Reaction Of Fluorine Atoms Research Articles

Infrared spectroscopic observation of the radical •XeF3 generated in solid argon

Xenon trifluoride radicals were generated by the solid-state chemical reaction of mobile fluorine atoms with XeF(2) molecules isolated in a solid argon matrix. On the basis of spectroscopic and kinetic FTIR measurements and performed quantum chemical calculations, two infrared absorption bands at 568 (strong) and 523 (very weak) cm(-1) have been assigned to asymmetric and symmetric Xe-F stretching vibrational modes of radical (*)XeF(3), respectively. Chemical reaction of fluorine atom with XeF(2) in a solid argon cage obeys specific kinetic behavior indicating the formation of a long-lived intermediate complex under the condition that the diffusing fluorine atom is attached to isolated XeF(2) at temperatures 20 K < T < 27 K. Subsequent thermally activated conversion in the complex is the main source of novel xenon-containing radical species (*)XeF(3). The rate constant and energy barrier are estimated for the reaction in an argon cage, [XeF(2)-F] --> (K(r)) [XeF(3)], as K(r) approximately 7 x 10(-5) c(-1) at 27 K and E approximately 1.2 kcal/mol, respectively. Quantum chemistry calculations reveal that radical (*)XeF(3) has a planar C(2v) structure. DFT calculations show that formation of the third Xe-F bond in the (*)XeF(3) radical is exothermic, and the binding energy of the third Xe-F bond is 8-20 kcal/mol.

State-to-state quasi-classical trajectory study of the F + CH 2D 2 reaction

The reaction of fluorine atoms with bideuterated methane can evolve along two paths, D- and H-abstraction. On an analytical potential energy surface (PES-2006) recently developed by our research group, quasi-classical trajectory (QCT) calculations were performed at different collision energies. The dynamics properties of the two paths – vibrational and rotational distributions, product energy partitioning, excitation functions and scattering distributions – are in general similar, suggesting the idea of an almost similar importance of the two mechanisms, with the H-abstraction being slightly favoured. Comparison with the scarce experimental information shows reasonable agreement, and the differences could be due to deficiencies of the PES, but also to the known limitations of the QCT method.

ChemInform Abstract: Dynamical Resonances in the Fluorine Atom Reaction with the Hydrogen Molecule

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Dynamical Resonances in the Fluorine Atom Reaction with the Hydrogen Molecule

[Reaction: see text]. The concept of transition state has played a crucial role in the field of chemical kinetics and reaction dynamics. Resonances in the transition state region are important in many chemical reactions at reaction energies near the thresholds. Detecting and characterizing isolated reaction resonances, however, have been a major challenge in both experiment and theory. In this Account, we review the most recent developments in the study of reaction resonances in the benchmark F + H 2 --> HF + H reaction. Crossed molecular beam scattering experiments on the F + H 2 reaction have been carried out recently using the high-resolution, highly sensitive H-atom Rydberg tagging technique with HF rovibrational states almost fully resolved. Pronounced forward scattering for the HF (nu' = 2) product has been observed at the collision energy of 0.52 kcal/mol in the F + H 2 (j = 0) reaction. Quantum dynamical calculations based on two new potential energy surfaces, the Xu-Xie-Zhang (XXZ) surface and the Fu-Xu-Zhang (FXZ) surface, show that the observed forward scattering of HF (nu' = 2) in the F + H 2 reaction is caused by two Feshbach resonances (the ground resonance and first excited resonance). More interestingly, the pronounced forward scattering of HF (nu' = 2) at 0.52 kcal/mol is enhanced considerably by the constructive interference between the two resonances. In order to probe the resonance potential more accurately, the isotope substituted F + HD --> HF + D reaction has been studied using the D-atom Rydberg tagging technique. A remarkable and fast changing dynamical picture has been mapped out in the collision energy range of 0.3-1.2 kcal/mol for this reaction. Quantum dynamical calculations based on the XXZ surface suggest that the ground resonance on this potential is too high in comparison with the experimental results of the F + HD reaction. However, quantum scattering calculations on the FXZ surface can reproduce nearly quantitatively the resonance picture of the F + HD reaction observed in the experiment. It is clear that the dynamics of the F + HD reaction below the threshold was dominated by the ground resonance state. Furthermore, the forward scattering HF (nu' = 3) channel from the F + H 2 ( j = 0) reaction was investigated and was attributed mainly to a slow-down mechanism over the centrifugal exit barrier, with small contributions from a shape resonance mechanism in a narrow collision energy range. A striking effect of the reagent rotational excitation on resonance was also observed in F + H 2 ( j = 1), in comparison with F + H 2 ( j = 0). From these concerted experimental and theoretical studies, a clear physical picture of the reaction resonances in this benchmark reaction has emerged, providing a textbook example of dynamical resonances in elementary chemical reactions.

