Sulfur hexafluoride, electronic

Sulfur hexafluoride [2551-62-4] 6 molecular weight 146.07, is a colorless, odorless, tasteless gas. It is not flammable and not particularly reactive. Its high chemical stabiUty and excellent electrical characteristics have led to widespread use in various kinds of electrical and electronic equipment such as circuit breakers, capacitors, transformers, microwave components, etc (see Electronic materials). Other properties of the gas have led to limited usage in a variety of unique appHcations ranging from medical appHcations to space research. 

Electrica.1 Properties. The electrical properties of SF stem primarily from its effectiveness as an electron scavenger. To accomplish electrical breakdown in a dielectric gas, primary electrons must gain sufficient energy to generate appreciable numbers of secondary electrons on molecular impact. Sulfur hexafluoride interferes with this process by capturing the primary electrons, resulting in the formation of SF or SF ions and F atoms (29) ... 

Sulfur hexafluoride may be analyzed chromatographicaHy using a molecular sieve or a Porapak QS column. Using an electron-capture detector, a sensitivity of 10 to lO " ppb is possible (51—53). 

The largest class of molecules to violate the octet rule consists of species in which the central atom is surrounded by more than four pairs of valence electrons. Typical molecules of this type are phosphorus pentachloride, PC15, and sulfur hexafluoride, SF6. The Lewis structures of these molecules are... 

A sulfur hexafluoride molecule, SF6, has six atoms attached to the central S atom and no lone pairs on that atom (8). According to the VSEPR model, the electron arrangement is octahedral, with four pairs at the corners of a square on the equator and the remaining two pairs above and below the plane of the square (see Fig. 3.2). An F atom is attached to each electron pair, and so the molecule is predicted to be octahedral. All its bond angles are either 90° or 180°, and all the F atoms are equivalent. 

Analysis of the halohydrocarbons, halocarbons, and sulfur hexafluoride is usually achieved by gas chromatography that is equipped with an electron capture detector. Complex metal anions, such as cobalt hexacyanide, are used as nonradioactive tracers in reservoir studies. The cobalt in the tracer compound must be in the complex anion portion of the molecule, because cationic cobalt tends to react with materials in the reservoir, leading to inaccurate analytic information [1226]. 

Sulfur hexafluoride is used as a gaseous insulator for electrical equipment and in electronic ultrahigh frequency devices. 

The results (electrostatic-fit charges based on Hartree-Fock 6-3IG wavefunctions) are ambiguous. Relative to dimethylsulfide as a normal-valent standard , the sulfur in oxygen loses about half an electron, and the sulfur in dimethylsulfone loses 1.7 electrons. This would seem to suggest that dimethylsulfoxide is halfway to being a zwitterion, but that dimethylsulfone is most of the way. Charges on sulfur in sulfur tetrafluoride and sulfur hexafluoride (relative to sulfur difluoride) show more modest effects, in particular for the latter. Overall, it appears that hypervalent molecules possess significant ionic character. 

Sulfur hexafluoride Oh). Six sulfur-fluorine cr bonds require 2 electrons in the valence shell. Six equivalent bonds require an octahedron and so sulfur wjU be hybridized sp d2 as shown in Fig. 6.lf. 

For a coulometric response, an ideal compound, sulfur hexafluoride, has remarkable sensitivity (10-1 g/sec). Again it should be emphasized that this value will be unique for a particular compound and depends on the species affinity for electron capture. 

For the analysis of sulfur hexafluoride an electron capture detector must be used. Analysis time for halothane, acetone, ether, cyclopropane, acetylenes, ethane, methane, and sulfur hexafluoride, under normal conditions, will average 8 min. Since the two detectors operate under different conditions samples containing both the hydrocarbons and sulfur hexafluoride are run through the gas chromatograph twice. By simultaneous use of separate ovens, the FID and ECD measurements can be made concurrently. Otherwise, the separate in jections must be made after switching the detectors. 

DISCUSSION. Figure 10.11 shows the separation of a group of gases from blood. The procedure described depends on complete equilibration between the gas and liquid in the syringe. The period to achieve equilibration depends on the gas, and can be as little as 1 min for acetone and as much as 30 min for sulfur hexafluoride. Linear calibration curves can be obtained when peak height is plotted against gas concentration for either the flame ionization or electron capture detector. The reproducibility (n = 10) for ethane gave a standard deviation of 23% of the mean concentration while it was 2.3% for halothane and 1.8% for ether. 

