is the dioxygen O2. Other allotropes are ozone O3 and the rare Tetra allotropes oxygen O4 and O8 Oktasauerstoff.
High-pressure phases of oxygen
At room temperature and pressures greater than 10 GPa oxygen converts, consisting of O2 molecules in a red solid order. This phase, which is apparently another allotrope next to dioxygen (O2) and ozone (O3) is also known as ε - oxygen or oxygen- red. It was originally thought because of infrared spectroscopic investigations in 1999, that this is Tetra oxygen O4. However, crystallographic analysis of the red oxygen from 2006 suggest novel O8 units, but differ from the S8 rings of sulfur. At 4800 ° C and 96 GPa, a phase transition occurs for ζ oxygen, which is electrically conductive and likely to be present in these metallic form in the interior of giant planets Jupiter and Saturn.
Tetra oxygen
Since 1911 there were first indications of the existence of O4, which was also referred to as Oxozon and was possibly in the ozone production by Carl Dietrich Harries. The then observed additions of four oxygen atoms at individual double bonds of organic compounds suggested the temporary presence of O4. Also Tetra oxygen was predicted in 1924 by Gilbert Newton Lewis in order to thus explain the failure of the Curie 's law for liquid oxygen can. In fact O4 aggregates were detected already in liquid oxygen. But these are to be regarded as (O2) 2, which are composed of two molecules, and O2 mol are a dissociation energy of 0.54 kJ / very unstable.
By means of mass spectrometry could be 2001 Tetra oxygen detected indirectly. Initially O2 molecules and positively charged O2 - ions were combined to O4 ions, whose existence could show the mass spectrometry. Subsequently, the ions were converted by absorption of electrons in neutral O4 molecules but could not be directly detected in the mass spectrometer. But since after a reionization again the O4 ions occurred in the meantime also stable, neutral O4 molecules must have existed. Theoretical calculations were talking so far for either a triangle of oxygen atoms with a fourth atom in the center, or for a diamond-shaped molecule. The results presented also suggest that the four oxygen atoms could form two dumbbell-shaped O2 molecules with loose binding to each other. The aggregation of the two O2 molecules is based on the occupation of two molecular orbitals (MO), which arose next two antibonding MOs of four degenerate π * orbitals and occupied by two pairs of electrons with antiparallel spins are (HOMO and HOMO- 1).
A possible technical applications due to the high energy density of O4 could be the use as a component in rocket fuels. It is expected that the combustion of hydrogen or hydrocarbons, is even more effective than liquid O2.
In the remote sensing Differential optical absorption spectroscopy exploits the fact that the collision complex O4 proportional occurs to the square of known oxygen concentration. On the Absorption structures of O4, it is then possible to draw conclusions on atmospheric properties.
Oktasauerstoff
Based on infrared spectroscopic measurements has been adopted since 1999 that they are Tetra oxygen molecules, ie O4 ε - oxygen. However, recent crystallographic studies have demonstrated the existence of O8 - aggregates in the ε - oxygen. The O8 structure but differs from the crown-shaped S8- rings of sulfur and resembles a slightly compressed cubes, sit with the two oxygen atoms at the four shorter edges. The bonds in the O2 units are 120 picometers (pm ) and thus correspond to the bond length of molecular oxygen under normal conditions ( 121 pm ), while the distances in the O4 -rings with 219 pm significantly smaller than the van der Waals distance are 304 pm for oxygen. The short distances of about 260 pm between the O8 - blocks indicate binding interactions.