Reaction Of Fluorine Atoms Research Articles

Reaction of F Atoms with Carbonyl Sulfide: Experimental and Theoretical Study.

The gas phase reaction of fluorine atoms with carbonyl sulfide, F + OCS → SF + CO (1), has been investigated over a wide temperature range, T = 220-960 K, in a discharge-flow system combined with mass spectrometry for the analysis of the reactive mixture. The reaction rate coefficient was determined as a function of temperature using both absolute (under pseudo-first-order conditions using an excess of F atoms) and relative rate methods: k1 = (8.95 ± 0.52) × 10-11 exp((102 ± 20)/T) cm3 molecule-1 s-1 (with an estimated total uncertainty of 15% independent of temperature). The SF radical was observed as the main primary reaction product. The experimental results are supported by theoretical calculations, which evidence that the reaction is exothermic with a weak negative temperature dependence.

Synthesis of substituted 5,6,7,8‐tetrafluoro‐1H‐benzo[f]indol‐4,9‐diones based on the reaction of hexafluoro‐1,4‐napthoquinone with methyl 3‐aminocrotonates

AbstractFor the first time, the possibility of using hexafluoro‐1,4‐naphthoquinone for the construction of condensed heterocyclic system was demonstrated. As a result, two‐step synthesis of previously unknown 5,6,7,8‐tetrafluoro‐1H‐benzo[f]indol‐4,9‐dione derivatives was elaborated. The suggested method includes initial interaction of hexafluoro‐1,4‐naphthoquinone with various methyl 3‐aminocrotonates and subsequent intramolecular cyclization into the target fluorinated benzo[f]indole‐4,9‐diones. The distinctive feature of considered protocol is regiospecific reaction of fluorine atoms in quinone fragment. Advantages of the presented approach are simple synthetic procedure avoiding chromatographic purification, atom economy and readily available starting materials. The structure of one of the synthesized compounds was established by x‐ray analysis.

Reaction of Atomic Fluorine with Benzene

Reaction of Atomic Fluorine with Benzene

Renormalized chemical kinetics and benchmark quantum mechanical rates: activation energies and tunnelling transitivities for the reactions of fluorine atoms with H2 and HD

Experimental, theoretical and computational chemical kinetics contribute to progress both in molecular and materials sciences and in biochemistry, exploring the gap between elementary processes and complex systems. Stationary state quantum mechanics and statistical thermodynamics provide interpretive tools and instruments for classical molecular dynamics simulations for stable or metastable structures and near-equilibrium situations. Chemical reaction kinetics plays a key role at the mesoscales: time-dependent and evolution problems are typically tackled phenomenologically, and reactions through intermediates and transition states need be investigated and modelled. In this paper, scaling and renormalization procedures are developed beyond the Arrhenius equation and the Transition State Theory, regarding two key observables in reaction kinetics, the rate “constant” as a function of temperature (and its reciprocal, the generalised lifetime), and the apparent activation energy (and its reciprocal, the transitivity function). Coupled first-order equations—dependent on time and on temperature—are formulated in alternative coupling scheme they link experimental results to effective modelling, or vice versa molecular dynamics simulations to predictions. The passage from thermal to tunnelling regimes is uniformly treated and applied to converged quantum mechanical calculations of rate constants available for the prototypical three-atom reactions of fluorine atoms with both H2 and HD: these are exothermic processes dominated by moderate tunnel, needing formal extension to cover the low-temperature regime where aspects of universal behaviour are shown to emerge. The results that have been validated towards experimental information in the 10–350 K temperature range, document the complexity of commonly considered “elementary” chemical reactions: they are relevant for modelling atmospheric and astrophysical environments. Perspectives are indicated of advances towards other types of transitions and to a global generality of processes of interest in applied chemical kinetics in biophysics and in astrochemistry.

Rate Coefficients of the Reactions of Fluorine Atoms with H2S and SH over the Temperature Range 220-960 K.

