Krypton difluoride

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Name = Krypton difluoride
ImageFile = Krypton-difluoride-2D-dimensions.png ImageName = Krypton difluoride
ImageFile1 = Krypton-difluoride-3D-vdW.png ImageName1 = Krypton difluoride
IUPACName = krypton(II) fluoride
OtherNames = krypton difluoride, krypton fluoride
Section1 = Chembox Identifiers
CASNo = 13773-81-4
RTECS =
Section2 = Chembox Properties
Formula = KrF₂
MolarMass = 121.7968 g mol⁻¹
Appearance = colourless solidpp. 442–443, Handbook of Inorganic Chemicals, Pradyot Patnaik, McGraw-Hill Professional, 2003. ISBN 0070494398.]
Density = 3.24 g/cm³, solid
Solubility =
MeltingPt =
BoilingPt =
Section3 = Chembox Structure
MolShape = linear
CrystalStruct = Body Centred Tetragonal
Dipole = 0 D
Section7 = Chembox Hazards
MainHazards =
FlashPt =
RPhrases =
SPhrases =
Section8 = Chembox Related
OtherCpds = Kr(OTeF₅)₂; XeF₂

Krypton difluoride, KrF₂, was the first compound of krypton discovered. [Grosse, A. V.; Kirschenbaum, A. D.; Streng, A. G.; Streng, L. V. "Krypton Tetrafluoride: Preparation and Some Properties" Science, 1963, volume 139, pages 1047-1048. doi|10.1126/science.139.3559.1047.] It is a volatile, colourless solid. The structure of the KrF₂ molecule is linear, with Kr−F distances of 188.9 pm. It reacts with strong Lewis acids to form salts of the KrF⁺ and Kr₂F₃⁺ cations. [Lehmann, J. F.; Dixon, D. A.; Schrobilgen, G. J. "X-ray Crystal Structures of α-KrF₂, [KrF] [MF₆] (M = As, Sb, Bi), [Kr₂F₃] [SbF₆] ^.KrF₂, [Kr₂F₃] ₂ [SbF₆] ₂^.KrF₂, and [Kr₂F₃] [AsF₆] ^. [KrF] [AsF₆] ; Synthesis and Characterization of [Kr₂F₃] [PF₆] ^.nKrF₂; and Theoretical Studies of KrF₂, KrF⁺, Kr₂F₃⁺, and the [KrF] [MF₆] (M = P, As, Sb, Bi) Ion Pairs” Inorganic Chemistry 2001, volume 40, pages 3002-3017. doi|10.1021/ic001167w]

ynthesis

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

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.Lehmann, John. F.; Mercier, Hélène P.A.; Schrobilgen, Gary J. The chemistry of Krypton. Coordination Chemistry Reviews. 2002, 233-234, 1-39] 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 0.25g/h can be achieved.Kinkead, S. A.; Fitzpatrick, J. R.; Foropoulos, J. Jr.; Kissane, R. J.; Purson, D. Photochemical and thermal Dissociation Synthesis of Krypton Difluoride. Inorganic Fluorine Chemistry: Toward the 21st Century, Thrasher, Joseph S.; Strauss, Steven H.: American Chemical Society. San Francisco, California, 1994. 40-54.] 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 1g/h. This is achieved by bombarding mixtures of Kr and F₂ with a proton beam that is operating at an energy level of 10MeV and at a temperature of about 133K. It is a fast method of producing relatively large amounts of KrF₂, it runs into difficulties in that it requires a source of α-particles which usually would come from a cyclotron.

Photochemical

The photochemical process for the production of KrF₂ involves the use of UV light and can produce under ideal circumstances 1.22g/h. The ideal wavelengths to use are in the range of 303-313nm. 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 170nm) produced on average 158mg/h, Vycor 7913 (UV cut off of 210nm) produced on average 204mg/h and Pyrex 7740 (UV cut off of 280nm) produced on average 507mg/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 680C. 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 6g/h. In order to achieve optimal yields the gap between the wire and the solid krypton should be 1cm, giving rise to a temperature gradient of about 900C/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.

Cystallographic Morphologies

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

Related compounds

* Xenon difluoride, XeF₂

References

General reading

*

External links

* [http://webbook.nist.gov/cgi/cbook.cgi?Units=SI&cTG=on&cIR=on&cTC=on&cMS=on&cTP=on&cES=on&cTR=on&cPI=on&cDI=on&ID=C13773814 NIST Chemistry WebBook: krypton difluoride]