Online Detection of Hydrogen Fluoride under Corona Discharge in Gas-Insulated Switchgear Based on Photoacoustic Spectroscopy

Internal discharge and overheating faults in sulfur hexafluoride (SF6) gas-insulated electrical equipment will generate a series of characteristic gas products. Hydrogen fluoride (HF) is one of the main decomposition gases under discharge failure. Because of its extremely corrosive nature, it can react with other materials in gas-insulated switchgear (GIS), resulting in a short existence time, so it needs to be detected online. Resonant gas photoacoustic spectroscopy has the advantage of high sensitivity, fast response, and no sample gas consumption, and can be used for the online detection of flowing gas. In this paper, a simulated GIS corona discharge experimental platform was built, and the HF generated in the discharge was detected online by gas photoacoustic spectroscopy. The absorption peak of HF molecule near 1312.59 nm was selected as the absorption spectral line, and a resonant photoacoustic cell was designed. To improve the detection sensitivity of HF, wavelength modulation and second-harmonic detection technology were used. The online monitoring of HF in the simulated GIS corona discharge fault was successfully realized. The experimental results show that the sensitivity of the designed photoacoustic spectroscopy detection system for HF is 0.445 μV/(μL/L), and the limit of detection (LOD) is 0.611 μL/L.

Electron Attachment to 5-Fluorouracil: The Role of Hydrogen Fluoride in Dissociation Chemistry

We investigate dissociative electron attachment to 5-fluorouracil (5-FU) employing a crossed electron-molecular beam experiment and quantum chemical calculations. Upon the formation of the 5-FU^- anion, 12 different fragmentation products are observed, the most probable dissociation channel being H loss. The parent anion, 5-FU^-, is not stable on the experimental timescale (~140 µs), most probably due to the low electron affinity of FU; simple HF loss and F^- formation are seen only with a rather weak abundance. The initial dynamics upon electron attachment seems to be governed by hydrogen atom pre-dissociation followed by either its full dissociation or roaming in the vicinity of the molecule, recombining eventually into the HF molecule. When the HF molecule is formed, the released energy might be used for various ring cleavage reactions. Our results show that higher yields of the fluorine anion are most probably prevented through both faster dissociation of an H atom and recombination of F^- with a proton to form HF. Resonance calculations indicate that F^- is formed upon shape as well as core-excited resonances.

Gold N-Heterocyclic Carbene Catalysts for the Hydrofluorination of Alkynes Using Hydrofluoric Acid: Reaction Scope, Mechanistic Studies and the Tracking of Elusive Intermediates

An efficient and chemoselective methodology deploying gold-N-heterocyclic carbene (NHC) complexes as catalysts in the hydrofluorination of terminal alkynes using aqueous HF has been developed. Mechanistic studies shed light on an in situ generated catalyst, formed by the reaction of Brønsted basic gold pre-catalysts with HF in water, which exhibits the highest reactivity and chemoselectivity. The catalytic system has a wide alkyl substituted-substrate scope, and stoichiometric as well as catalytic reactions with tailor-designed gold pre-catalysts enable the identification of various gold species involved along the catalytic cycle. Computational studies aid in understanding the chemoselectivity observed through examination of key mechanistic steps for phosphine- and NHC-coordinated gold species bearing the triflate counterion and the elusive key complex bearing a bifluoride counterion.

Hydrofluorination of Alkenes: A Review

In this minireview, we explore the different approaches used to perform the hydrofluorination reaction of alkenes. Contrary to other hydrohalogenation reactions, the hydrofluorination requires specific conditions due to the lower reactivity of HF. Over the years, many different approaches have been explored among which the use of HF complexes has particularly proved to be useful as these reagents are easier to handle. The enantioselective hydrofluorination has been demonstrated using electrophilic sources of fluorine, while radical fluorination proved compatible with a vast range of functional groups that are generally problematic with strong acids and some fluoride sources. This review will cover the different conditions developed through the years, starting with the first reported addition using gaseous HF, up to the most recent method described in October 2020.

