Boron monofluoride monoxide explained

Boron monofluoride monoxide or oxoboryl fluoride[1] or fluoroxoborane is an unstable inorganic molecular substance with formula FBO. It is also called boron fluoride oxide, or fluoro-oxoborane. The molecule is stable at high temperatures, but below 1000 °C condenses to a trimer (BOF)3 called trifluoroboroxin. FBO can be isolated as a triatomic non-metallic molecule in an inert gas matrix, and has been condensed in solid neon and argon.[2] When an attempt is made to condense the gas to a solid in bulk, a polymeric glass is formed, which is deficient in fluoride, and when heated forms a glassy froth like popcorn.[3] Boron fluoride oxide has been studied because of its production in high energy rocket fuels that contain boron and fluorine, and in the form of an oxyfluoride glass. BOF glass is unusual in that it can condense directly from gas.[4]

Properties

Monomer

The FBO molecule is linear with structure F-B=O. The F-B bond length is 1.283 Å, and B-O bond is 1.207 Å.[5]

The infrared spectrum of BFO has vibrational bands at 1900, 1050, and 500 cm−1.Spectroscopic constants of the 10BFO molecule are B=9349.2711 MHz D=3.5335 kHz and for 11BFO molecule they are B=9347.3843 MHz D=3.5273 kHz[6] The monomer is stable either at low pressures, or temperatures over 1000 °C. Below this temperature, the monomers associate to form a trimer[7] called trifluoroboroxole.[8]

Heat of formation ΔH is predicted to be -146.1 kcal/mol. Proton affinity 149.6 kcal/mol.[9]

Trimer

If a hot BFO gas is cooled slowly it dismutates back into B2O3 and BF3.[10] At room temperature this dismutation completes in an hour.[10]

Boron fluoride oxide forms a trimer with a ring composed of alternating oxygen and boron atoms, with fluorine bonded to the boron. (BFO)3. The ring structure puts it in the class of boroxols.[11] This is also called trifluoroboroxin. The trimer is the predominant form in gas at 1000K.[10] When heated to 1200K it mostly converts to the monomer BFO.[10] Boron oxyfluoride can be condensed from vapour to a fluorine deficient glass at temperatures below 190° by very rapid cooling. When heated this deposit has a temperature at which it loses more BF3 to form a frothy or porous glass that resembles popcorn. The glass deposited at lower temperatures has a higher proportion of fluorine. Deposits at -40 °C are predicted to have a 1:1 ratio of fluorine to oxygen.[12] Below -135° (BFO)3 is stable.[13]

The heat of formation of the trimer from the monomer (BFO)3 → 3BFO is 131 kcal/mol.[14]

Glass

Boron oxyfluoride glass is transparent and colourless. It is stable in dry air, but it is hygroscopic and in normal air becomes white and opaque. When heated the glass will encounter a glass transition temperature (Tg) at which it ceases to be a glass, and produces BF3 gas and a boron oxyfluoride with less fluorine is left behind. This glass transition temperature is determined from where the pressure of BF3 produced exceeds the strength of the glass. The hypothetical structure of BOF glass, is of long chains of B-O-B-O with fluorine attached to each boron. These can be considered as BO2F triangles linked in a chain by O atoms. These chains are tangled up like spaghetti in the glass. When the substance becomes fluorine deficient, crosslinks with oxygen form between the chains, and it becomes more two dimensional in structure.[15] BF3 is produced when the terminals of two linear chains join with each other.[4] These ends contain -O-BF2, and when two meet, BF3 can be eliminated and the chain extended with oxygen.[16]

Occurrence

BFO is expected to form in supernovae II output in gas between 1,000 and 2,000 °C and pressures around 10−7 bar.[17]

Preparation

Otto Ruff noticed that a mixture of BF3 and SiF4 passing over molten B2O3 produced some SiO2 and redistributed B2O3 into cold parts of the reaction tube. He speculated that there must be some heat stable intermediate that converted back into the original components on cooling.[18] Several years later, Paul Baumgarten and Werner Bruns made the boron oxyfluoride trimer by passing BF3 over solid B2O3 at 450 °C.[19] [20]

BFO is an intermediate in the hydrolysis of BF3 along with BF(OH)2, BF2OH and boric acid.

