A full dimensional direct ab initio dynamics study of the electron capture by SF6

The dynamics of electron capture by sulfur hexafluoride (SF₆) have been investigated by means of full dimensional direct ab initio dynamics calculations at the HF/6-311G(d)+sp (a diffuse sp-function is further added to sulfur atom) level. A rigid SF₆ structure and narrow Franck–Condon region for electron capture was assumed in choosing initial conditions for the trajectories. The direct ab initio dynamics calculations for the electron capture processes by SF₆ indicated that the SF⁻₆ ion formed by the vertical electron attachment to SF₆ decomposes into SF⁻₅ and F via short-lived complex formation (SF⁻₆). The lifetime of SF⁻₆ was distributed from 0.1–0.2 ps, which is quite short as lifetime of the intermediate complex. The dynamics calculations showed that 12% of the total available energy is partitioned into the relative translational mode between SF₅⁻ and F. The effect of solid phase on the dynamics has been examined by introducing a model bath-relaxation time for energy dispersion. There was a possibility that the SF₆⁻ anion exists in solid phase. The mechanism of the electron capture by SF₆ in gas phase and in solid phase was discussed on the basis of the results.     

Catalytic Degradation of Sulfur Hexafluoride by Rhodium Complexes

The development of a safe and efficient method for the degradation of SF₆ is of current environmental interest, because SF₆ is one of the most potent greenhouse gases. SF₆ is thermally and chemically extremely inert, and therefore, it has been used in various industrial applications. However, this inertness results in a major challenge for its depletion. We report on a process for a catalytic degradation of SF₆ in the homogeneous phase by using rhodium complexes as precatalysts. The SF₆ activation reactions feature mild reaction conditions, low catalyst loadings, and a high selectivity. The employment of phosphines and hydrosilanes for scavenging the sulfur and fluorine atoms of the SF₆ molecule allows the selective transformation of SF₆ into nongaseous and nontoxic compounds.     

A molecular orbital study of bond energies in compounds of sulfur and fluorine

CNDO/2 molecular orbital theory is employed in a study of the binding energies of the molecules SF, SF₂, SF₄, SF₆, their cations and anions, and of the molecules SSF₂, FSSF and S₂F₁₀. Computed energies, when rescaled according to energy partitioning concepts, compare favorably with available experimental data. Ionization energies and electron affinities are calculated for SF, SF₂, SF₄ and SF₆.

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Zusammenfassung Mit Hilfe der CNDO/2 Theorie werden die Bindungsenergien der SF, SF₂, SF₄ und SF₆ Moleküle, von deren positiven und negativen Ionen und von SSF₂, FSSF und S₂F₁₀ berechnet. Die berechneten Energien stimmen gut mit experimentellen Daten überein, wenn sie nach Energieaufteilungsprinzipien wiederberechnet werden. Ferner werden die Ionisierungsenergien und Elektronenaffinitäten für SF, SF₂, SF₄ und SF₆ angegeben.

     

Selective Dissociation of Sulfur Hexafluoride by Intense CO2 Laser Radiation in Pulsed Gas Dynamic Flow

Isotopically selective IR multiphoton dissociation (MPD) of SF₆ in a pulsed gas dynamic flow was studied. The dependence of the yield of the product SF₄ on the frequency of CO₂ laser radiation exciting SF₆ molecules was obtained. The ³⁴S enrichment coefficient in SF₄ was measured. The enrichment factor was found to agree well with the value predicted from comparison of spectral dependences for the SF₄ yield from ³²SF₆ and ³⁴SF₆. The obtained results are compared with the data on SF₆ dissociation in a low-temperature cell and in a molecular beam.     

Real-time monitoring traces of SF6 in near-source ambient air by ion mobility spectrometry

The leakage of sulphur hexafluoride (SF₆) gas threats the global climate changes and personnel safety. Monitoring the concentration of SF₆ in its application places is an industry regulation. In this study, ion mobility spectrometry (IMS) was developed for fast monitoring traces of SF₆ in near-source ambient air. Due to the water is an important part of the natural air and affects most atmospheric measurements, the operating parameters of IMS monitoring SF₆ were optimised for quantitative analysis of SF₆ at different relative humidity (RH). It is discovered two main product ions SF₆^? and SOF₄^? by IMS at different RH. The calibration curves of SF₆ were investigated by its relationship with the peak intensity of SOF₄^– for real application. The time resolution of the measurement was obtained less than 1 s and the limit of detection (LOD) achieved 0.16–0.68 ppm with a data averaging of 30 times. At last, the simulated application of monitoring SF₆ leakage was tested in the fume hood of our lab. The results showed a great potential application prospect of IMS in monitoring SF₆ in the ambient air of its application places.     