Aluminum Precursor Interactions with Alkali Compounds in Thermal Atomic Layer Etching and Deposition Processes

Surface fluorination and volatilization using hydrogen fluoride and trimethyaluminum (TMA) is a useful approach to the thermal atomic layer etching of Al₂O₃. We have previously shown that significant enhancement of the TMA etching effect occurs when performed in the presence of lithium fluoride chamber-conditioning films. Now, we extend this enhanced approach to other alkali halide compounds including NaCl, KBr, and CsI. These materials are shown to have varying capacities for the efficient removal of AlF₃ and ultimately lead to larger effective Al₂O₃ etch rates at a given substrate temperature. The most effective compounds allow for continuous etching of Al₂O₃ at substrate temperatures lower than 150 °C, which can be a valuable route for processing temperature-sensitive substrates and for improving the selectivity of the etch over other materials. The strong interaction between TMA and alkali halide materials also results in material-selective thin-film deposition at these reduced substrate temperatures. We discuss possible mechanisms of this etching enhancement and prospects for extending this approach to other material systems. The consequences of using TMA as an ALD and ALE precursor are discussed in the context of interface engineering for alkali-containing substrates such as lithium battery materials.

Kitamura Electrophilic Fluorination Using HF as a Source of Fluorine

This review article focused on the innovative procedure for electrophilic fluorination using HF and in situ generation of the required electrophilic species derived from hypervalent iodine compounds. The areas of synthetic application of this approach include fluorination of 1,3-dicarbonyl compounds, aryl-alkyl ketones, styrene derivatives, α,β-unsaturated ketones and alcohols, homoallyl amine and homoallyl alcohol derivatives, 3-butenoic acids and alkynes.

Development of mouse models for the study of chloropicrin and hydrogen fluoride ocular injury

The possibility of chemical terrorism within the United States is a rising concern, with the eye being one of the most sensitive tissues to toxicant exposure. We sought to develop mouse models of toxicant-induced ocular injury for the purpose of evaluating potential therapeutics. Chloropicrin (CP) and hydrogen fluoride (HF) were selected for the study owing to their reportedly high potential to induce ocular injury. Eyes of female BALB/c mice were exposed to CP or HF vapor in order to produce a moderate injury, as defined by acute corneal epithelial loss followed by progressive corneal pathology with the absence of injury to deeper eye structures. Clinical injury progression was evaluated up to 12 weeks postexposure, where a significant dose-dependent induction of corneal neovascularization was measured. Histopathology noted epithelial necrosis and stromal edema as early as 24 h after exposure but was resolved by 12 weeks. A significant increase in inflammatory cytokine concentrations was measured in the cornea 24 h after exposure and returned to baseline by day 14. The ocular injury models we developed here for CP and HF exposure should serve as a valuable tool for the future evaluation of novel therapeutics and the molecular mechanisms of injury.

Stabilizing a High-Voltage Lithium-Rich Layered Oxide Cathode with a Novel Electrolyte Additive

We report a novel electrolyte additive, bis(trimethylsilyl)carbodiimide, that effectively stabilizes high-voltage lithium-rich oxide cathode. Charge/discharge tests demonstrate that even trace amounts of bis(trimethylsilyl)carbodiimide in a baseline electrolyte improve the cycling stability of this cathode significantly, either in Li-based half cells or graphite-based full cells, where the capacity retention after 200 cycles between 2 and 4.8 V at 0.5C is enhanced from 40 to 72% and 49 to 77%, respectively. Analyses using physical characterization and theoretical calculations reveal that this additive not only builds a protective film on the cathode but also eliminates detrimental hydrogen fluoride via its strong coordination with hydrogen fluoride or protons.

Perfluoroalcohols: The Preparation and Crystal Structures of Heptafluorocyclobutanol and Hexafluorocyclobutane-1,1-diol

The first X-ray crystal structure of an α-fluoroalcohol is reported. Heptafluorocyclobutanol was obtained in quantitative yield from hexafluorocyclobutanone by HF addition in anhydrous hydrogen fluoride. The compound was characterized by its X-ray single crystal structure. Heptafluorocyclobutanol readily undergoes hydrolysis to hexafluorocyclobutane-1,1-diol, which was also structurally characterized by X-ray diffraction.

Collision-Induced Dissociation Study of Strong Hydrogen-Bonded Cluster Ions Y-(HF) n (Y=F, O2) Using Atmospheric Pressure Corona Discharge Ionization Mass Spectrometry Combined with a HF Generator

