Fluorine Sites Research Articles

Fluorine edge decoration on zigzag silicene nanoribbons

Abstract We used the density functional theory (DFT) to explore the effects of fluorine (F) edge decoration on zigzag silicene nanoribbons (ZSiNR). Due to the strong electron affinity of fluorine atoms, a strong chemical bond between Si atoms and F atoms is formed. The formation energy also indicated that the fluorination configurations had higher stability than for the ZSiNR. The fluorination sites at the edges efficiently modulate the electronic and magnetic properties of the ZSiNR. In particular, we found a significant change from a metallic to a semiconductor characteristic in asymmetric edge fluorinated ZSiNR, in a mono-fluorinated system with one single edge, which was attributed to the weakening of the σ-π mixing effect. In contrast, a half-metallic character with a 100% spin polarization was induced in another other two full asymmetric fluorination systems due to the contribution from the F-Py, F-Pz, and Si-Py orbitals. All asymmetric fluorinations maintained a ferromagnetic character. Symmetric edge fluorinated ZSiNR maintained a ferromagnetic metallic characteristic or an antiferromagnetic semi-conductor characteristic due to the Coulomb repulsion from both edges. The interesting and useful physical properties in ZSiNR with edge fluorination including the half-metallic character and the spin polarization implied that edge-fluorinated ZSiNR can be effectively used in spintronic devices.

Self-diffusion and ionic exchange in mechanosynthesized, nanocrystalline solid solutions of PbF2 and CaF2[sbnd]19F 2D NMR visualizes the flourine hopping preferences

Ionic conductors based on PbF2, being one of the archetypes to study ion dynamics, serve as excellent model substances to study the elementary steps of translational anion dynamics in the solid state. Here, high-energy ball milling was employed to prepare a structurally disordered, metastable solid solution by mixing PbF2 with CaF2. The cation-mixed lattice in cubic Ca0.4Pb0.6F2 provides a range of magnetically distinct fluorine sites. We used 2D 19F magic angle spinning nuclear magnetic resonance (NMR) exchange spectroscopy to reveal the site preference of the F anions when hopping through the lattice. It turned out that the F anions located in sites coordinated by Pb cations are more mobile than those residing near or in Ca-rich environments. Conductivity measurements and variable-temperature 19F NMR spin-lattice relaxation experiments provide activation energies (0.24 eV, 0.40 eV) from which we deduce that in cation-mixed Ca0.4Pb0.6F2 anion diffusivity, on the angstrom length-scale, is mainly determined by vacant F− sites. Macroscopic ion transport has to be characterized by an activation energy of 0.55 eV suggesting that the F anions also use interstitial sites for long-range ionic transport.

Origin of Difference in the Reactivity of Aliphatic and Aromatic Guanidine‐containing Pharmaceuticals Toward [18F]Fluorination: Coulombic Forces and Hydrogen Bonding

Quantum chemical analysis is presented to elucidate the origin of difference in the reactivity of aliphatic vs. aromatic guanidine‐containing pharmaceuticals toward [18F]fluorination. We focus on the position (near to or far away from the site of reaction) of F− nucleophile in pre‐reaction complexes, as determined by intricate interplay of the Coulombic forces between the ionic species and hydrogen bonding with the –Boc protected guanidine. In [18F]fluorination of aliphatic guanidine compounds, the freely moving nucleophile F− is positioned close to the site of fluorination irrespective of the length of side chain, in agreement with the observed similar reaction yields for –CH2OMs and –CH2CH2CH2OMs side chains. As for the effects of positions of –Boc protection, we predict that the effects would to be contrary to the corresponding aromatic case, with the N, N″‐bis‐Boc protected guanidine compound being much more reactive than the N, N′‐bis‐Boc protected guanidine compound.

Investigation on structural, optical, thermal and gamma photon shielding properties of zinc and barium doped fluorotellurite glasses

Fluorotellurite glasses based on zinc and barium fluorides with total dopant concentration limited to 40 mol% in tellurite glass matrix were obtained by melt quenching technique. Explicit effects of both the zinc and barium fluorides on the tellurite glass network were observed from their response to various studies. The tendency of fluorine atoms to break the network was observed from the formation of more non-bridging sites instead of bridging sites from Raman measurements. Formation of these non-bridging fluorine sites instead of non-bridging oxygen led to an increase in the optical band gap of these samples from 3.36 eV to 3.52 eV. The reduction in the glass transition temperature from 316 °C to 299 °C observed with zinc fluoride doping is due to the breakage in the rigid network structure, while the increase from 316 °C to 329 °C with barium fluoride doping is correlated to the effect of size of barium atoms in the network. Radiation shielding properties calculated in terms of parameters like mean free path, mass attenuation coefficient etc., showed that the presence of more heavily doped elements like barium in the glass matrix led them to exhibit better shielding properties which can be utilized as replacement materials for concrete and lead based glasses as efficient shielding materials.