Reaction F + H2S→SH + HF (1) is an effective source of SH radicals and an important intermediate in atmospheric and combustion chemistry. We employed a discharge-flow, modulated molecular beam mass spectrometry technique to determine the rate coefficient of this reaction and that of the secondary one, F + SH→S + HF (2), at a total pressure of 2 Torr and in a wide temperature range 220-960 K. The rate coefficient of Reaction (1) was determined directly by monitoring consumption of F atoms under pseudo-first-order conditions in an excess of H2S. The rate coefficient of Reaction (2) was determined via monitoring the maximum concentration of the product of Reaction (1), SH radical, as a function of [H2S]. Both rate coefficients were found to be virtually independent of temperature in the entire temperature range of the study: k1 = (1.86 ± 0.28) × 10-10 and k2 = (2.0 ± 0.40) × 10-10 cm3 molecule-1 s-1. The kinetic data from the present study are compared with previous room temperature measurements.

Vibrational Energy Relaxation of Deuterium Fluoride in d-Dichloromethane: Insights from Different Potentials.

Vibrationally excited deuterium fluoride (DF) formed by fluorine atom reaction with a solvent was found (Science, 2015, 347, 530) to relax rapidly (less than 10 ps) in acetonitrile-d3 (CD3CN) and dichloromethane-d2 (CD2Cl2). However, insights into how CD2Cl2 facilitates this energy relaxation have so far been lacking, given the weak interaction between DF and a single CD2Cl2. In this work, we report the results of reactive simulations with a two-state reactive empirical valence bond (EVB) potential to study the energy deposited into nascent DF after transition-state passage and of nonequilibrium molecular dynamics simulations using multiple different potential energy functions to model the relaxation dynamics. For these second simulations, we used the standard Merck molecular force field (MMFF) potential, an MMFF-based covalent-ionic empirical valence bond (EVB) potential (EVBCI), a newly developed potential [referred to as MMFF(rDF)] which extends upon the MMFF potential by making the DF/CD2Cl2 interaction depend on the value of the D-F bond stretching coordinate and by taking the anisotropic charge distribution of the solvent molecules into account, the polarizable atomic multipole optimized energetics for biomolecular applications (AMOEBA) potential, and the quantum mechanics/molecular mechanics (QM/MM) potential. The relaxation is revealed to be highly sensitive to the potential used. Neither standard MMFF nor EVBCI reproduces the experimentally observed rapid relaxation dynamics, and they also fail to provide a good description of the interaction potential between DF and CD2Cl2 as calculated using CCSD(T)-F12. This is attributed to the use of a point-charge model for the solute and to failing to model the anisotropic electrostatic properties of CD2Cl2. The MMFF(rDF), AMOEBA, and QM/MM potentials all reproduce the CCSD(T)-F12 two-body DF---CD2Cl2 interaction potential rather well but only with the QM/MM approach is fast vibrational relaxation obtained (lifetimes of ∼288, ∼186, and ∼8 ps, respectively), which we attribute to differences in the solute-solvent local structure. With QM/MM, a unique "many-body" interaction pattern in which DF is in close contact with two solvent Cl atoms and more than three solvent D atoms is found, but this structure is not seen with other potentials. The QM/MM dynamics also display enhanced solute-solvent interactions with vibrationally excited DF that induce a DF band redshift and hence a resonant overlap with solvent C-D modes, which facilitate the intermolecular energy transfer. Our work also suggests that potentials used to model energy relaxation need to capture the fine structure of solute-solvent interactions and not just the two-body part.