Observation and assignment of the first photoelectron band of the CH 3CO (X 2A) radical

HeI photoelectron spectra have been recorded for the reaction of fluorine atoms with acetaldehyde at different reaction times. A structured band associated with a short-lived primary reaction product was recorded at a reactant mixing distance of 6 mm above the photon beam. The vertical ionization energy was measured as (8.39 ± 0.05 eV), and regularly spaced vibrational structure was observed in this band with an average vibrational separation of (1605 ± 30 cm −1). The band was assigned to ionization of the CH 3CO (X 2A) radical on the basis of first vertical ionization energies computed for the two possible primary products, CH 3CO and CH 2CHO, using ab-initio calculations. The calculations carried out on CH 3CO (X 2A), and its singlet ground cationic state, revealed that the observed vibrational structure can be assigned to excitation of the C O stretching mode in the ion. The measured adiabatic ionization energy, 7.21 ± 0.05 eV, is higher than the value expected (6.94 ± 0.03 eV) from evidence in the literature and calculated values in this work. This difference is assigned to poor Franck–Condon factors in the region of the band onset which means that the true adiabatic ionization energy was not observed.

Formation and Decomposition of Chemically Activated Cyclopentoxy Radicals from the c-C5H9 + O Reaction

The formation and the decomposition of chemically activated cyclopentoxy radicals from the c-C5H9 + O reaction have been studied in the gas phase at room temperature. Two different experimental arrangements have been used. Arrangement A consisted of a laser-flash photolysis set up combined with quantitative Fourier transform infrared spectroscopy and allowed the determination of the stable products at 4 mbar. The c-C5H9 radicals were produced via the reaction c-C5H10 + Cl with chlorine atoms from the photolysis of CFCl3; the O atoms were generated by photolysis of SO2. Arrangement B, a conventional discharge flow-reactor with molecular beam sampling, was used to determine the rate coefficient. Here, the hydrocarbon radicals (c-C5H9, C2H5, CH2OCH3) were produced via the reaction of atomic fluorine with c-C5H10, C2H6, and CH3OCH3, respectively, and detected by mass spectrometry after laser photoionization. For the c-C5H9 + O reaction, the relative contributions of intermediate formation (c-C5H9O) and direct abstraction (c-C5H8 + OH) were found to be 68 +/- 5 and 32 +/- 4%, respectively. The decomposition products of the chemically activated intermediate could be identified, and the following relative branching fractions were obtained: c-C5H8O + H (31 +/- 2%), CH2CH(CH2)2CHO + H (40 +/- 5%), 2 C2H4 + H + CO (17 +/- 5%), and C3H4O + C2H4 + H (12 +/- 5%). Additionally, the product formation of the c-C5H8 + O reaction was studied, and the following relative yields were obtained (mol %): C2H4, 24%; C3H4O, 18%; c-C5H8O, 30%; c-C5H8O, 23%; 4-pentenal, 5%. The rate coefficient of the c-C5H9 + O reaction was determined relative to the reactions C2H5 + O and CH3OCH2 + O leading to k = (1.73 +/- 0.05) x 10(14) cm3 mol(-1) s(-1). The experimental branching fractions are analyzed in terms of statistical rate theory with molecular and transition-state data from quantum chemical calculations, and high-pressure limiting Arrhenius parameters for the unimolecular decomposition reactions of C5H9O species are derived.