Hydrogen fluoride (HF) was produced by a homemade HF generator in order to investigate the properties of strong hydrogen-bonded clusters such as (HF) _n . The HF molecules were ionized in the form of complex ions associated with the negative core ions Y^- produced by atmospheric pressure corona discharge ionization (APCDI). The use of APCDI in combination with the homemade HF generator led to the formation of negative-ion HF clusters Y^-(HF) _n (Y=F, O₂), where larger clusters with n≥4 were not detected. The mechanisms for the formation of the HF, F^-(HF) _n , and O₂^-(HF) _n species were discussed from the standpoints of the HF generator and APCDI MS. By performing energy-resolved collision-induced dissociation (CID) experiments on the cluster ions F^-(HF) _n (n=1-3), the energies for the loss of HF from F^-(HF)₃, F^-(HF)₂, and F^-(HF) were evaluated to be 1 eV or lower, 1 eV or higher, and 2 eV, respectively, on the basis of their center-of-mass energy (E_CM). These E_CM values were consistent with the values of 0.995, 1.308, and 2.048 eV, respectively, obtained by ab initio calculations. The stability of [O₂(HF) _n ]^- (n=1-4) was discussed on the basis of the bond lengths of O₂H-F^-(HF) _n and O₂^-H-F(HF) _n obtained by ab initio calculations. The calculations indicated that [O₂(HF)₄]^- separated into O₂H and F^-(HF)₃.

Protonation of Nitramines: Where Does the Proton Go?

The reactions of nitramine, N-methyl nitramine, and N,N-dimethyl nitramine with anhydrous HF and the superacids HF/MF₅ (M=As, Sb) were investigated at temperatures below -40 °C. In solution, exclusive O-protonation was observed by multinuclear NMR spectroscopy. Whereas no solid product could be isolated from the neat HF solutions even at -78 °C, in the HF/MF₅ systems, protonated nitramine MF₆^- salts were isolated for the first time as moisture-sensitive solids that decompose at temperatures above -40 °C. In the solid state, depending on the counterion, O-protonated or N-protonated cations can be formed, in accord with theoretical calculations which show that the energy differences between O-protonation and N-protonation are very small. The salts [H₂ N-NO₂ H][AsF₆ ], [H₃ N-NO₂ ][SbF₆ ], [MeHNNO₂ H][SbF₆ ], and [Me₂ NNO₂ H][SbF₆ ] were characterized by their X-ray crystal structures.

Evaluation of Efficient and Practical Methods for the Preparation of Functionalized Aliphatic Trifluoromethyl Ethers

The “chlorination/fluorination” technique for aliphatic trifluoromethyl ether synthesis was investigated and a range of products with various functional groups was prepared. The results were compared with oxidative desulfurization-fluorination of xanthates with the same structure.

Isolation of the simplest hydrated acid

Dissociation of an acid molecule in aqueous media is one of the most fundamental solvation processes but its details remain poorly understood at the distinct molecular level. Conducting high-pressure treatments of an open-cage fullerene C₇₀ derivative with hydrogen fluoride (HF) in the presence of H₂O, we achieved an unprecedented encapsulation of H₂O·HF and H₂O. Restoration of the opening yielded the endohedral C₇₀s, that is, (H₂O·HF)@C₇₀, H₂O@C₇₀, and HF@C₇₀ in macroscopic scales. Putting an H₂O·HF complex into the fullerene cage was a crucial step, and it would proceed by the synergistic effects of "pushing from outside" and "pulling from inside." The structure of the H₂O·HF was unambiguously determined by single crystal x-ray diffraction analysis. The nuclear magnetic resonance measurements revealed the formation of a hydrogen bond between the H₂O and HF molecules without proton transfer even at 140°C.

Thermal Atomic Layer Etching of SiO2 by a "Conversion-Etch" Mechanism Using Sequential Reactions of Trimethylaluminum and Hydrogen Fluoride

The thermal atomic layer etching (ALE) of SiO₂ was performed using sequential reactions of trimethylaluminum (TMA) and hydrogen fluoride (HF) at 300 °C. Ex situ X-ray reflectivity (XRR) measurements revealed that the etch rate during SiO₂ ALE was dependent on reactant pressure. SiO₂ etch rates of 0.027, 0.15, 0.20, and 0.31 Å/cycle were observed at static reactant pressures of 0.1, 0.5, 1.0, and 4.0 Torr, respectively. Ex situ spectroscopic ellipsometry (SE) measurements were in agreement with these etch rates versus reactant pressure. In situ Fourier transform infrared (FTIR) spectroscopy investigations also observed SiO₂ etching that was dependent on the static reactant pressures. The FTIR studies showed that the TMA and HF reactions displayed self-limiting behavior at the various reactant pressures. In addition, the FTIR spectra revealed that an Al₂O₃/aluminosilicate intermediate was present after the TMA exposures. The Al₂O₃/aluminosilicate intermediate is consistent with a "conversion-etch" mechanism where SiO₂ is converted by TMA to Al₂O₃, aluminosilicates, and reduced silicon species following a family of reactions represented by 3SiO₂ + 4Al(CH₃)₃ → 2Al₂O₃ + 3Si(CH₃)₄. Ex situ X-ray photoelectron spectroscopy (XPS) studies confirmed the reduction of silicon species after TMA exposures. Following the conversion reactions, HF can fluorinate the Al₂O₃ and aluminosilicates to species such as AlF₃ and SiO_xF_y. Subsequently, TMA can remove the AlF₃ and SiO_xF_y species by ligand-exchange transmetalation reactions and then convert additional SiO₂ to Al₂O₃. The pressure-dependent conversion reaction of SiO₂ to Al₂O₃ and aluminosilicates by TMA is critical for thermal SiO₂ ALE. The "conversion-etch" mechanism may also provide pathways for additional materials to be etched using thermal ALE.

