Dissociation Of Fluorine Research Articles

Gas-Phase Alcoholysis of Benzylic Halides in the Atmospheric Pressure Ionization Source.

The present study investigates the gas-phase alcoholysis reaction of benzylic halides under atmospheric pressure chemical ionization (APCI) conditions. The APCI corona discharge is used to initiate the novel reaction, which is monitored by ion trap mass spectrometry (IT-MS). The model compound α,α,α-trifluorotoluene is applied to observe the cascade methoxylation reaction during the +APCI-MS analysis, resulting in the formation of [PhC(OCH3)2]+. Based on the results of isotopic labeling and substrate expansion experiments, an addition-elimination mechanism is proposed: initially, the reaction was initiated by the dissociation of fluorine from PhCF3 under APCI condition, leading to the formation of [PhCF2]+; subsequently, two methanol molecules nucleophilicly attack [PhCF2]+ stepwisely, accompanied by the elimination of HF, yielding the product ion [PhC(OCH3)2]+. The proposed mechanism was further corroborated by theoretical calculations. The results of substrate scope expansion experiments suggest that this in-source reaction has the potential to differentiate the positional isomers of alcohols and phenols.

Degradation of Perfluorooctanoic Acid on Aluminum Oxide Surfaces: New Mechanisms from Ab Initio Molecular Dynamics Simulations.

Perfluorooctanoic acid (PFOA) is a part of a large group of anthropogenic, persistent, and bioaccumulative contaminants known as per- and polyfluoroalkyl substances (PFAS) that can be harmful to human health. In this work, we present the first ab initio molecular dynamics (AIMD) study of temperature-dependent degradation dynamics of PFOA on (100) and (110) surfaces of γ-Al2O3. Our results show that PFOA degradation does not occur on the pristine (100) surface, even when carried out at high temperatures. However, introducing an oxygen vacancy on the (100) surface facilitates an ultrafast (<100 fs) defluorination of C-F bonds in PFOA. We also examined degradation dynamics on the (110) surface and found that PFOA interacts strongly with Al(III) centers on the surface of γ-Al2O3, resulting in a stepwise breaking of C-F, C-C, and C-COO bonds. Most importantly, at the end of the degradation process, strong Al-F bonds are formed on the mineralized γ-Al2O3 surface, which prevents further dissociation of fluorine into the surrounding environment. Taken together, our AIMD simulations provide critical reaction mechanisms at a quantum level of detail and highlight the importance of temperature effects, defects, and surface facets for PFOA degradation on reactive surfaces, which have not been systematically explored or analyzed.

Graphene Nanoribbon Synthesis with a Fluorinated Molecular Precursor

The surface-assisted reaction is a key process for graphene nanoribbons (GNRs) synthesis. So far, anthracene based compounds, as represented by 10,10’-dibromo-9,9’-bianthracene, are most-used molecular precursors for GNRs synthesis. However, even the simple anthracene based precursor molecules exhibit a variety of reactivity because the on-surface synthesis includes hierarchical reactions such as dehalogenation, polymerization, and cyclodehydrogenation. Considering that the reactivity of the small precursor molecules determines the final structure of GNRs, the key to success for synthesizing tailored GNRs is the deep understanding the relationship between the reactivity of the molecules and the geometry on the catalytic metal surface. Here, we investigated the reactivity of a fluorinated precursor molecule during the GNR formation. The fluorine atoms were introduced at 2,3,6,7-positions of an anthracene sandwiched by 9-bromoanthracenes in order that the effect of fluorine substitution at the edge positions of polyanthrylene and GNRs would be investigated. Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) indicate the formation of the edge-fluorinated polyanthrylenes. Interestingly, the following cyclodehydrogenation yields GNRs, dissociating the carbon-fluorine bonds at the edge positions of polyanthrylenes. Namely, the strong C-F bond survived during the polymerization step, but the following cyclodehydrogenation cleaved the C-F bond. Theoretical calculation implies that the structure of the intermediate state during the cyclodehydrogenation played an important role in the fluorine dissociation. Thus, the lessons from the dissociation of the strong C-F bond, which is believed as one of the strongest bonding in the solution chemistry, would provide the synthetic access to the proper design of precursor molecules for edge-functionalized GNRs.

