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Phosphorus monoxide is an unstable
radical Radical may refer to: Politics and ideology Politics *Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...
inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...
with molecular formula P O. Phosphorus monoxide is notable as one of the few molecular compounds containing phosphorus that has been detected outside of Earth. Other phosphorus containing molecules found in space include PN, PC, PC2, HCP and PH3. It was detected in the
circumstellar shell A circumstellar envelope (CSE) is a part of a star that has a roughly spherical shape and is not gravitationally bound to the star core. Usually circumstellar envelopes are formed from the dense stellar wind, or they are present before the formatio ...
of VY Canis Majoris and in the star forming region catalogued as AFGL 5142. The compound has been found to have been initially produced in star-forming regions, and speculated to be carried by interstellar comets throughout outer space, including to the early Earth. Phosphorus monoxide plays a role in the phosphorescence of phosphorus.


Discovery

In 1894 W. N. Hartley was the first to report an observation of ultraviolet emission from a phosphorus compound, that was later expanded on by Geuter. The source of the spectral lines and bands were known to be related to phosphorus, but the exact nature was unknown. In 1927 H. J. Emeléus and R. H. Purcell determined that the cause was a phosphorus oxide. But it was in 1921 that P. N. Ghosh and G. N. Ball determined that the oxide was phosphorus monoxide. Phosphorus monoxide is believed to be the most abundant phosphorus-containing molecule found in interstellar clouds. Phosphorus was identified as a cosmically abundant element in 1998 after researchers found a cosmic ratio of phosphorus to hydrogen (P/H) of about 3×10−7. Even with the prevalence of phosphorus in interstellar clouds, very few phosphorus bearing molecules had been identified and found in very few sources; phosphorus nitride, PN, and the free radical CP were found in a carbon rich envelope of IRC +10215 in 1987. This suggested that more phosphorus containing molecules had to be found in interstellar space. While examining the oxygen-rich shell of the supergiant star VY Canis Majoris (VY CMa) the presence of PO was detected. VY CMa was studied using the
Submillimeter Telescope Submillimetre astronomy or submillimeter astronomy (see spelling differences) is the branch of observational astronomy that is conducted at submillimetre wavelengths (i.e., terahertz radiation) of the electromagnetic spectrum. Astronomers plac ...
(SMT) of the Arizona Radio Observatory (ARO). The telescope was able to observe the rotational frequencies of PO. ARO's 10 m SMT was able to measure the rotational transitions of PO showing J=5.5→4.5 at 240 GHz and J=6.5→5.5 at 284 GHz toward the evolved star, each consisting of well-defined lambda-doublets. Since the detection of PO towards the envelope of the VY CMa supergiant in 2001, PO has been found in many more interstellar clouds and is found in abundance around oxygen-rich shells.


Formation

PO is formed when phosphorus is burnt in oxygen or ozone. It is a transient molecule observed in hot flames, or can be condensed into noble gas matrix. PO can be formed in an inert gas matrix in the
photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...
of P4S3O, a
phosphorus oxysulfide Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Earth ...
. On Earth, phosphorus monoxide can be prepared for study by spraying phosphoric acid into a flame. Because commercial
acetylene Acetylene (systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure ...
gas contains some phosphine, an oxy-acetylene flame will have weak PO emission bands in its spectrum also. In the flame, PO oxidises back to P4O10.


Reactions


Phosphorescence

As white phosphorus oxidises it gives out a greenish-white glow. The glow happens as PO is oxidised by one of these reactions: PO + O → PO2; or PO + O2 →PO2 + O. The possible ways that PO appears in this process is by breakup of the P2O molecule which in turn may come from P4O.


Ligand

Phosphorus monoxide can act as a ligand on transition elements such as
molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...
, ruthenium and
osmium Osmium (from Greek grc, ὀσμή, osme, smell, label=none) is a chemical element with the symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mos ...
. The phosphorus forms a triple bond with the metal. The first to be discovered was on a nickel- tungsten cluster. The WNi2P2 cluster was oxidised by a peroxide to yield a μ3-coordination, where each phosphorus atom is bound to three metal atoms.


Properties


Bond

Phosphorus monoxide is a free radical with phosphorus double bonded to oxygen with phosphorus having an unpaired valence electron. The
bond order In chemistry, bond order, as introduced by Linus Pauling, is defined as the difference between the number of bonds and anti-bonds. The bond order itself is the number of electron pairs (covalent bonds) between two atoms. For example, in diat ...
is about 1.8. The P=O bond in PO has a dissociation energy of 6.4 eV. The bond length of the PO double bond is 1.476 Å, and free PO shows an infrared vibrational frequency of 1220 cm−1 due to the stretching of the bond. The free radical nature of PO makes it highly reactive and unstable compared to other phosphorus oxides that have been further oxidized.


Spectrum

The visible to ultraviolet spectrum of phosphorus monoxide has three important bands. There is a continuum band near 540 nm. The β-system near 324 nm is due to the D2Σ→2Π transition. The γ-system has bands near 246 nm due to a A2Σ→2Π transition. Peaks in this band occur at 230, 238, 246, 253, and 260 nm in the ultraviolet. All these bands can be emission, absorption, or fluorescence depending on the method of illumination and temperature. There is also a C'2Δ state. The γ-system band can be broken down into sub-bands based on the different vibrational transitions. (0,0), (0,1) and (1,0) are designations for the sub-bands produced by the transition between two vibration states, as the electronic transition occurs. Each of these contains eight series termed branches. These are oP12, P2, Q2, R2, P1, Q1, R1 and sR21.


Molecule

The ionisation potential of PO is 8.39 eV. When ionised, PO forms the cation PO+. The adiabatic electron affinity of PO is 1.09 eV. On gaining an electron the PO ion forms. re in the ground state is 1.4763735 Å. The dipole moment of the molecule is 1.88 D. The phosphorus atom has a slight positive charge calculated as 0.35 of the electron.


See also

*
Nitric oxide Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its che ...
*
Phosphorus mononitride Phosphorus mononitride is an inorganic compound with the chemical formula P N. Containing only phosphorus and nitrogen, this material is classified as a binary nitride. It is the first identified phosphorus compound in the interstellar medium. ...


References

{{portal bar, Astronomy, Biology, Chemistry, Science Phosphorus oxides Free radicals