Understanding the effects of oxygen defects on the redox reaction pathways in LiVPO4F by combining ab initio calculations with experiments

Asymmetric redox reaction pathways in LiVPO4F can be understood by a presence of oxygen defects in fluorine sites.

Investigations of Uranyl Fluoride Sesquihydrate (UO2F2·1.57H2O): Combining 19F Solid-State MAS NMR Spectroscopy and GIPAW Chemical Shift Calculations.

High-resolution 19F magic-angle spinning (MAS) NMR spectra were obtained for the uranium-bearing solid uranyl fluoride sesquihydrate (UO2F2·1.57H2O). While there are seven distinct crystallographic fluorine sites, the 19F NMR spectrum reveals six peaks at -33.3, 9.1, 25.7, 33.0, 39.0, and 48.2 ppm, with the peak at 33.0 ppm twice the intensity of all the others and therefore corresponding to two sites. To assign the peaks in the experimental spectra to crystallographic sites, 19F chemical shifts were calculated using the gauge including projector augmented waves (GIPAW) plane-wave pseudopotential approach for a DFT-optimized crystal structure. The peak assignments from DFT are consistent with two-dimensional double-quantum 19F MAS NMR experiments.

Mechanochemical synthesis of PbF2 by high energy ball milling

Lead(II)-fluoride was successfully prepared by high energy ball milling and for comparison by classical thermal solid state chemical reaction. The influence of different starting materials, fluorinating agents and synthesis methods on the formation of the orthorhombic α-phase and the cubic β-phase was investigated. XRD analysis provided insight of which phase was formed under varying synthesis conditions. 19F and 207Pb MAS NMR measurements delivered an even further insight into the local structures. Additional simulations of the 19F spectra were performed to identify the 19F-207Pb coupling constants of the four-fold coordinated fluorine sites in both phases.

Synthesis of pH indicators for Cerenkov imaging by electrophilic substitution of bromine by fluorine in an aromatic system

Direct electrophilic fluorination using molecular fluorine gas is used in organic synthesis to create novel fluorine-containing compounds with potential beneficial activity that could not be obtained by nucleophilic substitution. In this paper, we report a novel electrophilic substitution of bromine by fluorine in an aromatic system. The mechanism of this type of fluorination was explored using the reaction between bromothymolsulfonphthalein (Bromothymol Blue) and dilute fluorine gas under acidic conditions. Substitution occurs in the bromine atoms located in the ortho-position relative to the hydroxyl group. A similar electrophilic fluorination of thymolsulfonphthalein (Thymol Blue) leads to a substitution of hydrogen atoms in the same position (ortho to hydroxyl). NMR spectroscopy was used to confirm the fluorination sites. NMR spectra of thymolsulfonphthalein and its derivatives under basic conditions can be explained by considering the absence of resonance between the two phenolic rings. Both dibromothymol blue and fluorobromothymol blue intermolecularly attenuate Cerenkov radiation selectively near their maximum absorbance in a pH dependent manner.

Proton Exchange Membrane Fuel Cell With Enhanced Durability Using Fluorinated Carbon As Electrocatalyst

This study evaluates the fluorination of a carbon aerogel and its effects on the durability of the resulting electrocatalyst for Proton Exchange Membrane Fuel Cell (PEMFC). Fluorine has been introduced before or after platinum deposition. The different electrocatalysts are physico-chemically and electrochemically characterized, and the results discussed by comparison with commercial Pt/XC72 from E-Tek. The results demonstrate that the level of fluorination of the carbon aerogel can be controlled. The fluorination modifies the texture of the carbons by increasing the pore size and decreasing the specific surface area, but the textures remain appropriate for PEMFC applications. Two fluorination sites are observed, leading to both high covalent C-F bond and weakened ones, the quantity of which depends on whether the treatment is done before or after platinum deposition. The order of the different treatments is very important. The presence of platinum contributes to the fluorination mechanism, but leads to amorphous platinum rather inactive towards the Oxygen Reduction Reaction. Finally, a better durability was demonstrated for the fluorinated then platinized catalyst compared both to the same but not fluorinated catalyst and to the reference commercial material (based on the loss of the electrochemical real surface area after accelerated stress tests).