Fluorination of the diamond surface by photoinduced dissociation ofC60F48

The effect of synchrotron beam irradiation on ${\mathrm{C}}_{60}$F${}_{48}$ adlayers absorbed on the hydrogen-terminated C(100) diamond surface has been examined using core level photoelectron spectroscopy. Irradiation causes the dissociation of fluorine from the ${\mathrm{C}}_{60}$F${}_{48}$ and the formation of a fluorine-terminated surface with a (2\ifmmode\times\else\texttimes\fi{}1) surface reconstruction. This is accompanied by a loss in the ${\mathrm{C}}_{60}$F${}_{48}$-induced surface transfer doping of the diamond substrate. The simplicity of this process makes it highly applicable to monofluoride surface patterning for nanoelectronic and surface functionalization applications of diamond.

A tractable and accurate electronic structure method for static correlations: The perfect hextuples model

We present the next stage in a hierarchy of local approximations to complete active space self-consistent field (CASSCF) model in an active space of one active orbital per active electron based on the valence orbital-optimized coupled-cluster (VOO-CC) formalism. Following the perfect pairing (PP) model, which is exact for a single electron pair and extensive, and the perfect quadruples (PQ) model, which is exact for two pairs, we introduce the perfect hextuples (PH) model, which is exact for three pairs. PH is an approximation to the VOO-CC method truncated at hextuples containing all correlations between three electron pairs. While VOO-CCDTQ56 requires computational effort scaling with the 14th power of molecular size, PH requires only sixth power effort. Our implementation also introduces some techniques which reduce the scaling to fifth order and has been applied to active spaces roughly twice the size of the CASSCF limit without any symmetry. Because PH explicitly correlates up to six electrons at a time, it can faithfully model the static correlations of molecules with up to triple bonds in a size-consistent fashion and for organic reactions usually reproduces CASSCF with chemical accuracy. The convergence of the PP, PQ, and PH hierarchy is demonstrated on a variety of examples including symmetry breaking in benzene, the Cope rearrangement, the Bergman reaction, and the dissociation of fluorine.

Characteristics improvement of HfO 2/Ge gate stack structure by fluorine treatment of germanium surface

Chemical reactivity of fluorine molecule (F 2)–germanium (Ge) surface and dissociation of fluorine (F)–Ge bonding have been simulated by semi-empirical molecular orbital method theoretically, which shows that F on Ge surface is more stable compared to hydrogen. Ge MIS (metal insulator semiconductor) capacitor has been fabricated by using F 2-treated Ge(1 0 0) substrate and HfO 2 film deposited by photo-assisted MOCVD. Interface state density observed as a hump in the C– V curve of HfO 2/Ge gate stack and its C– V hysteresis were decreased by F 2-treatment of Ge surface. XPS (X-ray photoelectron spectroscopy) depth profiling reveals that interfacial layer between HfO 2 and Ge is sub-oxide layer (GeO x or HfGeO x ), which is believed to be origin of interface state density. F was incorporated into interfacial layer easily by using F 2-treated Ge substrate. These results suggest that interface defect of HfO 2/Ge gate stack structure could be passivated by F effectively.

Novel bromine oxyfluorides: structures, thermochemistry and electron affinities of BrOF n /BrO (n = 1–5)

Four selected density functional theory (DFT) and hybrid Hartree-Fock/DFT methods have been used to carry out an investigation of the molecular structures, thermochemistry and the neutral-anion energy separations of the bromine oxygen fluorides BrOF n /BrO (n = 1–5). Double-ζ plus polarization quality basis sets augmented with the diffuse functions, labelled as DZP++, were adopted. Thirty-four stationary points have been located on their potential energy hypersurfaces. Several previously unknown and novel structures are discovered in this research. For example, a beautiful, nearly octahedral C4v structure is predicted to be a local minimum for BrOF5. Most global minima are found to be Br-centred, O-terminal structures, except for OBrF5, which has a higher energy than Br(OF)F4 by ∼6 kcal mol−1. The adiabatic electron affinities (EAad) with ZPVE correction are predicted by BP86 to be 3.46 eV (BrOF), 3.90 eV (BrOF2), 4.02 eV (BrOF3), 6.23 eV (BrOF4) and 5.04 eV (BrOF5). Except for BrOF4, these EAad values are smaller than those for the corresponding Br2F n , BrClF n , and BrF n +1 species. The first fluorine dissociation energies for the BrOF n and BrO (n = 1–5) species are reported.

