Fluorine Migration Research Articles

Why is acyl fluoride favored in NHC/photoredox catalysis?

Recent advances in N-heterocyclic carbene (NHC)/photoredox catalyzed acylation have underlined the special reactivity of acyl fluoride compared with other acyl halides. Though safer and more convenient, acyl chloride is rarely used with NHC in known references. In a recent work, Studer et al. confirmed that acyl chloride and bromide are inactive in NHC/photoredox acylation, yet reason remains unknown. Herein, computational results showed that [1,2]-fluorine migration intermediate should be responsible for the unique role of acyl fluoride.

Fluorine-assisted asphalts waste carbonization to Ni-F-C electrocatalyst by mechanochemistry for efficient oxygen evolution

Low-cost Ni-F-C electrocatalysts were gram-scale synthesized through mechanochemical synthesis accompanied by asphalt and e-waste polytetrafluoroethylene (PTFE) upgrading. The high-value utilization of bulk industrial solid waste and the design of high-performance oxygen evolution electrocatalyst hold immense importance for sustainable environment and energy economy. The overpotential at 10 mA cm−2 (1 M KOH) for the optimized Ni-F-C catalyst was low as 269 mV. First, PTFE as a pore-forming agent generated plenty of pores inside the Ni-F-C catalyst (promoting mass transfer and surface wettability). Second, the structural self-reconstruction caused by fluorine migration made the electrode surface undergo in-situ transformation from Ni species to Ni-OOH/OH phase during OER. Additionally, DFT calculations demonstrated that the F atoms could trigger charge redistribution, effectively activate adjacent Ni atoms and reduce the energy barrier of the rate-determining step (*OH→*O). The migration of fluorine species and the infiltration of electrolyte combine to yield an increased number of active sites, improved conductivity and hydrophilicity, enhanced catalytic activity and durability.

Surface Reconstruction Facilitated by Fluorine Migration and Bimetallic Center in NiCo Bimetallic Fluoride Toward Oxygen Evolution Reaction.

Oxygen evolution reaction (OER) is a critical anodic reaction of electrochemical water splitting, developing a high-efficiency electrocatalyst is essential. Transition metal-based catalysts are much more cost-effective if comparable activities can be achieved. Among them, fluorides are rarely reported due to their low aqueous stability of coordination and low electric conductivity. Herein, a NiCo bimetallic fluoride with good crystallinity is designed and constructed, and significantly enhanced catalytic activity and conductivity are observed. The inevitable oxidation of transition metal ions at high potential and the dissociation of F- are attributed to the low aqueous stability of coordination. The theoretical researches predicte that transition metal fluorides should have a strong tendency to electrochemical reconstruction. Therefore, based on the observations on their electrochemical behavior, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and bode plots, it is further demonstrated that surface reconstruction occurred during the electrochemical process, meanwhile a significant increase of electrochemically active area, which is created by F migration, are also directly observed. Additionally, DFT calculation results show that the electronic structure of the catalysts is modulated by the bimetallic centers, and this reconstruction helps optimizing the adsorption energy of oxygen-containing species and improves OER activity.

Fluorine migration and in-situ doping for regeneration of Ni-rich LiNi0.9Co0.05Mn0.05O2 cathode material from spent lithium-ion batteries

Fluorine migration and in-situ doping for regeneration of Ni-rich LiNi0.9Co0.05Mn0.05O2 cathode material from spent lithium-ion batteries

In situ fluorine migration in ZIF-67/F-TiO2 Z-type heterojunction and its photocatalytic degradation mechanism of tetracycline hydrochloride

The advancement of photocatalytic technology depends on the creation of effective photocatalysts. In this research, a series of ZIF-67/F-TiO2 composite catalysts with various ZIF-67 content were made using a straightforward hydrothermal approach, and their photocatalytic performance was examined. The analysis of element content and crystal structure of the composites showed that F element in TiOF2/TiO2 migrated during the hydrothermal process, resulting in F-TiO2 with special surface fluorine. The existence of special surface fluorine can act as electron trapping sites to promote oxygen adsorption and accelerate the hole migration to the surface of TiO2, to achieve better electron hole separation efficiency and enhance photocatalytic activity. Density functional theory (DFT) calculations verified the formation of heterojunction. Tetracycline hydrochloride (TCH) was broken down by ZIF-67/F-TiO2 composites with different mass ratios under simulated sunlight to determine their photocatalytic effectiveness. The results indicated that ZIF-67/F-TiO2 composite photocatalyst with 10 % ZIF-67 content had the highest removal efficiency (87 %) of tetracycline hydrochloride (20 mg/L) within 1 h. The following are the causes of the increased photocatalytic activity of ZIF-67/F-TiO2 composites: More electron capture sites are made available by the surface F produced by in situ F migration, which also encourages photogenerated electron-hole separation. A Z-type heterojunction, on the other hand, lengthens the lifetime of photogenerated carriers. This study introduces a fresh idea: F-TiO2 with special surface fluorine was prepared by in situ F migration, and the photocatalytic degradation of wastewater containing tetracycline hydrochloride by F-TiO2 composites was investigated.