The magnetic structure on the ground state of the equilateral triangular spin tube

The ground state of the frustrated equilateral triangular spin tube CsCrF4 is still hidden behind a veil though NMR spectrum broaden into 2 T at low temperature. In order to investigate the spin structure in an ordered state by 19F-NMR, we have determined the anisotropic hyperfine coupling tensors for each three fluorine sites in the paramagnetic state. The measurement field was raised up to 10 T to achieve highest resolution. The preliminary analysis using the obtained hyperfine tensors has shown that the archetypal 120°-type structure in ab-plane does not accord with the NMR spectra of ordered state.

Insights into the Complexity of Weak Intermolecular Interactions Interfering in Host-Guest Systems.

The recognition properties of heteroditopic hemicryptophane hosts towards anions, cations, and neutral pairs, combining both cation-π and anion-π interaction sites, were investigated to probe the complexity of interfering weak intermolecular interactions. It is suggested from NMR experiments, and supported by CASSCF/CASPT2 calculations, that the binding constants of anions can be modulated by a factor of up to 100 by varying the fluorination sites on the electron-poor aromatic rings. Interestingly, this subtle chemical modification can also reverse the sign of cooperativity in ion-pair recognition. Wavefunction calculations highlight how short- and long-range interactions interfere in this recognition process, suggesting that a disruption of anion-π interactions can occur in the presence of a co-bound cation. Such molecules can be viewed as prototypes for examining complex processes controlled by the competition of weak interactions.

First Insight into Fluorinated Pt/Carbon Aerogels as More Corrosion-Resistant Electrocatalysts for Proton Exchange Membrane Fuel Cell Cathodes

This study evaluates the fluorination of a carbon aerogel and gives first insights into its durability when used as platinum electrocatalyst substrate for proton exchange membrane fuel cell (PEMFC) cathodes. Fluorine has been introduced before or after platinum deposition. The different electrocatalysts are physico-chemically and electrochemically characterized, and the results discussed by comparison with commercial Pt/XC72 from E-Tek. The results demonstrate that the level of fluorination of the carbon aerogel can be controlled. The fluorination modifies the texture of the carbons by increasing the pore size and decreasing the specific surface area, but the textures remain appropriate for PEMFC applications. Two fluorination sites are observed, leading to both high covalent C-F bonds and weakened ones, the quantity of which depends on whether the treatment is done before or after platinum deposition. The order of the different treatments is very important. Indeed, the presence of platinum contributes to the fluorination mechanism, but leads to amorphous platinum, which is demonstrated rather inactive towards the oxygen reduction reaction. On the contrary, a better durability was demonstrated for the fluorinated and then platinized catalyst compared both to the same but not fluorinated catalyst and to the reference commercial material (based on the loss of the electrochemical real surface area after accelerated stress tests).

Fluorination of graphene enhances friction due to increased corrugation.

The addition of a single sheet of carbon atoms in the form of graphene can drastically alter friction between a nanoscale probe tip and a surface. Here, for the first time we show that friction can be altered over a wide range by fluorination. Specifically, the friction force between silicon atomic force microscopy tips and monolayer fluorinated graphene can range from 5-9 times higher than for graphene. While consistent with previous reports, the combined interpretation from our experiments and molecular dynamics simulations allows us to propose a novel mechanism: that the dramatic friction enhancement results from increased corrugation of the interfacial potential due to the strong local charge concentrated at fluorine sites, consistent with the Prandtl-Tomlinson model. The monotonic increase of friction with fluorination in experiments also demonstrates that friction force measurements provide a sensitive local probe of the degree of fluorination. Additionally, we found a transition from ordered to disordered atomic stick-slip upon fluorination, suggesting that fluorination proceeds in a spatially random manner.