Rate constant for heterogeneous dissociation of fluorine in a temperature range of 700–900 K on a nickel surface

The aim of the present work was the study of dissociation kinetics of molecular fluorine on the hot surface of a nickel catalyst. The concentration of fluorine atoms was measured by EPR spectroscopy using molecular oxygen as a reference. The fluorine atom concentration was measured with an random error of about 3%. Molecular fluorine dissociation on the walls of a heated nickel tubes was measured. For process simulation and analysis of the experimental curves a one-dimensional kinetic model of the reactor which did not take into account axial and radial diffusion of the gas in the reactor and radial gas speed distribution was used, but account was taken of the pressure drop in the reactor and tubes between the reactor and ESR spectrometer. In the experiments we measured the fluorine atom concentration dependencies on the gas flow velocity and fluorine pressure (10–30 Torr). The dissociation constant on the surface of nickel foil is k d =(2.4±0.7)×10 4 exp [−(19 000±500)/RT] cm/s (at 700–900 K), the probability of heterogeneous recombination on the nickel surface γ = (2.2 ± 0.5) × 10 −4, rate constant of the three-body gas phase recombination on molecular fluorine as `third' body at room temperature k rec = (4.7 ± 1.2) × 10 −34 cm 6/s. This last value coincides with literature value. Our measurements confirmed the value of the equilibrium constant of the fluorine dissociation process: K = 1.11 × 10 25 exp(−37 840/ RT)cm −3 ( K is expressed in cal/mole).

Epitaxial growth mechanisms and structure of CaF2/Si(111).

The early stages of interface formation between ${\mathrm{CaF}}_{2}$ and Si(111) have been studied, in situ, by a combination of reflection high-energy electron diffraction, x-ray diffraction, and core-level photoemission. The results are combined with ex situ transmission-electron-microscopy measurements to show that the initial growth mode changes from Volmer-Weber to Stranski-Krastanow, depending on the substrate temperature. The crossover is correlated with a submonolayer transition from the Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) to a (3\ifmmode\times\else\texttimes\fi{}1) reconstruction. This is accompanied by fluorine dissociation at the interface. Both initial growth modes can lead to a uniform ${\mathrm{CaF}}_{2}$ epilayer and subsequent growth on this surface is layer by layer. Using x-ray crystal truncation-rod analysis, we have examined the ${\mathrm{CaF}}_{2}$/Si(111) surface and interface structure. For films grown at temperatures above the (7\ifmmode\times\else\texttimes\fi{}7)\ensuremath{\rightarrow}(3\ifmmode\times\else\texttimes\fi{}1) transition, the Ca atom in the CaF layer at the interface is located in a single ${\mathit{T}}_{4}$ bonding site. Finally, we have observed a structural transition at the interface from the as-grown structure to a (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} reconstruction, which appears to be incommensurate. The dynamics of this transition and the possible mechanisms will be discussed.

Study of the CaF2/Si interface using x-ray photoelectron spectroscopy

The interfacial properties of CaF2 on Si(111) under two different deposition conditions are studied using x-ray photoelectron spectroscopy (XPS). The CaF2/Si sample grown at a fixed temperature of 600 °C exhibits B-type epitaxy. For the second sample, the Si substrate was ramped from 100 to 600 °C in 5 min and then held at 600 °C until the end of the deposition. Under these conditions, the film exhibits A-type epitaxy, with very different electrical properties. The XPS analysis shows that for both samples, Si is bonded with both Ca and F at the interface. Significant differences are observed, however, for the chemical environment of Ca in the interfacial region for the two samples. In the first sample, Ca–Si bonding is relatively more pronounced, while in the second sample Ca–O bonding is observed to be predominant. In addition, for the sample grown under ramped temperature conditions, the fluorine dissociation is observed to be less and more F–Si bonds form at the interface. The results of this study help to explain the widely different electrical properties of the two films.