Rate Constant for the Reaction H + C2H5 at T = 150−295 K

The reaction between the hydrogen atom and the ethyl (C2H5) radical is predicted by photochemical modeling to be the most important loss process for C2H5 radicals in the atmospheres of Jupiter and Saturn. This reaction is also one of the major sources for the methyl radicals in these atmospheres. These two simplest hydrocarbon radicals are the initial species for the synthesis of larger hydrocarbons. Previous measurements of the rate constant for the H + C2H5 (1) reaction varied by a factor of 5 at room temperature, and some studies showed a dependence upon temperature while others showed no such dependence. In addition, the previous studies were at higher temperatures and generally higher pressures than that needed for use in planetary atmospheric models. The rate constant for the reaction H + C2H5 has been measured directly at T = 150, 202, and 295 K and at P = 1.0 Torr He for all temperatures, and additionally at P = 0.5 and 2.0 Torr He at T = 202 K. The measurements were performed in a discharge−fast flow system. The decay of the C2H5 radical in the presence of excess hydrogen was monitored by low-energy electron impact mass spectrometry under pseudo-first-order conditions. H atoms and C2H5 radicals were generated rapidly and simultaneously by the reaction of fluorine atoms with H2 and C2H6, respectively. The total rate constant was found to be temperature and pressure independent. The measured total rate constants at each temperature are: k1(295 K) = (1.06 ± 0.25) × 10-10, k1(202 K) = (1.05 ± 0.23) × 10-10, and k1(150 K) = (0.94 ± 0.21) × 10-10, all in units of cm3 molecule-1 s-1. The total rate constant derived from all the combined measurements is k1 = (1.07 ± 0.18) × 10-10 cm3 molecule-1 s-1. At room-temperature our results are about a factor of 2 higher than the recommended rate constant and a factor of 3 lower than the most recently published study.

Assignment of the first photoelectron band of the CH3CHCl radical using ab initio quantum mechanical calculations

HeI photoelectron spectra have been recorded for the reaction of atomic fluorine with ethyl chloride at different reaction times. A structured band associated with a short-lived primary reaction product has been recorded with adiabatic and vertical ionization energies of (7.84±0.02) and (8.18±0.02) eV respectively. An average vibrational separation of (680±30) cm−1was observed in this band. Comparison between the experimental vertical and adiabatic ionization energies and ionization energies computed for CH3CHCl (X2A) and CH2CH2Cl (X2A) at different levels of theory led to the assignment of the observed first photoelectron band to the ionization of CH3CHCl (X2A). The observed vibrational structure was assigned to excitation of the C–Cl stretching mode in CH3CHCl+(X1A).

Mid-Infrared Spectrum of the Gas-Phase Ethyl Peroxy Radical: C2H5OO

The mid-IR spectrum of gas-phase C2H5OO has been measured over the spectral range of 800−4000 cm-1. The employed experimental technique was long-path-length absorption in which a White cell was coupled to an FTIR spectrometer. Radicals were generated in a mixing manifold prior to the absorption cell by passing an F2/He mixture through a microwave discharge to create fluorine atoms followed by the reaction of atomic fluorine with an ethane/oxygen mixture. A number of absorption bands were observed to increase in intensity with increasing atomic fluorine concentration. Verification that the observed bands originated from the ethyl peroxy radical was done first by comparing measured band frequencies with the spectra of possible secondary reaction products and second by titrating C2H5OO• with excess nitric oxide and observing the nitrogen dioxide and ethyl nitrite products that ultimately result from the titration reaction. Bands were assigned to normal modes of C2H5OO• through comparing the observed band positions and intensities to the results of ab initio calculations and to the previous work of Chettur and Snelson (Chettur, G.; Snelson, A. J. Phys. Chem. 1987, 91, 3484) and Pushkarsky, Zalyubovsky, and Miller (Pushkarsky, M.B.; Zalyubovsky, S. J.; Miller, T. A. J. Chem. Phys. 2000, 112, 10695).