Solid solution in the fluorapatite-chlorapatite binary system: High-precision crystal structure refinements of synthetic F-Cl apatite

Apatite sensu lato, Ca 10 (PO 4 ) 6 (F,OH,Cl) 2 , is the tenth most abundant mineral on Earth, and is fundamentally important in geological processes, biological processes, medicine, dentistry, agriculture, environmental remediation, and material science. The steric interactions among anions in the [0,0, z ] anion column in apatite make it impossible to predict the column anion arrangements in solid solutions among the three end-members. In this work we report the measured atomic arrangements of synthetic apatite in the F-Cl apatite binary with nominal composition Ca 10 (PO 4 ) 6 (F 1 Cl 1 ), synthesized in vacuum at high temperature to minimize both hydroxyl- and oxy-component of the apatite. Four crystals from the high-temperature synthesis batch were prepared to assess the homogeneity of the batch and the precision of the location of small portions of an atom in the apatite anion column by single-crystal X-ray diffraction techniques. Crystals were ground to spheres of 80 μm diameter, and full-spheres of Mo K α diffraction data were collected to θ = 33o, with average redundancies >16. Final R 1 values ranged from 0.0145 to 0.0158; the lattice parameters ranged from a = 9.5084(2)–9.5104(3), c = 6.8289(3)–6.8311(2) A. Based on this study, solid solution in P 6 3 / m apatites along the F-Cl join is attained by creation of an off-mirror fluorine site at (0,0,0.167), a position wherein the fluorine atom relaxes away from its normal position within the {00 l } mirror plane in P 6 3 / m apatites; that relaxation is coupled with relaxation of a chlorine atom at the adjacent mirror plane away from the off-mirror fluorine, allowing acceptable F-Cl distances in the anion column. There are a total of four partially occupied anion positions in the anion column, including two for fluorine [(0,0,1/4) and (0,0,0.167)] and two for chlorine [(0,0,0.086) and (0,0,0)]; the chlorine site at the origin was previously postulated but not observed in calcium apatite solid solutions.

Comprehensive Solid-State Characterization of Rare Earth Fluoride Nanoparticles

The combination of multinuclear solid-state NMR spectroscopy and powder X-ray diffraction has been applied to characterize the octahedron-shaped crystalline nanoparticle products resulting from an inverse micelle synthesis. Rietveld refinements of the powder X-ray diffraction data from the nanoparticles revealed their general formula to be (H3O)Y3F10·xH2O. 1H magic-angle spinning (MAS) NMR experiments provided information on sample purity and served as an excellent probe of the zeolithic incorporation of atmospheric water. 19F MAS NMR experiments on a series of monodisperse nanoparticle samples of various sizes yielded spectra featuring three unique 19F resonances arising from three different fluorine sites within the (H3O)Y3F10·xH2O crystal structure. Partial removal of zeolithic water from the internal cavities and tunnels of the nanoparticles led to changes in the integrated peak intensities in the 19F MAS NMR spectra; the origin of this behavior is discussed in terms of 19F longitudinal relaxation. 19F–89Y variable-amplitude cross-polarization (VACP) NMR experiments on both stationary samples and samples under MAS conditions indicated that two distinct yttrium environments are present, and on the basis of the relative peak intensities, the population of one of the two sites is closely linked to the nanoparticle size. Both 19F MAS and 19F–89Y VACP/MAS experiments indicated small amounts of an impurity present in certain nanoparticles; these are postulated to be spherical amorphous YF3 nanoparticles. We discuss the importance of probing molecular-level structure in addition to microscopic structure and how the combination of these characterization methods is crucial for understanding nanoparticle design, synthesis, and application.

Nitrogen and fluorine anionic substitution in lithium phosphate glasses

Abstract Phosphate based glasses containing simultaneously fluorine and nitrogen in significant amounts have been prepared according to a new synthesis route described in this paper. The results obtained on glass series based on a 55Li2O·45P2O5 composition (mol %) allow, for the first time, the investigation of the effect of fluorine introduction on lithium phosphate and lithium phosphorous oxynitride glasses. New structural features, including the presence of an unreported fluorine site and a change in the Q2 groups/fluorinated species ratio which depends on the lithium fluoride content, were derived from 1D 19F and 31P solid state NMR essays. Distinction between pure phosphate and fluorinated phosphate groups was established using the 2D 31P{19F} CP-HETCOR experiments. The evolution of the glass structure was studied as a function of the lithium fluoride content. The study of the properties of these glasses showed an increase of the ionic conductivity and a decrease of the glass transition temperature due to the phosphate network depolymerisation when lithium fluoride is introduced in the formulation.