Another way in which BFO can be made is to vapourise B2O3 with BF3.[12]

When BF3 is heated with air, BFO gas predominates from 2800° to 4000 °C, being a maximum at 3200°. Above 4000 °C BO dominates.[22]

Hot BF3 passed over some oxides such as SiO2 forms BFO.[23] Other oxides that can yield boron oxyfluoride are magnesium oxide, titanium dioxide, carbonates or alumina.[24]

In the plasma phase HF reacts with BO2H, B2OH+, B3O, B2O, B2O, B2OH+ to make FBO, and other products including FBOH and FBO+.[25]

Related

The B-O-F molecule theoretically exists but it releases energy when it rearranges to F-B-O.[26] [27] A related molecule is BOF2.[28] Molecules related to the trimer include B3O3ClF2, B3O3Cl2F, and (BOCl)3.[29]

FBO is predicted to be able to insert noble gas atoms between the fluorine and boron atom yielding FArBO, FKrBO and FXeBO. The molecules are predicted to be linear.[30]

Uses

Boron oxyfluoride could be used in boriding steel. By using a gas, sticking solids onto the steel is avoided. Also this method allows control of the boron concentration, and mostly forms Fe2B instead of the more brittle FeB.[24] Burning boron releases much energy, so its use in explosives or fuel is being researched. To maximise energy output, both fluorine and oxygen are used to react, and thus FBO and related molecules are formed and may be in the exhaust.[25]

