Krypton difluoride, KrF₂, was the first compound

Chemical compound

A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 of krypton

Krypton

Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...

 discovered. It is a volatile

Volatility (chemistry)

In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...

, colourless solid. The structure of the KrF₂ molecule is linear, with Kr−F distances of 188.9 pm. It reacts with strong Lewis acid

Lewis acid

]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...

s to form salts of the KrF⁺ and Kr₂F₃⁺ cations.

Synthesis

Krypton difluoride can be synthesized using many different methods including electrical discharge, photochemical, hot wire, and proton bombardment.

It can also be prepared by irradiating krypton with ultraviolet rays in a fluorine-argon gas mixture at liquid helium temperature. The product can be stored at −78 °C without decomposition.

Electrical discharge

The first method used to make krypton difluoride and the only one ever reported to produce krypton tetrafluoride was the electrical discharge method. The electrical discharge method involves having 1:1 to 2:1 mixtures of F₂ to Kr at a pressure of 40 to 60 torr and then arcing large amounts of energy between it. Rates of almost 0.25 g/h can be achieved. The problem with this method is that it is unreliable with respect to yield.

Proton bombardment

Using proton bombardment for the production of KrF₂ has a maximum production rate of about 1 g/h. This is achieved by bombarding mixtures of Kr and F₂ with a proton beam that is operating at an energy level of 10 MeV and at a temperature of about 133 K. It is a fast method of producing relatively large amounts of KrF₂, but requires a source of α-particles which usually would come from a cyclotron

Cyclotron

In technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...

.

Photochemical

The photochemical process for the production of KrF₂ involves the use of UV light and can produce under ideal circumstances 1.22 g/h. The ideal wavelengths to use are in the range of 303-313 nm. It is important to note that harder UV radiation is detrimental to the production of KrF₂. In order to avoid the harder wavelengths, simply using Pyrex glass or Vycor or quartz will significantly increase yield because they all block harder UV light. In a series of experiments performed by S. A Kinkead et al., is was shown that a quartz insert (UV cut off of 170 nm) produced on average 158 mg/h, Vycor 7913 (UV cut off of 210 nm) produced on average 204 mg/h and Pyrex 7740 (UV cut off of 280 nm) produced on average 507 mg/h. It is clear from these results that higher energy ultra violet light reduces the yield significantly. The ideal circumstances for the production KrF₂ by a photochemical process appear to occur when krypton is a solid and fluorine is a liquid which occur at 77K. The biggest problem with this method is that is requires the handling of liquid F₂ and the potential of it being released if it becomes over pressurized.

Hot wire

The hot wire method for the production of KrF₂ involves having the krypton in a solid state with a hot wire running a few centimeters away from it as fluorine gas is then run past the wire. The wire has a large current, causing it to reach temperatures around 680 °C. This causes the fluorine gas to split into its radicals which then can react with the solid krypton. Under ideal conditions, it has been known to reach a maximum yield of 6 g/h. In order to achieve optimal yields the gap between the wire and the solid krypton should be 1 cm, giving rise to a temperature gradient of about 900 °C/cm. The only major downside to this method is the amount of electricity that has to be passed through the wire thus making it dangerous if not properly set up.

Structure

Krypton difluoride can exist in one of two possible crystallographic morphologies: α-phase and β-phase. β-KrF₂ generally exists at above −80 °C, while the α-KrF₂ is more stable at lower temperatures. The unit cell of α-KrF₂ is body-centred tetragonal.

Chemistry

Krypton difluoride is primarily a powerful oxidising and fluorinating agent. It can oxidise gold

Gold

Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

 to its highest-known oxidation state, +5:

7 (g) + 2 Au (s) → 2 KrFAuF (s) + 5 Kr (g)

KrFAuF decomposes at 60°C into gold(V) fluoride

Gold(V) fluoride

Gold fluoride is the inorganic compound with the formula Au2F10. This fluoride compound features gold in its highest known oxidation state. This red solid dissolves in hydrogen fluoride but these solutions decompose, liberating fluorine....

 and krypton and fluorine gases:

KrFAuF → (s) + Kr (g) + (g)

can also directly oxidise xenon

Xenon

Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...

 to xenon hexafluoride

Xenon hexafluoride

Xenon hexafluoride is a noble gas compound with the formula XeF6 and the highest of the three binary fluorides of xenon, the other two being XeF2 and XeF4. All are exergonic and stable at normal temperatures. XeF6 is the strongest fluorinating agent of the series...

:

3 + Xe → + 3 Kr

is used to synthesize the highly reactive BrF cation. reacts with to form the salt KrFSbF; the KrF cation is capable of oxidising both

Bromine pentafluoride

Bromine pentafluoride, BrF5, is an interhalogen compound and a fluoride of bromine. It is a strong fluorination reagent.It melts at −61.30 °C and boils at 40.25 °C. BrF5 finds use in oxygen isotope analysis. Laser ablation of solid silicates in the presence of bromine pentafluoride releases O2 for...

 and

Chlorine pentafluoride

Chlorine pentafluoride is an interhalogen compound with formula ClF5. It was first synthesized in 1963.Its square pyramidal structure with C4v symmetry was confirmed by its high resolution19F NMR spectrum.-Preparation:...

to BrF and ClF, respectively.

External links

• NIST Chemistry WebBook: krypton difluoride