Geochemical evidence of fluoride behavior in loess and its influence on seepage characteristics: An experimental study

High background levels of fluorine in groundwater and soil in arid and semi-arid loess regions pose a severe threat to socio-economic development and human health, necessitating the evaluation of fluorine migration in loess. In this study, static leaching and dynamic seepage tests as well as scanning electron microscopy, mercury intrusion porosimetry, and X-ray fluorescence analyses were conducted using loess as the porous medium. Additionally, simulations using PHREEQC software were performed. The results indicated that the studied loess had a high background level of fluorine. Geochemical processes closely related to fluorine include dissolution of gypsum and dolomite, precipitation/dissolution of calcite and fluorite, and ion exchange between CaX and NaX. Under seepage of water with high fluorine levels, soil particles flocculated and formed aggregates; furthermore, the contact area between soil particles reduced, resulting in point-to-point contact between particles. Consequently, pores changed from small (intra- and inter-particle pores) to large (intra- and inter-granular pores) scale pores. Permeability initially decreased rapidly, then remained relatively stable, and subsequently rapidly increased with the passage of time. This was attributed to the dissolution of calcium minerals in loess, yielding Ca2+, which induced the precipitation of fluorite and promoted the dissolution of carbonates. Cation exchange and dissolution of other components were also important factors influencing permeability. The findings of this study can elucidate the coupling between loess microstructure, seepage behavior, and geochemical actions under the influence of high‑fluorine water, and are of great significance for in-situ regional research in loess areas.

Exploring α-electron-deficiency-induced [1,2]-fluorine migration.

The present study investigates the standard model of [1,2]-fluorine migration and that triggered by the rearrangement of cyclopropyl-substituted fluoroepoxides. The [1,2]-fluorine migration reaction proceeds via a synchronous concerted, tight-ion-pair mechanism. When coupled with other reaction coordinates, the whole reaction follows an asynchronous mechanism, while the [1,2]-fluorince migration unit still retains its tight-ion-pair feature and the reaction coordinates of two C-F distances vary synchronously. A general reaction program for α-electron-deficiency-induced [1,2]-fluorine migration is proposed through an analysis of the intermediates generated from nucleophilic addition. The reaction mechanisms associated with α-electron deficiency and the rearrangement are scrutinized using computational chemistry. Two additional reaction programs for [1,2]-fluorine migration are identified. The Gibbs free energy change of [1,2]-fluorine migration exhibits a linear dependence on the value of the Fukui function of the substrate, which could lead to the production of the desired α-monofluoroketone and enhance the utilization of fluorine atoms.

Nitrogen Trifluoride Complexes of Group 10 Transition Metals M(NF3) (M = Pd, Pt).

The group 10 transition metal atoms Pd and Pt react with nitrogen trifluoride (NF3) forming N-coordination M(NF3) complexes in solid neon and argon matrices. The M(NF3) complexes isomerize to more stable fluoronitrenoid FNMF2 isomers via fluorine migration upon blue LED (λ = 470 nm) light irradiation. These products are characterized on the basis of infrared absorption spectroscopy with isotopic substitutions and theoretical frequency calculations. The analysis of the electronic structure of nitrogen trifluoride complexes indicates that the bonding between metal and nitrogen trifluoride can be described as σ donation from the HOMO of nitrogen trifluoride to the empty metal dz2 orbital and π back-donation from the metal dxz/yz orbitals to the LUMO of nitrogen trifluoride, the latter of which stabilized the metal ligand bond and destabilized the ligand N-F bond. In FNMF2, the FN ligand doubly bonded to the metal and bear imido character.

Fluorine and sodium depletion followed by refractive index modification imprinted on fluorophosphate glass surface by thermal poling

Thermal poling using microstructured electrodes on oxyfluoride glasses have been investigated and the impact on optical properties was evaluated. A micrometer refractive index modification based on the sodium and fluorine migration has been evidenced, through the creation of a layer with different chemical composition as compared to the original glass. Topological and optical properties modifications after poling treatment are discussed. For the first time, we described the migration of negative fluorine ion in the direction of the cathode from the glass network. Na and F migration led to a layer richer in oxides with a strong contrast of + 0.06 of the refractive index in the imprinted structures.