Etching of Si3N4 by SF6/H2 and SF6/D2 plasmas

Selective plasma etching of silicon nitride (Si3N4) over silicon oxide (SiO2) is one of critical steps in the nanofabrication processes, where a direct plasma etching of Si3N4 is required for some applications. It was reported recently that Si3N4 etching by remote plasma source can be initiated by the vibrationally excited HF(v) molecules at the low concentration of atomic fluorine. Generally, the main source of HF(v) in plasma is reaction of atomic fluorine with H2 molecule. Due to this fact, the kinetic isotope effect should be observed during Si3N4 etching if replace the H2 source gas on deuterium gas (D2). The data presented here are the Si3N4 and SiO2 etching by the SF6/H2 and SF6/D2 plasmas. A different amount of H2 and D2 have been added to the SF6 discharge during the Si3N4 and SiO2 etching. It was shown that at low H2/D2 flow rate the Si3N4 etch rate is lower in the SF6/D2 discharge. This one confirms the hypothesis about Si3N4 etching by HF(v) and indicates on the fact that HF(v) gives contribution to Si3N4 etching by the direct plasma (not only by the remote plasma) at relatively high concentration of atomic fluorine.

Mass Spectrometry Study of the Reaction of Fluorine Atoms with Benzene

The kinetics of the reaction between fluorine and benzene is studied experimentally at T = 293 K. Competitive reactions are used to find a reaction rate constant. The interaction is studied in a low-pressure flow reactor, and mass spectrometry is used to control the concentration of reagents and products. The reaction between fluorine atoms and methane is used as a competitive one. The ratio of rate constants of the studied reaction (k1) and a competitive one (k2) is found for the first time: k1/k2 = 4.4 ± 0.3. A rate constant is calculated for the reaction of fluorine atoms with benzene using literature data: k1 (293 K) = (2.7 ± 0.5) × 10‒10 cm3 molecule−1 s−1.

Statistical insights into the reaction of fluorine atoms with silicon

The dependences of silicon etching rate on the concentration of F atoms are investigated theoretically. The nonlinear regression analysis of the experimental data indicates that the reaction of F atoms with silicon is 2nd overall order reaction. The relationship between overall reaction order and kinetic reaction order is established using the etching rate equation. It is found that kinetic reaction order monotonically decreases with the increase in concentration of F atoms due to the increased surface coverage. Surface passivation by the reaction products is not observed under the investigated experimental conditions.

Atomic layer etching of Silicon Oxide

The atomic layer etching process for Silicon oxide is studied. The process consists of deposition step and activation step separated by pumping and purging steps. The deposition step is performed in C4F8 plasma and produces a polymer fluorocarbon film with 0.2-3 nm thickness. Then activation step is performed in Argon or Oxygen plasma with DC bias applied to the waferholder. Accelerated ions enhance reaction of fluorine atoms of the polymer film with underlaying Silicon oxide which leads to controllable saturated etching.

The reaction of fluorine atoms with methanol: yield of CH3O/CH2OH and rate constant of the reactions CH3O + CH3O and CH3O + HO2.

Xenondifluoride, XeF2, has been photolysed in the presence of methanol, CH3OH. Two reaction pathways are possible: F + CH3OH → CH2OH + HF and F + CH3OH → CH3O + HF. Both products, CH2OH and CH3O, will be converted to HO2 in the presence of O2. The rate constants for the reaction of both radicals with O2 differ by more than 3 orders of magnitude, which allows an unequivocal distinction between the two reactions when measuring HO2 concentrations in the presence of different O2 concentrations. The following yields have then been determined from time-resolved HO2 profiles: φCH2OH = (0.497 ± 0.013) and φCH3O = (0.503 ± 0.013). Experiments under low O2 concentrations lead to reaction mixtures containing nearly equal amounts of HO2 (converted from the first reaction) and CH3O (from the second reaction). The subsequent HO2 decays are very sensitive to the rate constants of the reaction between these two radicals and the following rate constants have been obtained: k(CH3O + CH3O) = (7.0 ± 1.4) × 10-11 cm3 s-1 and k(CH3O + HO2) = (1.1 ± 0.2) × 10-10 cm3 s-1. The latter reaction has also been theoretically investigated on the CCSD(T)//M06-2X/aug-cc-pVTZ level of theory and CH3OH + O2 have been identified as the main products. Using μVTST, a virtually pressure independent rate constant of k(CH3O + HO2) = 4.7 × 10-11 cm3 s-1 has been obtained, in good agreement with the experiment.