Theoretical study of penetration reaction of fluorine atoms and ions into hydrogen-terminated Si(111) thin film

The interaction of fluorine atoms and ions with hydrogen-terminated Si(111) thin film has been studied using periodic ab initio electronic structure calculations. Penetration of fluorine atoms into the Si surface is found to be quite easy, and an interstitial fluorine atom can be situated directly behind the SiH bond while the original lattice structure, as well as the SiH bond itself, remains intact. The fluorine atom in this position is negatively charged by accumulating electron charges from the nearest silicon atoms. The penetration reaction of the fluorine ions into the Si surface is also found to be easy.

Determining the heat of a surface plasmochemical reaction by scanning calorimetry

A method for experimental determination of the heat of an exothermic reaction, which proceeds on a solid surface under the action of a chemically active plasma and is accompanied by the production of volatile compounds, is proposed. Scanning calorimetry in a discharge is used in order to determine the contribution due to the heat release of the chemical reaction to the total heat flux onto the surface. It is shown how necessary it is to consider the contributions of different heat exchange mechanisms when determining the reaction heat in the case of a significant difference between the temperatures of the active and inert calorimeters. The heat of the exothermic chemical reaction of atomic fluorine with a unit mass of single-crystal silicon in a CF4+O2 plasma is 31±2 kJ/g or 9 eV per Si atom.

EPR spectroscopy of the radical–molecular complex NH 2–HF formed in low temperature chemical reaction of fluorine atoms with NH 3 molecules trapped in solid argon

Chemical reactions of F atoms with NH 3 molecules in a solid argon matrix were studied with electron paramagnetic resonance (EPR) spectroscopy. Fluorine atoms were generated by UV photolysis of F 2 molecules in dilute solutions of F 2 and NH 3 in solid Ar. To distinguish reactions of photogenerated translationally excited F atoms (at 15 K) from reactions of diffusing thermal F atoms (at T>20 K ) experiments were conducted at different temperatures. The radical–molecular complex NH 2–HF is observed for the first time as an intermediate product in reactions involving both types of F atoms. The hyperfine (hf) constants of the NH 2–HF complex: a N =1.20 mT , a H =2.40 mT , and a F =0.70 mT , were determined from analysis of EPR spectra in isotopic substituted samples ( 14 N→ 15 N , and H→D). The performed calculations revealed that complex NH 2–HF has a planar C 2v structure and a binding energy of 12.3 kcal/mol. Optimisation of arrangement of the complex in an argon lattice shows that the complex suffers a minor distortion in crystalline lattice relative to its equilibrium geometry in a gas phase. Calculated hf constants of the complex are in good agreement with those observed in EPR experiments.

A Kinetic and Mechanistic Study of the Low-Temperature Fluorine-Initiated Copolymerization of Tetrafluoroethylene with Oxygen

The fluorine-initiated reaction of tetrafluoroethylene with oxygen, in a liquid phase at low temperature, leads to almost quantitative yields of polymeric perfluoroethers made by CF2CF2O and CF2O units and containing variable amounts of peroxy bonds. This chain reaction is studied in a series of semibatch experiments, and the results are interpreted on the basis of a rather simplified reaction mechanism involving a sequence of elementary steps of different free-radical species. A most unexpected experimental finding is the positive action exerted by the oxygen pressure on the initiation rate; this effect is attributed to a competitive reaction of fluorine atoms with molecular oxygen to form FO2 reactive species which are able to produce a substantial number of chain-initiating biradicals. The kinetic relationships deduced from the overall reaction model are proved to fairly describe, in a quantitative way, the correlation between the operating variables and the various polymer structure indexes. The value...