The main regularities of atomic fluorine interaction with gases in condensed state

The interaction between intensive flows (10 16÷10 18s −1cm −1) of atomic fluorine with condesed: krypton (77 K), xenon (77÷115 K), xenon difluoride (100÷200 K), nitrogen dioxide (77÷150 K), nitrosyl fluoride (77÷150 K), chlorine (77÷140 K), chlorine trifluoride (77÷190 K) and chlorine(I) oxide were investigated. The fluorination of condensed samples were carried out in reaction volume, where zone of atomic fluorine generation was separated in space from the zone where the reaction took place. Atomic fluorine was formed by means of molecular fluorine dissociation on the heated surface of nickel catalyst and then transported to the surface of the sample to be fluorinated by diffusion. The loss of atomic fluorine during transportation was less then 10%. It was found out that for investigated reaction the following two parameters has the main influence on fluorination kinetics: the sample temperature and intensity of atomic fluorine flow. It was shown, that depending on these two parameters the interaction between atomic fluorine and the sample may be proceeded in one of the three following regimes: 1. the rate fluorination doesn't depend upon its starting thickness and degree of conversion; 2. the fluorinatin to the certain thickness, which decreases while intensity of F-atom flow increases; 3. the rate of fluorination depends upon the degree of conversion but doesn't depend upon the starting thickness. The optimum conditions of certain inorganic fluorides preparative synthesis using atomic fluorine were determined.

Effect of the potential-energy surface on the diatom dissociation rate constant for F2in Arat3000 K

Gas-phase dissociation of fluorine (1Ʃ+g) molecules in an agron bath at 3000 K was studied by using the 3D Monte Carlo classical trajectory (3DMCCT) method. To assess the importance of the potential energy surface (PES) in such calculations, three surfaces, with a fixed, experimentally determined F2dissociation energy, were constructed. These surfaces span the existing experimental uncertainties in the shape of the F2potential. The first potential was the widest and softest; in the second potential the anharmonicity was minimized. The intermediate potential was constructed to ‘localize’ anharmonicity in the energy range in which the collisions are most reactive. The remaining parameters for each PES were estimated from the best available data on interatomic potentials. By using the single uniform ensemble (SUE) method (Kutz, H. D. &amp; Burns, G. J.chem. Phys. 72, 3652-3657 (1980)), large ensembles of trajectories (LET) were generated for the PES. Two such ensembles consisted of 30000 trajectories each and the third of 26200. It was found that the computed one-way-flux equilibrium rate coefficients (Widom, B.Science148, 1555-1560 (1965)) depend in a systematic way upon the anharmonicity of the potential, with the most anharmonic potential yielding the largest rate coefficient. Steady-state reaction-rate constants, which correspond to experimentally observable rate constants, were calculated by the SUE method. It was determined that this method yields (for a given trajectory ensemble, PES and translational temperature) a unique steady-state rate constant, independent of the initial, arbitrarily chosen, state (Tolman, R. C.The principles of statistical mechanics, p. 17. Oxford University Press (1938)) of the LET, and consequently independent of the corresponding initial value of the reaction rate coefficient. For each initial state of the LET, the development of the steady-state rate constant from the equilibrium rate coefficient was smooth, monotonic, and consistent with the detailed properties of the PES. It was found that, although the increased anharmonicity of the F2potential enhanced the equilibrium rate coefficients, it also enhanced the non-equilibrium effects. As a result, the steady-state rate constants were found to be insensitive to the variation of the PES. Thus, the differences among the steady-state rate constants for the three potentials were of the order of their standard errors, which was about 15% or less. On the other hand, the calculated rate constants exceeded the experimental rate constant by a factor of five to six. Because within the limitations of classical mechanics the calculations wereab initio, it was tentatively concluded that the discrepancy of five to six is due to the use of classical mechanics rather than details of the PES structure.