Role of rice cultivation on fluorine distribution behavior in soda saline-alkali land

Fluorine stability in the soil is crucial to protecting the groundwater and the food chain from pollution by fluorine, which was abundantly present (762.78–1330.66 mg/kg) in soda saline-alkali soil areas of western Jilin Province. This study investigated the fluorine distribution and pollution level in soil. Two representative areas in Zhenlai County, namely Chagan and Hatu villages, were selected to gather soda saline-alkali soil profiles (0–20, 20–40, and 40–60 cm) from different cultivation years (i.e. 3, 5, 8, 10, 13, 15, 18, and 20). The results revealed that fluorine content in soil decreased with increasing cultivation years. In the soil profiles, fluorine and water-soluble fluorine concentrations followed the order of 40–60 > 20–40 > 0–20 cm, while the contents of exchangeable and organic matter-bound fluorine decreased with the increasing profile depth. Rice planting increased the content of macroaggregates with diameters >2 and 2–0.25 mm, which significantly improved the aggregate structure of the soil. The fluorine content was the greatest in microaggregate with particle sizes <0.053 mm in the aggregates' sizes. Rice planting in saline-alkali soil reduced the contents of water-soluble and exchangeable fluorine, while enhancing the content of organic matter-bound fluorine, thereby reducing the bioavailability of fluorine in the soil and the risk of fluorine migration through the groundwater. The degree of soil pollution decreased with increasing plantation years, and the soil became slightly polluted after 15 years of rice planting. The rice plants grew normally after years of improving fluorine-containing saline-alkali land, and the fluorine content was within the safe limit.

Fluorine Migration and Controlling for Regeneration of Ni-Rich LiNi0.9Co0.05Mn0.05O2 Cathode Material From Spent Lithium-Ion Batteries

Fluorine Migration and Controlling for Regeneration of Ni-Rich LiNi0.9Co0.05Mn0.05O2 Cathode Material From Spent Lithium-Ion Batteries

Fluorine Migration from Carbon to Iron and Fluorine–Iron Dative Bonds in Octafluorocyclohexadiene Iron Carbonyl Chemistry

The geometries and energies of mononuclear and binuclear octafluorocyclohexadiene iron carbonyl derivatives have been studied using density functional theory. The lowest energy C6F8Fe(CO)3 tricarbonyl structure is the octafluoro-1,3-cyclohexadiene derivative known experimentally. Isomeric structures containing the other two possible C6F8 hexagon ligands lie at higher energies. Unprecedented oxidative addition involving fluorine migration from carbon to iron is theoretically predicted in the C6F8Fe(CO)4 tetracarbonyl systems to give low-energy C6F7Fe(CO)4F structures. A similar energetically favored fluorine migration, also from carbon to iron, converts the coordinatively unsaturated (η4-C6F8)Fe(CO)2 dicarbonyl complex to the coordinatively saturated heptafluorocyclohexadienyl complex (η5-C6F7)Fe(CO)2F. For C6F8Fe2(CO)8, trans isomers with two Fe(CO)4 moieties on opposite sides of the C6F8 ring coordinated to C═C bonds are the lowest energy structures. These are similar to the experimentally known hexafluorocyclopentadiene complex C5F6[Fe(CO)4]2. The corresponding cis isomers with the Fe(CO)4 moieties on the same size of the C6F8 ring are high-energy structures, possibly a consequence of the steric hindrance between the two closely spaced Fe(CO)4 moieties. The low-energy C6F8Fe2(CO)7 structures may be viewed as the substitution products of the experimentally known Fe2(CO)9, with the two carbonyl groups replaced by a terminal or bridging C6F8 ligand donating four electrons to one or both iron atoms of the central Fe2 unit. The low-energy singlet structures in the unsaturated C6F8Fe2(CO)6 system have C6F8 ligands donating electrons to the central Fe2 system not only through Fe–C bonds but also through F→Fe dative bonds involving fluorine lone pairs.

Facile synthesis of fluorine doped single crystal Ni-rich cathode material for lithium-ion batteries

Fluorine doped single crystal Ni-rich cathode material with excellent electrochemical properties were successfully synthesized. X-ray diffraction (XRD) revealed that the fluorine substitution process did not change the hexagonal layer structure, but it promoted the unit cell expanded along the c-direction which was conductive to improving the kinetic of Li+ transmission. Energy dispersive X-ray Spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) further proved the fluorine doped into the cathode material, which simultaneously triggered partial Ni3+ reduction to Ni2+. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) demonstrated that fluorine substitution can effectively decrease polarization and suppress migration resistance of lithium ions. The lithium ions diffusion rate (DLi+) indicated that lithium ions migrated more smoothly in fluoride doped sample. Both the substitution effects of fluorine and short migration path way for lithium ions in single crystal particles account for excellent electrochemical performance of the Ni-rich lithium cathode material. This study provided some new thought into synthesis of high performance fluorine doped single crystal Ni-rich lithium cathode materials.