Enhanced performances in primary lithium batteries of fluorinated carbon nanofibers through static fluorination

Using galvanostatic discharges, the electrochemical properties of fluorinated carbon nanofibers (CNF) have been investigated. Several methods of fluorination are compared, (i) direct process using a flux of pure molecular fluorine F2 (dynamic process), (ii) controlled fluorination using atomic fluorine released continuously by the thermal decomposition of a solid fluorinating agent (TbF4), and (iii) static fluorination by the filling of a closed reactor with undiluted molecular fluorine as reactive gas. Electrochemical performances of the resulting materials are compared highlighting significant improvement using the static method. The discharge potential increases from 2.27 V (vs. Li+/Li°) for materials obtained by the direct route to a medium 2.40 V by the static route resulting from a two steps discharge mechanism. Owing to a complete characterization of each fluorinated materials by X-ray diffraction (XRD), solid state nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM), the increased average potential of fluorinated sample through the static way has been explained by the peculiar distribution of fluorine and carbon sites in the nanomaterial. Then, in order to understand the two steps discharge mechanism of the latter material, its discharge mechanism through galvanostatic measurements at different depths of discharge has been investigated in different electrolytes. The discharged materials have been studied owing to 19F MAS NMR, XRD and scanning electron microscopy characterizations. The texture and cristallinity of lithium fluoride particles are key parameters acting on the ionic diffusion of Li+ and F− ions and as a consequence on the electrochemical performances. Its high solubility in EC/PC/3DMC solvent mixture prevents from overvoltage phenomenon occurring during the discharge of fluorinated carbon nanofibers in primary lithium battery.

NbF5 and TaF5: Assignment of 19F NMR resonances and chemical bond analysis from GIPAW calculations

The 19F isotropic chemical shifts (δiso) of two isomorphic compounds, NbF5 and TaF5, which involve six nonequivalent fluorine sites, have been experimentally determined from the reconstruction of 1D 19F MAS NMR spectra. In parallel, the corresponding 19F chemical shielding tensors have been calculated using the GIPAW method for both experimental and DFT-optimized structures. Furthermore, the [M4F20] units of NbF5 and TaF5 being held together by van der Waals interactions, the relevance of Grimme corrections to the DFT optimization processes has been evaluated. However, the semi-empirical dispersion correction term introduced by such a method does not show any significant improvement. Nonetheless, a complete and convincing assignment of the 19F NMR lines of NbF5 and TaF5 is obtained, ensured by the linearity between experimental 19F δiso values and calculated 19F isotropic chemical shielding σiso values. The effects of the geometry optimizations have been carefully analyzed, confirming among other matters, the inaccuracy of the experimental structure of NbF5. The relationships between the fluorine chemical shifts, the nature of the fluorine atoms (bridging or terminal), the position of the terminal ones (opposite or perpendicular to the bridging ones), the fluorine charges, the ionicity and the length of the M-F bonds have been established. Additionally, for three of the 19F NMR lines of NbF5, distorted multiplets, arising from 1J-coupling and residual dipolar coupling between the 19F and 93Nb nuclei, were simulated yielding to values of 93Nb-19F 1J-coupling for the corresponding fluorine sites.

An ab initio density functional theory calculations on the K2YF5 crystal containing hydroxyl impurities

High frequency absorption spectral lines not matching any of the chemical constituents were observed while analyzing the infrared experimental spectrum of a K2YF5:Tb+3 sample. We ascribe these lines to the presence of impurities that inadvertently contaminated the crystal compound during synthesis, whose mass and electronegativity apparently indicate OH substitutional ions occupying fluorine sites. In this report we have performed ab initio calculations by means of a solid state computational code, applied to a model consisting of a potassium-yttrium-double fluoride structure where OH ion aggregates are introduced on F sites, which indeed confirm such assignment.

GET-SERF, a new gradient encoded SERF experiment for the trivial edition of 1H–19F couplings

A new spatially encoded heteronuclear 1H–19F selective refocusing NMR experiment (GET-SERF) is proposed. This sequence allows editing in one single 2D experiment all couplings between a selected fluorine site and all the proton nuclei of the molecule. Its efficiency is illustrated in the case of diflunisal, a difluorinated anti-inflammatory drug, in isotropic and anisotropic media.