Gaseous actinometry measurements in a flashlamp-pumped pulsed chemical laser

A method is described for applying the well-known technique of gaseous actinometry to determine the initial degree of fluorine dissociation in a flashlamp-pumped chemical laser system. The method is based on a model which takes into account multiple reflections and different path lengths for the incident light in a flashlamp-pumped chemical laser. The use of the method is illustrated with measurements made on a ∼2-liter pulsed chemical laser, and the results are compared with previously reported measurements using other techniques.

Energy lost in formation of fluorine atoms in the course of electron-beam dissociation of fluorine and fluoride molecules

It was found that the composition of a mixture used in a chemical HF laser influenced the energy of formation eF of a fluorine atom when fluorine and fluoride molecules were dissociated by an electron beam. The relative values of eF were determined for SF6, C5F12, and CF4.

Raman spectroscopy of atomic fluorine

We have measured the Raman scattering cross section of atomic fluorine for transitions between the ground fine-structure electronic states. The fluorine was contained a in heated, static chamber. Gas temperature, determined from the rotational Raman spectrum of molecular fluorine, was used as an input to calculate the degree of fluorine dissociation (assuming thermodynamic and chemical equilibrium). The Raman frequency shift and depolarization ratio were also determined. Our results indicate that Raman scattering can be used as a probe for atomic fluorine.

Mixing of plane laminar relaxing gas jets with radiation taken into account

The influence of a chain pumping reaction mechanism in a diffusion-type continuous-action chemical amplifier on an RF molecule is investigated. The actual flow picture is simulated by boundary-layer equations taking account of coherent radiation. It is shown that the efficiency of a chemical amplifier in a mode with partial fluorine dissociation is sufficiently high under the condition of the presence of a line with 0 ≤v≤5.

Dissociation of fluorine by electron impact

Using a previously developed F-atom diagnostic technique, the efficiency of F2 dissociation by an electron beam or an electron-beam-sustained discharge was systematically studied. By monitoring the transient concentration of ions generated during the electron-beam-sustained discharge, we have determined the F−+F+2 ion-ion recombination and F2 dissociative attachment rate constants at various E/P conditions. The dissociative attachment rate constant of F2 at an average electron energy of 1 eV was found to be about (2.3±0.3) ×10−9 cm3/molecule sec−1 and increased with decreasing electron energy. The F−+F+2→3F recombination rate constant at an E/P of 18 kV/cm/atm was found to be about (1.5±0.2) ×10−8 cm3/molecule sec−1 and also increases with decreasing E/P values. The sustained-discharge enhancement of F-atom production from these experiments is F2 is small, ∼1.7.

3088. Dissociation of fluorine by electron impact

3088. Dissociation of fluorine by electron impact

WB6 - DF-CO2transfer laser development

Power extraction and chemiluminescence experiments have been conducted in the large-scale DF-CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> transfer chemical laser (TCL) (IRIS-I and IRIS-II) facility at Bell Aerospace Company (BAC). The modular design of the device allowed testing to be conducted with both a supersonic nozzle bank and also in subsonic flow with sonic injection for the deuterium. Power levels of 15 kW at 10.6 μ were obtained in IRIS-I (subsonic) employing an unstable resonator with a 50-percent output coupling ratio and cavity pressure of 35 torr. For IRIS-II (supersonic) somewhat lower power was obtained. In both systems the fluorine dissociation (α = F/F + 2F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ) was less than 0.01. Chemi-luminescent emission from HF and DF was monitored under zero-power conditions along an axis parallel to the laser-mirror axis. From the measured DF-concentration profiles it can be inferred that vibrationally excited DF is being produced throughout the cavity and, accordingly, the production of DF must be attributed largely to the chain reaction.

Electron beam dissociation of fluorine

A high-energy pulsed electron beam has been used to dissociate fluorine molecules in fluorine, helium, CO2, and hydrogen chloride gaseous mixtures. By using a probe HF laser and monitoring the amount of ir absorption by hydrogen fluoride resulting from the rapid F + HCl → HF + Cl reaction in these mixtures, the fluorine atom concentration was determined. It was found that an upper limit of approximately three fluorine atoms is produced from each electron-molecule ionizing collision.