The Existence of Tricyanomethane

Calcium tricyanomethanide reacts with hydrogen fluoride under formation of tricyanomethane and Ca(HF2)2. Tricyanomethane is stable below -40 °C and was characterized by IR, Raman, and NMR spectroscopy. The vibrational spectra were compared to the quantum-chemical frequencies at the PBE1PBE/6-311G(3df,3dp) level of theory and confirm the predicted C(3v) symmetry of the molecule with regular C-H (109.8 pm), C-C (146.7 pm), and C≡N (114.7 pm) bonds.

Sirtuin1 and autophagy protect cells from fluoride-induced cell stress

Sirtuin1 (SIRT1) is a nicotinamide adenine dinucleotide (NAD(+))-dependent deacetylase functioning in the regulation of metabolism, cell survival and organismal lifespan. Active SIRT1 regulates autophagy during cell stress, including calorie restriction, endoplasmic reticulum (ER) stress and oxidative stress. Previously, we reported that fluoride induces ER-stress in ameloblasts responsible for enamel formation, suggesting that ER-stress plays a role in dental fluorosis. However, the molecular mechanism of how cells respond to fluoride-induced cell stress is unclear. Here, we demonstrate that fluoride activates SIRT1 and initiates autophagy to protect cells from fluoride exposure. Fluoride treatment of ameloblast-derived cells (LS8) significantly increased Sirt1 expression and induced SIRT1 phosphorylation resulting in the augmentation of SIRT1 deacetylase activity. To demonstrate that fluoride exposure initiates autophagy, we characterized the expression of autophagy related genes (Atg); Atg5, Atg7 and Atg8/LC3 and showed that both their transcript and protein levels were significantly increased following fluoride treatment. To confirm that SIRT1 plays a protective role in fluoride toxicity, we used resveratrol (RES) to augment SIRT1 activity in fluoride treated LS8 cells. RES increased autophagy, inhibited apoptosis, and decreased fluoride cytotoxicity. Rats treated with fluoride (0, 50, 100 and 125ppm) in drinking water for 6weeks had significantly elevated expression levels of Sirt1, Atg5, Atg7 and Atg8/LC3 in their maturation stage enamel organs. Increased protein levels of p-SIRT1, ATG5 and ATG8/LC3 were present in fluoride-treated rat maturation stage ameloblasts. Therefore, the SIRT1/autophagy pathway may play a critical role as a protective response to help prevent dental fluorosis.

Asymmetry in angular rigidity of hydrogen-bonded complexes

The asymmetry in angular rigidity of the proton donor and proton acceptor of hydrogen-bonded hydrogen fluoride binary complexes is investigated. The intermolecular bending frequency of HF, as the proton donor, is linearly proportional to the square root of the dissociation energy, whereas that of the proton acceptor is always much lower. The asymmetry, measured by the ratio of bending elastic constants of HF to that of the proton acceptor, is generally >2, and varies pronouncedly with the acceptors reaching values >20. Molecules with nitrogen as the bridged acceptor atom show an angular rigidity nearly one order of magnitude greater than the group with oxygen as the proton acceptor.

An Experimental Search for Gaseous Reactivity Between AlF3 and HF Near 1200 K

Using an accurate transpiration method, AlF₃ was sublimed near 1200 K into argon containing 0, 0.02, and 0.76 atm of HF, but no reaction between AlF₃ and the HF was detected within the precision (about 1%). Two alternative structures of HAlF₄ are postulated. An upper bound for extent of reaction corresponds to Δ H 298 ∘ > - 33 kcal (- 138 kJ) for AlF₃ (g) + HF (g) = HAlF₄ (g); this indicates a far lower stability of HAlF₄ (g) than that of LiAlF₄ (g) or NaAlF₄ (g) when formed similarly.