Kinetics and Products of the Reactions of Fluorine Atoms with ClNO and Br2 from 295 to 950 K.

The kinetics and products of the reactions of F atoms with Br2 and ClNO have been studied in a flow reactor coupled with an electron impact ionization mass spectrometer at nearly 2 Torr total pressure of helium and over a wide temperature range, T = 295-950 K. The rate constant of the reaction F + ClNO → products (1) was determined under pseudo-first order conditions, monitoring the kinetics of F atom consumption in excess of ClNO. The measured temperature independent rate constant, k1 = (1.29 ± 0.13) × 10-10 cm3 molecule-1 s-1 (T = 299-950 K), was found to be in excellent agreement with the only previous low temperature study which allowed to recommend the value of k1 in an extended temperature range, 228-950 K. FCl and Cl atoms were observed as the reactions products (corresponding to two reaction pathways: Cl-atom abstraction and replacement with fluorine atom, respectively) with the independent of temperature, in the range 295-948 K, yields of 0.68 ± 0.10 and 0.32 ± 0.05, respectively. Rate constant of the reaction F + Br2 (2), k2 = (1.28 ± 0.20) × 10-10 cm3 molecule-1 s-1, determined using both absolute and relative rate methods, was found to be independent of temperature at T = 299-940 K.

Review Article: Reactions of fluorine atoms with silicon, revisited, again

Chemical etching of silicon by fluorine atoms in the absence of ion bombardment is reviewed. Controversies on the identity of etching products and reaction probabilities are discussed. Attempts are made to explain the apparent presence of SiF2 as a primary product in many studies, dating back to 1980, but not in others, including those of Harold Winters from as early as 1979. Reported estimates of reaction probabilities (here defined as the probability of removing a Si atom from the substrate per incident F atom) vary by a factor of 2000. When these values, with some corrections and reasonable adjustments, are plotted as a function of F atom flux, most of them fall on a “universal curve” that reveals a large (∼30-fold) decrease in the reaction probability with increasing F flux, from 0.03 at a F flux 1012 cm−2 s−1 to 0.001 at a flux of 1020 cm−2 s−1. These values were extracted from beam experiments with F atoms generated from cracking of F2, including those by Harold Winters, from isotropic etching in plasma experiments (both in-plasma and downstream) with F2, CF4/10%O2, and NF3 feed gases, as well as from molecular dynamics simulations. Reaction coefficients derived from chemical etching rates in SF6 plasmas do not follow this trend, however, suggesting a large enhancement in the F reaction probability (∼20 to 100-fold at F fluxes of 1018–1019 cm−2 s−1), due to the presence of sulfur.

REACTION DYNAMICS. Spectroscopic observation of resonances in the F + H2 reaction.

Photodetachment spectroscopy of the FH2(-) and FD2(-) anions allows for the direct observation of reactive resonances in the benchmark reaction F + H2 → HF + H. Using cooled anion precursors and a high-resolution electron spectrometer, we observe several narrow peaks not seen in previous experiments. Theoretical calculations, based on a highly accurate F + H2 potential energy surface, convincingly assign these peaks to resonances associated with quasibound states in the HF + H and DF + D product arrangements and with a quasibound state in the transition state region of the F + H2 reaction. The calculations also reveal quasibound states in the reactant arrangement, which have yet to be resolved experimentally.

Crossed-beam DC slice imaging of fluorine atom reactions with linear alkanes.

We report the reaction dynamics of F atom with selected alkanes studied by crossed beam scattering with DC slice ion imaging. The target alkanes are propane, n-butane, and n-pentane. The product alkyl radicals are probed by 157 nm single photon ionization following reaction at a collision energy of ∼10 kcal mol(-1). The analyzed data are compared with the corresponding theoretical studies. Reduced translational energy distributions for each system show similar trends with little of the reaction exoergicity appearing in translation. However, the pentane reaction shows a somewhat smaller fraction of available energy in translation than the other two, suggesting greater energy channeled into pentyl internal degrees of freedom. The center-of-mass angular distributions all show backscattering as well as sharp forward scattering that decreases in relative intensity with the size of the molecule. Possible reasons for these trends are discussed.