As was true in the case of five charge clouds, different shapes are possible for molecules having atoms with six charge clouds, depending on whether the clouds are of bonding or nonbonding electrons. Sulfur hexafluoride, for example, has all six positions around sulfur occupied by fluorine atoms ... 

The great kinetic stability of sulfur hexafluoride 73>74>, like that of carbon tetrafluoride 73>, particularly toward nucleophilic reagents, may be viewed as arising from the presence about the central atom s kernel (and about the kernels of the fluorine atoms) of a nearly complete, protective sheath of electrons with no pockets 52> of sufficient depth (orbitals of sufficiently low energy) to permit effective coordination with the unshared electrons of an entering nucleophile. The possibility remains, however, of attack by electrophilic reagents, e.g., by strong Lewis acids, such as sulfur trioxide 74>. 

This type of hybridization is known as sp3d (or dsp3, depending on where you look). The remaining type of hybridization, known as sp3cf (cfsp3), can be seen in the hybridization of sulfur in the compound sulfur hexafluoride, SF6. In Figure 7.21 below, note how two electrons are promoted to the d orbitals ... 

Sulfur hexafluoride sublimes at -64 °C to produce a dense gas (6.14 g L-1). Under a pressure of 2 atm, the melting point is -51 °C. The molecule has the expected octahedral structure and a dipole moment of zero. The compound is so inert that it is used as a gaseous insulator, and rats allowed to breathe a mixture of SF6 and oxygen show no ill effects after several hours of exposure. This inertness is a result of the molecule having no vacant bonding site or unshared electron pairs on sulfur to initiate a reaction and the fact that six fluorine atoms shield the sulfur atom from attack. Consequently, there is no low-energy pathway for reactions to occur, and the compound is inert even though many reactions are thermodynamically favored. 

Sulfur hexafluoride is reported to be reduced to the pentafluoride by the hydrated electron and that SF5 has a significant lifetime (29). A potential for the reaction... 

Some atoms appear to exceed the octet rule. This behavior is observed only for those elements in Period 3 of the periodic table and beyond. To see how this arises, we will consider the Lewis structure for sulfur hexafluoride (SF6). The sum of the valence electrons for SF6 is... 

A nmnber of ions have been studied by the magnetron technique, and, wherever possible, the parent electron acceptors have been derived from several different substrates. To emphasize the potentialities of the method, the results will be considered according to the class of substrate, rather than the electron acceptor, and any discussion of the individual acceptors be kept to the end. No members of the first class of substrate have yet been observed, although it would appear probable that sulfur hexafluoride, sulfur trioxide, nitrogen dioxide, and certain organic compounds such as tetracyanoethylene should be able to capture electrons directly. 

Sulfur, unlike oxygen, has the capacity to expand its valence shell beyond the normal octet of electrons to form hypervalent compounds such as sulfur tetrafluoride (SF4) with 10 electrons in the outermost shell and sulfur hexafluoride (SF6) containing 12 electrons in the valence shell.63,7 The chemistry of hypervalent sulfur started in 1873 with the discovery of the unstable compound sulfur tetrachloride (SCI4). The existence of hypervalent sulfur compounds is an important feature of the chemistry of sulfur and the precise nature of the bonding in these molecules has remained a puzzling problem. 

Figure 4.5 Negative-ion mass spectrometry data plotted as In KT312 versus 1,000/7 for nitrobenzene and sulfur hexafluoride. The data exhibit one a and one P region. The magnitude was scaled to the value of the k for SF6 at room temperature. The curves are calculated using the measured electron affinities of SFg and C6H5NC>2. The data determine the Qan values to he 1.0. The responses were obtained by injecting a solution with a known amount of the two compounds into the mass spectrometer.

Figure 5.15 Timeline for the electron affinities of sulfur hexafluoride. The lower values can be assigned to excited states. The one higher value is known to have a larger random uncertainty.

The Group VIA elements below oxygen form some covalent compounds of the ABg type by sharing their six valence electrons with six other atoms. Sulfur hexafluoride, SFg (mp — 51 °C), an unreactive gas, is an example. Sulfur hexafluoride molecules are nonpolar octahedral molecules. The hexafluorophosphate ion, PFg, is an example of a polyatomic ion of the type AB. ... 

Many molecules are known in which a central atom forms bonds with six other atoms that are arranged about it at the corners of a regular octahedron. An example is sulfur hexafluoride, SF6. In this octahedral molecule the sulfur atom uses the six electrons of its outer shell to form six bonds, one with each of the six fluorine atoms. This molecule is described as involving octahedral coordination of the six fluorine atoms about the sulfur atom or octahedral ligation of the six fluorine atoms to the sulfur atom. 

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