Nucleophilic Activation of Sulfur Hexafluoride: Metal‐Free,Selective Degradation by Phosphines

The development of new methods for the chemical activation of the extremely inert greenhouse gas sulfur hexafluoride (SF₆) not only is of current environmental interest, but also offers new opportunities for applications of SF₆ as a reagent in organic synthesis. We herein report the first nucleophilic activation of SF₆ by Lewis bases, namely by phosphines, which results either in its complete degradation to phosphine sulfides and difluorophosphoranes or in the selective conversion of SF₆ into a bench‐stable, crystalline salt containing the SF₅^? anion. Quantum chemical calculations reveal a nucleophilic substitution mechanism (S_N2) for the initial fluorine abstraction from SF₆ by the phosphine. Furthermore, a scalable one‐pot procedure for the complete decomposition of SF₆ into solid, nonvolatile products is presented based on cheap and commercially available starting materials.     

Hot-atom initiated chemical laser study of the H* + SF6 reaction

Chemical laser methods have been used to study the “hot-atom” reaction H* + SF₆ → products at KE_rel < 102 kcal mole^?1. Collision-induced dissociation of SF₆ by H* is the dominant reaction channel. Internal excitation of SF₆ may be required to promote the abstraction reaction: H + SF₆ → HF + SF₅.     

Nucleophilic Activation of Sulfur Hexafluoride: Metal‐Free,Selective Degradation by Phosphines

The development of new methods for the chemical activation of the extremely inert greenhouse gas sulfur hexafluoride (SF₆) not only is of current environmental interest, but also offers new opportunities for applications of SF₆ as a reagent in organic synthesis. We herein report the first nucleophilic activation of SF₆ by Lewis bases, namely by phosphines, which results either in its complete degradation to phosphine sulfides and difluorophosphoranes or in the selective conversion of SF₆ into a bench‐stable, crystalline salt containing the SF₅⁻ anion. Quantum chemical calculations reveal a nucleophilic substitution mechanism (S_N2) for the initial fluorine abstraction from SF₆ by the phosphine. Furthermore, a scalable one‐pot procedure for the complete decomposition of SF₆ into solid, nonvolatile products is presented based on cheap and commercially available starting materials.     

DC plasma etching of silicon by sulfur hexafluoride. Mass spectrometric study of the discharge products

Polycrystalline silicon wafers were etched in dc discharges of SF₆. SF_x species were extracted from the discharges and measured with a mass spectrometer. A systematic procedure was used to measure the SF _x ⁺ signals such that they are indicators of events in the discharge close to the sample undergoing etching. The picture that emerges is remarkably simple and shows the relative stability of several SF_x species including SF₆, SF₄, SF₂, and SF which are shown to be extracted from the discharge both in the presence and absence of the silicon sample. When silicon is being etched on the cathode of the discharge cell, the only significant additional products are SiF₄ and S₂F₂. A comparison of blank and sample data for opposite substrate polarities shows that there is only a small cation-assisted etching effect and suggests that ions do not play an important role in the etching of silicon by SF₆ discharges.     

Heat of Adsorption of Pure Sulfur Hexafluoride on Micro-Mesoporous Adsorbents

The isotherms and the isosteric heats of adsorption of pure SF₆ were measured on two microporous zeolites (NaX and Silicalite), one mesoporous alumina, and two activated carbons (BPL and PCB) at 305 K. The adsorption isotherms were Type I by Brunauer classification. The PCB carbon adsorbed SF₆ most strongly and the alumina adsorbed SF₆ most weakly. The adsorption of SF₆ on the other three materials were comparable in the low pressure region despite their drastic differences in the physicochemical properties. The heat of adsorption of SF₆ on the silicalite and the alumina remained practically constant over a large range of coverage. The heat of adsorption of SF₆ increased with increasing adsorbate loading on the NaX zeolite in the high coverage region. The heat of adsorption of SF₆ on the activated carbons decreased with increasing adsorbate loading before leveling off in the high coverage region.     

Vibrational energy distribution of CsF produced by reactive beam scattering of Cs by SF6 and SF4

The molecular beam electric resonance method has been used to measure the vibrational energy distribution of CsF produced in the chemical reactions Cs + SF₆ and SF₄. The reaction Cs + SF₆ yields a perfect Boltzmann distribution for the products CsF. For the reaction Cs + SF₄ a more complicated distribution has been found which, according to the molecular structure of SF₄, can be explained as a superposition of two Boltzmann distributions of different vibrational temperatures.     

Nucleophilic Activation of Sulfur Hexafluoride by N-Heterocyclic Carbenes and N-Heterocyclic Olefins: A Computational Study

Sulfur hexafluoride (SF₆) is considered as a potent greenhouse gas, whose effective degradation is challenging. Here we report a computational study on the nucleophilic activation of sulfur hexafluoride by N-heterocyclic carbenes and N-heterocyclic olefins. The result shows that the activation of SF₆ is both thermodynamically and kinetically favorable at mild condition using NHOs with fluoro-substituted azolium and sulfur pentafluoride anion being formed. The Gibbs free energy barrier during the activation of SF₆ has a linear relationship with the energy of HOMO of substrates, which could be a guideline for applying those compounds that feature higher energy in HOMO to activate SF₆ in high efficiency.     