Improvement on Fluorine Migration from SF6 to SiF4 by an Efficient Mediator of Fe2O3/Cr2O3 Composites.

An economic and facile method was urgently required for the degradation of SF6 to replace the high-energy excitation treatment. Both theoretical calculations and experimental observations were conducted to reveal the synergy of Cr/Fe/Si composites on a new technique of SF6 degradation through reacting silicon dioxide. Density functional theory (DFT) calculations show that strong adsorption of SF6 on Cr2O3, and then the fast F/O exchange between CrF3 and Fe2O3 (energy barrier was 1.45 eV) as well as FeF3 and SiO2 (energy barrier was 1.69 eV) enhanced mediated efficiency from SF6 to SiF4. The fluorine (F) migration between solid interfaces in Cr2O3&Fe2O3@SBA15 was responsible for efficient SF6 removal. The F migration route was composed of SF6 to CrF3, CrF3 to FeF3, and FeF3 to SiF4 with the lowest thermodynamic driving. Enhanced specific accumulative converted amount (SACA) of SF6 on Cr2O3&Fe2O3@SBA15 was achieved and the highest SACA was 13.98 mmol/g within 7 h, significantly higher than that on Fe2O3@SBA15 (5.74 mmol/g) and Cr2O3@SBA15 (2.71 mmol/g). Moreover, X-ray diffractometry and X-ray photoelectron spectroscopy were performed to support DFT calculations, including ion intensities detected using mass spectroscopy and composition analysis of the mediator during the reaction. Therefore, our work put forward a novel approach for economic and efficient SF6 decomposition through reacting with silicon dioxide under the mediation of Cr2O3&Fe2O3. This method was also potentially used in effective degradation of refractory non-metal halides.

Single and double fluorine migration in third row transition metal dialkylaminodifluorosulfane complexes

Previous theoretical studies indicate that metal trifluorosulfane complexes are generally energetically disfavored with respect to fluorine migration from sulfur to the metal atom. This accounts for the limitation of currently known experimental metal trifluorosulfane chemistry to the single complex (Et3P)2Ir(CO)(Cl)(F)(SF3) having octahedral coordination of the central iridium atom. In a search for systems where such fluorine migrations are energetically disfavored, the density functional theoretical methods M06-L, BP86, and BP86-D3 have been used to study the related dimethylaminodifluorosulfane complexes of the third row transition metals, namely [M](SF2NMe2) ([M] = Ta(CO)5, CpW(CO)2, Re(CO)4, CpOs(CO), Ir(CO)3, CpPt). However, the complexes containing intact Me2NSF2 ligands, like the corresponding [M](SF3) complexes, are found to be energetically disfavored with respect to migration of one or two fluorine atoms from sulfur to the metal to give the monofluorides [M](SFNMe2)F and the difluorides [M](SNMe2)F2. Isomeric structures containing intact dihapto η2-Me2NSF2 ligands lie at significantly higher energies. Even higher energy [M](SF2)NMe2 structures with separate SF2 and NMe2 ligands are also found. An octahedral Re(CO)4(η2-SF2NMe2) structure with a relatively long SN bond can be viewed as a frozen intermediate in the rupture of an SN bond in an Re(CO)4(SF2NMe2) structure with an intact Me2NSF2 ligand to give a Re(CO)4(SF2)NMe2 structure with separate SF2 and Me2N ligands.

Nitrogen Engineering in the Ultrathin SiO2 Interface Layer of High- ${k}$ CMOS Devices: A First-Principles Investigation of Fluorine, Oxygen, and Boron Defect Migration

The further development of future semiconductor devices necessitates methods for characterization on an atomic scale. This ab initio investigation reveals consequences of nitrogen treatment of the state-of-the-art high- $k$ gate-stacks. The model allows a profound characterization of the SiO2 interface layer for different impurity concentrations. The presented results explain recent experimental observations qualitatively as well as quantitatively. Beyond that, a fundamental understanding is given, which can be used as an essential instrument for future reliability engineering.

1,2-Fluorine Radical Rearrangements: Isomerization Events in Perfluorinated Radicals.