Notes and References

  1. Book: Kuchitsu. Kozo. Structure of Free Polyatomic Molecules: Basic Data. Springer Science & Business Media. 9783642457487. 28. en. 2013-03-09.
  2. Jacox. Marilyn E.. The vibrational energy levels of small transient molecules isolated in neon and argon matrices. Chemical Physics. December 1994. 189. 2. 149–170. 10.1016/0301-0104(94)00143-X. 1994CP....189..149J.
  3. Boussard-Plédel. Catherine. Vapor Phase Deposition of Foaming Boron Oxyfluoride Vitreous Materials. Le Floch. Marie. Fonteneau. Gilles. Lucas. Jacques. Materials Research Bulletin. July 1997. 32. 7. 805–811. 10.1016/S0025-5408(97)00050-0.
  4. Polishchuk. S. A.. Ignat’eva. L. N.. Marchenko. Yu. V.. Bouznik. V. M.. Oxyfluoride glasses (A review). Glass Physics and Chemistry. 5 March 2011. 37. 1. 1–20 (14). 10.1134/S108765961101010X. 97609959.
  5. Kawashima. Yoshiyuki. Kawaguchi. Kentarou. Endo. Yasuki. Hirota. Eizi. Infrared diode laser and microwave spectra and molecular structure of an unstable molecule, FBO. The Journal of Chemical Physics. 1987. 87. 4. 2006. 10.1063/1.453175. 1987JChPh..87.2006K.
  6. Gatehouse. Bethany. Müller. Holger S.P.. Gerry. Michael C.L.. Hyperfine Constants and Nuclear Shieldings from the Microwave Spectra of FBO, ClBO, and FBS. Journal of Molecular Spectroscopy. July 1998. 190. 1. 157–167. 10.1006/jmsp.1998.7565. 9645936. 1998JMoSp.190..157G.
  7. Farber. M.. Blauer. J.. The heat of formation and entropy of BOF. Transactions of the Faraday Society. 1962. 58. 2090. 10.1039/TF9625802090.
  8. Web site: Thevenot. Francois H. J.. Goeuriot. Patrice M. V.. Driver. Julian H.. Lebrun. Jean-Paul R.. Apparatus for the boronizing of pieces made of metal or cermet and surface-bornished pieces. 1982.
  9. Nguyen. Minh Tho. Vanquickenborne. L. G.. Sana. Michel. Leroy. Georges. Heats of formation and proton affinities of some oxoborons (R-B≡O) and sulfidoborons (R-B≡S) with R = hydrogen, fluorine, chlorine, and methyl group. The Journal of Physical Chemistry. May 1993. 97. 20. 5224–5227. 10.1021/j100122a010.
  10. Siegel. B. The oxyhalides of the III-B elements. Inorganica Chimica Acta Reviews. December 1968. 2. 137–146. 10.1016/0073-8085(68)80019-1.
  11. Goubeau. J.. Keller. H.. Über Boroxol-Verbindungen Darstellung, physikalische und chemische Eigenschaften. Zeitschrift für anorganische und allgemeine Chemie. December 1952. 267. 1–3. 1–26. de. 10.1002/zaac.19522670102.
  12. Boussard-Plédel. Catherine. Le Floch. Marie. Fonteneau. Gilles. Lucas. Jacques. Sinbandhit. Sourisak. Shao. J.. Angell. C.A.. Emery. Joël. Buzaré. J.Y.. The structure of a boron oxyfluoride glass, an inorganic cross-linked chain polymer. Journal of Non-Crystalline Solids. February 1997. 209. 3. 247–256. 10.1016/S0022-3093(96)00548-0. 1997JNCS..209..247B.
  13. Web site: Fisher. H. D.. Kiehl. J.. Cane. A.. Infrared Spectra and Thermodynamic Properties of Trifluoroboroxine (FBO)3 Final Report HTC-61-90. https://web.archive.org/web/20161227130517/http://www.dtic.mil/get-tr-doc/pdf?AD=AD0274243. dead. December 27, 2016. Hughs Tool Company Aircraft Division. 27 November 2016. Culver City, California. June 1961.
  14. Hildenbrand. Donald L.. Theard. Lowell P.. Saul. Albert M.. Transpiration and Mass Spectrometric Studies of Equilibria Involving BOF(g) and (BOF)3(g). The Journal of Chemical Physics. 1963. 39. 8. 1973. 10.1063/1.1734569. 1963JChPh..39.1973H.
  15. Boussard-Plédel. Catherine. Fonteneau. Gilles. Lucas. Jacques. Boron oxyfluoride glasses in the B-O-F system: new polymeric spaghetti-type glasses. Journal of Non-Crystalline Solids. July 1995. 188. 1–2. 147–152. 10.1016/0022-3093(95)00183-2.
  16. Lucas. Jacques. Non-conventional spaghetti-type glasses. Journal of Non-Crystalline Solids. May 1995. 184. 21–24. 10.1016/0022-3093(95)00087-9. 1995JNCS..184...21L.
  17. Hoppe. P.. Lodders. K.. Katharina Lodders. Strebel. R.. Amari. S.. Lewis. R. S.. Boron in Presolar Silicon Carbide Grains from Supernovae. The Astrophysical Journal. 10 April 2001. 551. 1. 478–485. 10.1086/320075. 2001ApJ...551..478H. free.
  18. Ruff. Otto. Braida. A.. Bretschneider. O.. Menzel. W.. Plaut. H.. Die Darstellung, Dampfdrucke und Dichten des BF3, AsF5 und BrF3. Zeitschrift für anorganische und allgemeine Chemie. 18 May 1932. 206. 1. 59–64. de. 10.1002/zaac.19322060108.
  19. Baumgarten. Paul. Bruns. Werner. Über die Umsetzung von Borfluorid mit Bortrioxyd, Boraten, Carbonaten und Nitraten und zur Kenntnis eines mutmaßlichen Boroxyfluorides (BOF)3. Berichte der Deutschen Chemischen Gesellschaft (A and B Series). 6 September 1939. 72. 9. 1753–1762. 10.1002/cber.19390720921. de.
  20. Baumgarten. Paul. Bruns. Werner. Über die Umsetzung von Borfluorid mit Aluminiumoxyd, Siliciumdioxyd, Titandioxyd und Silikaten und die mögliche Verwendung dieser Reaktionen für den Aufschluß aluminiumhaltiger Silikate zwecks Herstellung für die Aluminiumgewinnung verwendbarer Ausgangsmaterialien. Berichte der Deutschen Chemischen Gesellschaft (A and B Series). de. 9 July 1941. 74. 7. 1232–1236. 10.1002/cber.19410740717.
  21. Zhang. Lei. Zhang. Weijiang. Xu. Jiao. Ren. Xin. Synthesis of enriched 10B boric acid of nuclear grade. Transactions of Tianjin University. 7 December 2014. 20. 6. 458–462. 10.1007/s12209-014-2303-x. 91517483.
  22. Yoder. M.John. High temperature arc studies of infrared radiation from boron and tungsten oxides. Journal of Quantitative Spectroscopy and Radiative Transfer. December 1974. 14. 12. 1317–1328. 10.1016/0022-4073(74)90100-9. 1974JQSRT..14.1317Y.
  23. Goeuriot. Patrice. Thévenot. François. Driver. Julian H.. Magnin. Thierry. Methods for examining brittle layers obtained by a boriding surface treatment (Borudif). Wear. April 1983. 86. 1. 1–10. 10.1016/0043-1648(83)90083-2.
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  25. Web site: Smolanoff. Jason. Lapicki. Adom. Anderson. Scott L.. Sowa-Resat. Marianne. A Cluster Beam Study of Boron Oxide Chemistry With HF. 1 December 2016. 26 Dec 1994.
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  27. Soto. Maribel R.. Rate Constant Determinations for HBO + F Channels from ab Initio Reaction Path Calculations. The Journal of Physical Chemistry. April 1995. 99. 17. 6540–6547. 10.1021/j100017a039.
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  29. Latimer. B.. Devlin. J.P.. Vibrational spectra of fluorine and chlorine derivatives of boroxine—II. Spectrochimica Acta Part A: Molecular Spectroscopy. January 1967. 23. 1. 81–88. 10.1016/0584-8539(67)80210-1. 1967AcSpA..23...81L.
  30. Lin. Tsun-Yi. Hsu. Jeng-Bin. Hu. Wei-Ping. Theoretical prediction of new noble-gas molecules OBNgF (Ng=Ar, Kr, and Xe). Chemical Physics Letters. February 2005. 402. 4–6. 514–518. 10.1016/j.cplett.2004.12.090. 2005CPL...402..514L.

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