Laser isotope separation in SF6

Laser induced isotope separation in SF₆ and SF₆ mixtures has been investigated in a collisionally dominated pressure regime. Experimental results with SF₆/rare gas mixtures point out the importance of collision induced dissociation following the initial collisionless dissociation. Rotational relaxation induced by rare gases and H₂ is shown to play a signficant role in the dissociation of both isotopic species. Total vibrational relaxation (V-T/R) induced by H₂ as a collision partner is shown to dominate the dissociation efficiency of SF₆/H₂ mixtures.     

TEA CO2-Laser Induced SF6 + Ba Beam-Surface Ionization

A TEA CO₂-laser induced SF₆ + Ba beam-surface ionization process has been studied when vibrational excitation of SF₆ molecules was carried out at (and near) the polished surface of the electrically heated up to 675 K polycristalline Ba. Electron emission and negative molecular ion signal were detected. The dependence of the molecular ion signal on laser fluence and frequency (on SF₆ molecular absorption) as well as on the SF₆ gas pressure in the nozzle were studied. The results reveal a nonlinear, probably multiphoton character of the molecular ion formation and a clear vibrational selectivity i.e. vibrational enhancement, of the SF₆ + Ba beam-surface ionization process. Possible mechanisms of the negative molecular ion formation in the IR laser induced SF₆ + Ba beam-surface ionization process are discussed.     

Electron scattering and dissociative attachment by SF6 and its electrical-discharge by-products

Discrete electron-molecule processes relevant to SF₆ etching plasmas are examined. Absolute, total scattering cross sections for 0.2–12-eV electrons on SF₆, SO₂, SOF₂, SO₂F₂, SOF₄, and SF₄, as well as cross sections for negative-ion formation by attachment of electrons, have been measured. These are used to calculate dissociative-attachment rate coefficients as a function ofE/N for SF₆ by-products in SF₆.     

The σ‐Hole Interaction Between Sulfur Hexafluoride and Ammonia Characterised by Broadband Rotational Spectroscopy

A weakly‐bound complex of SF₆ and NH₃ is generated within an expanding gas jet and characterised by broadband rotational spectroscopy. The spectra of isotopologues ³²SF₆???¹⁴NH₃, ³²SF₆???¹⁴ND₃, ³²SF₆???¹⁵NH₃ and ³⁴SF₆???¹⁵NH₃ are observed and assigned to determine the spectroscopic parameters. These parameters are consistent with the complex having a C_3v symmetric rotor geometry, in which the nitrogen atom of NH₃ coordinates to SF₆ such that the C_3v axis of the NH₃ sub‐unit is aligned with a local C₃ axis on the SF₆ sub‐unit. The geometry of the complex is rationalized in terms of a σ‐hole interaction. The observed spectra and ab initio calculations also reveal evidence of internal dynamics involving internal rotation of one monomer sub‐unit with respect to the other about the symmetry axis of the complex.     

Examination of the electronic structure of gaseous sulfur fluorides by soft X-ray spectroscopy

X-ray absorption spectra in the region of the L edge of S are reported for SF₄ and SF₆. These are interpreted in terms of transitions from the L sublevels of S to free MO [molecular absorption mechanism]. A semiempirical electron-structure calculation is reported for SF₆. The bond types are discussed.     

Calculation of thermodynamic properties and transport coefficients for SF6-N2 mixtures in the temperature range 1,000–30,000 K

We have computed the equilibrium composition, the transport coefficients (viscosity, electrical and thermal conductivities), the thermodynamic properties (Gibbs and Helmholtz potentials, entropy, enthalpy, specific heats), and the derived quantities (mass density, sound velocity) for SF₆-N₂ mixtures in conditions relevant to circuit-breaker arcs: temperatures between 1000 and 30,000 K, pressures in the range 1–10 atm. The validity of our computation has been checked by a detailed comparison of our results with those available in the literature concerning pure SF₆ and pure N₂. In SF₆-N₂ mixtures the chemical reactions (dissociation, ionization) have a strong influence on thermal conduction and heat capacities. The effect of SF₆ on the properties of such mixtures is elucidated: in a mixture containing 40% SF₆, the amplitude of the thermal conduction peak appearing around 7500 K is reduced by a factor of 4 relative to that of pure N₂. The influence of pressure on the properties of the plasma between 1 and 10 atm is relatively low.     

Electron attachment to sulphur hexafluoride: Formation of stable SF6− at low pressure

Electron attachment to SF₆ has been studied at low pressure in the gas phase by ion cyclotron resonance (ICR) spectroscopy, Formation of stable SF₆^? under collision-free conditions is rationalized on the basis of radiative relaxation by (SF₆^?)^*. The utility of ICR for investigating electron attachment at extremely low pressures is noted.     

Rapid interspecies energy flow between vibrationally excited SF6 and the asymmetric stretch of N2O

Rapid, selective collision-dependent excitation of N₂O following pumping of SF₆ with a CO₂ laser is reported. The N₂O fluorescence rise depends on the pressure of each component and is dominated by the SF₆-dependent contribution of 2290 ms^?1 Torr^?1. The subsequent fall is governed by V→V processes among SF₆ vibrational modes.