Kinetics and Mechanism of the Reaction of Fluorine Atoms with Pentafluoropropionic Acid

The kinetics of the reaction between fluorine atoms and pentafluoropropionic acid has been studied experimentally at T = 262-343 K. The overall reaction rate constant decreases with temperature: k1(T) = 6.1 × 10(-13) exp(+1166 K)/T) cm(3) molecule(-1) s(-1). The potential energy surface of the reaction has been studied using quantum chemistry. The results were used in transition state theory calculations of the temperature dependences of the rate constants of the two channels of the reaction. The abstraction channel ultimately producing HF, C2F5, and CO2 is dominant at the experimental temperatures; the addition-elimination channel producing C2F5 and CF(O)OH becomes important above 1000 K.

Computational study on fluorine atom reaction with silane molecule (SiH4)

The fluorine atom’s reaction with silane molecule (SiH4) is investigated in this work. Two reaction channels which form SiH3+HF and SiH3F+H are discussed in the microscopic level. The analyses of transition states show that the SiH3+HF channel proceeds through a direct hydrogen abstract mechanism and the SiH3F+H channel could take place via the substitution mechanism. The energetic information of the potential energy surface has been obtained using high-level ab initio molecular orbital theory. A dual-level direct dynamics method is employed to calculate the rate constants of the title reaction. The rate constants of the hydrogen abstraction channel are much larger than the substitution channel. The calculated rate constants are in best agreement with available experimental result.

F + (H2O)2 Reaction: The Second Water Removes the Barrier

In light of controversy concerning the classical barrier for the F + H2O → HF + OH reaction, higher level theoretical methods are applied. Both aug-cc-pV5Z CCSD(T) and cc-pVQZ full CCSDT methods predict a low classical barrier of about 2 kcal/mol. For the analogous water dimer reaction, the presence of the second water molecule erases this barrier entirely. An entrance complex F···(H2O)2 is found lying 7.3 kcal/mol below separated F + (H2O)2. A barrier follows this complex on the reaction coordinate, but lying 3.1 kcal/mol below the reactants. There is also an exit complex HF···H2O3 lying 11.0 kcal/mol below the separated products HF + H2O3. Thus the reactions of atomic fluorine with the water monomer and dimer are qualitatively different.

Kinetics and mechanism of the reaction of fluorine atoms with trifluoroacetic acid

The kinetics of the reaction between fluorine atoms and trifluoroacetic acid has been studied experimentally at T=258–343K. The overall reaction rate constant decreases with temperature: k1(T)=1.09×10−12 exp((+1034K)/T) cm3molecule−1s−1. The potential energy surface of the reaction has been studied using quantum chemistry. The results were used in transition state theory calculations of the temperature dependences of the rate constants of the two channels of the reaction. The abstraction channel ultimately producing HF is dominant at the experimental temperatures; the addition–elimination channel producing CF3 and CF(O)OH becomes important above 1000K.

Dual-level direct dynamics studies on the hydrogen abstraction reactions of fluorine atoms with CF3CH2X(X=F, Cl)

The kinetic properties of the hydrogen abstraction reactions of CF3CH2F + F → CF3CHF + HF (R1) and CF3CH2Cl + F → CF3CHCl + HF (R2) have been studied by dual-level direct dynamics method. Optimized geometries and frequencies of all the stationary points and extra points along the minimum-energy path (MEP) were obtained at the B3LYP/6-311 + G(2d,2p) level. Two complexes with energies less than that of the reactants were located in the reactant side of each reaction. The energy profiles were further refined with the interpolated single-point energies (ISPE) method at the G3(MP2) level of theory. Using canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) method, the rate constants were evaluated over a wide temperature range of 200–2,000 K. Our calculations have shown that C–H bond activity decreases when one hydrogen atom of CF3CH3 is substituted by a fluorine atom, than when substituted with a chlorine atom. This is in good agreement with the experimental results.