Krypton difluoride
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Krypton difluoride, KrF2, was the first compound of krypton discovered.[1] 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.[2]
Synthesis[edit | edit source]
Krypton difluoride can be synthesized using many different methods including electrical discharge, photochemical, irradiation, hot wire and proton bombardment.
Electrical discharge[edit | edit source]
The first method used to make krypton difluoride and the only one ever reported to produce krypton tetrafluoride was the electrical discharge method.[3] 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.[3] Rates of almost 0.25g/h can be achieved.[4] The problem with this method is that it is unreliable with respect to yield.
Proton Bombardment[edit | edit source]
Using proton bombardment for the production of KrF2 has a maximum production rate of about 1g/h.[3] 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.[3] 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.[3]
Photochemical[edit | edit source]
The photochemical process for the production of KrF2 involves the use of UV light and can produce under ideal circumstances 1.22g/h.[3] The ideal wavelengths to use are in the range of 303-313nm.[3] It is important to note that harder UV radiation is detrimental to the production of KrF2.[4] 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.[4] 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.[4] 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 Kr is a solid and Fluorine is a liquid which occur at 77K.[4] 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.[3]
Hot Wire[edit | edit source]
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.[4] The wire has a large current, causing it to reach temperatures around 680C.[3] This causes the fluorine gas to split into its radicals which then can react with the solid krypton.[3] Under ideal conditions, it has been known to reach a maximum yield of 6g/h.[4] 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.[4] 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.[3]
Cystallographic Morphologies[edit | edit source]
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.[3] The unit cell of α-KrF2 is body centred tetragonal.
Related compounds[edit | edit source]
- Xenon difluoride, XeF2
References[edit | edit source]
- ↑ 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.
- ↑ 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
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 Lehmann, John. F.; Mercier, Hélène P.A.; Schrobilgen, Gary J. The chemistry of Krypton. Coordination Chemistry Reviews. 2002, 233-234, 1-39
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 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.
General reading[edit | edit source]
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