Devising effective degradation technologies for perfluoroalkyl substances (PFASs) is an active area of research, where the molecular mechanisms involving both oxidative and reductive pathways are still elusive. One commonly neglected pathway in PFAS degradation is fluorine atom migration in perfluoroalkyl radicals, which was largely assumed to be implausible because of the high C-F bond strength. Using density functional theory calculations, it was demonstrated that 1,2-F atom migrations are thermodynamically favored when the fluorine atom migrated from a less branched carbon center to a more branched carbon center. Activation barriers for these rearrangements were within 19-29 kcal/mol, which are possible to easily overcome at elevated temperatures or in photochemically activated species in the gas or aqueous phase. It was also found that the activation barriers for the 1,2-F atom migration are lowered as much as by 10 kcal/mol when common oxidative degradation products such as HF assisted the rearrangements or if the resulting radical center was stabilized by vicinal π-bonds. Natural bond orbital analyses showed that fluorine moves as a radical in a noncharge-separated state. These findings add an important reaction to the existing knowledge of mechanisms for PFAS degradation and highlights the fact that 1,2-F atom shifts may be a small channel for isomerization of these compounds, but upon availability of mineralization products, this isomerization process could become more prominent.

Role of NO in highly selective SiN/SiO2 and SiN/Si etching with NF3/O2 remote plasma: Experiment and simulation

This paper describes the study of mechanisms of highly selective silicon nitride etching, in particular, the role of NO in silicon nitride etching by atomic fluorine. This paper presents experimental and simulation data about SiN, SiO2, and Si etching with NF3/O2 remote plasma. Quantum chemistry simulations show that NO reacts with the F–N bond in a SiN cluster with lower activation energy than the F-atom. Thus, NO increases the rate of fluorine migration on the silicon nitride surface from the nitrogen atom to the silicon atom during the etching process. In the absence of NO, such migration proceeds with relatively high activation energy, which limits the etching rate. The analytical model based on the SiN etching mechanism taking into account the fluorine migration is proposed. The results of calculations with the analytical model show a good agreement with the experimental SiN, SiO2, and Si etch rates.

Isolation of an Antiaromatic Singlet Cyclopentadienyl Zwitterion.

The reaction of triplet tetrachlorocyclopentadienylidene with BF3 in rare gas matrices yields a zwitterion consisting of a cyclopentadienyl cation bearing a positive charge and a negatively charged BF3 unit. IR and UV-vis spectra as well as the absence of EPR signals demonstrate a singlet ground state of the zwitterion, and its calculated geometry and magnetic properties clearly reveal a strong antiaromatic character. The zwitterion is highly labile and by visible or IR irradiation rearranges via a 1,2-fluorine migration from boron to carbon. Interaction with a second molecule of BF3 stabilizes the zwitterion and suppresses the fluorine migration, thus providing a convenient and efficient synthesis of an antiaromatic molecule under very mild conditions.

Tetrafluoroethylene versus trifluoromethylfluorocarbene complexes of cobalt carbonyl

The reaction of tetrafluoroethylene with Co2(CO)8 under mild conditions was reported to give (OC)4CoCF2CF2Co(CO)4. This compound readily undergoes decarbonylation with accompanying fluorine migration to give the trifluoromethylfluorocarbene complex (μ-CF3CF)Co2(CO)6(μ-CO) and eventually the cluster CF3CCo3(CO)9. In order to understand the chemistry of these cobalt carbonyl complexes obtained from tetrafluoroethylene, the structures and thermochemistry of the (C2F4)Co2(CO)n (n = 8, 7, 6, 5) systems have been investigated by density functional theory. The lowest energy (C2F4)Co2(CO)8 isomer is the experimentally observed (OC)4CoCF2CF2Co(CO)4, lying ∼6 kcal/mol in energy below the isomeric (CF3CF)[Co(CO)4]2. Loss of CO from (OC)4CoCF2CF2Co(CO)4 with accompanying fluorine migration to give (μ-CF3CF)Co2(CO)6(μ-CO) is essentially thermoneutral within ∼1 kcal/mol. The higher energy of ∼20 kcal/mol for the isomeric (μ-CF2CF2)Co2(CO)6(μ-CO) structure, where fluorine migration has not occurred, suggests a significant activation energy for this process. Further loss of CO from (μ-CF3CF)Co2(CO)6(μ-CO) gives low-energy (μ-CF3CF)Co2(CO)n(μ-CO) isomers (n = 5, 4) containing CoCo multiple bonds and/or vacant coordination sites. Such structures are possible intermediates to form CF3CCo3(CO)9 by reaction with excess Co2(CO)8 followed by Co(CO)nF elimination.