Laser-induced fluorescence of the CD2CFO radical

The laser-induced fluorescence spectrum of the B̃ 2A″→X̃ 2A″ transition of the CD2CFO radical has been observed in the region 316–335 nm. The radical was produced by 193 nm photolysis or by fluorine atom reaction with acetyl-d3 fluoride. The spectrum of CD2CFO was similar to that of CH2CFO reported previously except for small isotope shifts in the range 7–343 cm−1. The isotope shifts support the assignment of these spectra to fluorinated vinoxy radicals, and rule out the alternate assignment to FCO proposed by others. The X̃→B̃ electronic transition energy (T0) for CD2CFO was measured to be 29 867 cm−1, which is only 7 cm−1 lower than that for CH2CFO. From an analysis of the laser-induced single vibronic level fluorescence, some of the vibrational frequencies can be assigned for the ground electronic state; ν3(CO str.)=1735; ν4(CD2 sciss.)=1043; ν5(CF str.)=1248; ν6(CD2 rock.)=774; ν7(CC str.)=863; ν8(CCF bend)=597; and ν9(CCO bend)=370 cm−1. For the B̃ 2A″ state, ν3=1772; ν4=1073; ν5=1241; ν6=783; ν7=827; ν8=530; and ν9=370 cm−1. These assignments are supported by ab initio calculations. Among these fundamental frequencies, the ν4 and ν6 modes showed the largest isotope shifts, although isotope effects were observed in all the above vibrational fundamentals. The radiative lifetimes of the excited CD2CFO and the quantum yield of formation of the CH2CFO radical from photolysis of CH3CFO at 193 nm are also reported.

Laser-Induced Fluorescence of Methyl Substituted Vinoxy Radicals and Reactions of Oxygen Atoms with Olefins

Laser-induced fluorescence spectra in the 330−370 nm region are assigned to the five expected methyl substituted vinoxy radicals: 1-methylvinoxy, 2-methylvinoxy, 1,2-dimethylvinoxy, 2,2-dimethylvinoxy, and 1,2,2-trimethylvinoxy radicals. Substituted vinoxy radicals were produced by photolyses or by chlorine or fluorine atom reactions with ketones, aldehydes, or ethers. These radicals were also observed when reacting oxygen atoms with olefins such as propene, cis- and trans-2-butene, isobutene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene. These results extend our knowledge of the replacement mechanism in O + olefin reactions, i.e., a methyl radical or a hydrogen atom can be expelled directly from the triplet biradical formed by an O atom addition to an olefin. Methyl substituted vinoxy radicals can also be produced by the release of a methyl radical from the 2-position carbon of an energized singlet aldehyde or ketone formed by a 1,2-migration of a hydrogen atom in the triplet biradical.

Reaction of the fluorine atom and molecule with the hydrogen-terminated Si(111) surface

To establish the self-limiting reaction process that is necessary to achieve the atomic layer-by-layer etching for the damageless fabrication of nanometer-electronics devices, the initial reaction of fluorine (F) atoms and F2 molecules with hydrogen (H)-terminated Si(111) was studied employing a combined system of Fourier transform infrared (FTIR)-attenuated total reflection (ATR) and x-ray photoelectron spectroscopy (XPS). In the ATR measurement, peaks of 2086 cm−1 (B2) and 2090 cm−1 (B3) newly appeared instead of a decrease in the original Si–H peak at 2083 cm−1 (B1) with initial exposure of XeF2. The sum area of B1, B2, and B3 peaks until ∼2000 L was almost constant. This implies that B2 and B3 peaks also resulted from Si–H bonds. The XPS measurement revealed that the initial exposure of XeF2 generated nonbonded F atoms at first, followed by SiF1 bonds. Based on the good correspondence between ATR and XPS results, first the F atoms penetrate just underneath the Si–H bond, generating the B2 peak. After further exposure the B3 peak appears arising from the bonding of an F atom with a Si–H bond at the five-coordination state. However, further exposure of F atoms caused higher order SiFx (x=1,2,3) products. Hence, an F2 gas that was less reactive than F atoms was investigated. It was found that the exposure of H-terminated Si(111) to 5% F2/He reached a plateau value at 5×105 L, where terminated H atoms completely disappeared. The SiF monolayer corresponded exactly to the formation of an atomic layer of Si(111). This indicates that the self-limiting process for the Si/F system is realized first.

HF infrared emission from the reactions of atomic fluorine with methylcyanide, methylisocyanide, dimethylsulfide and dimethyldisulfide

In the reactive systems F + CH 3CN, F + CH 3NC, F + CH 3SCH 3 and F + CH 3SSCH 3 infrared emission spectra were recorded from HF in the fundamental region under relaxation-free conditions. The reported vibrational distributions are for F+CH 3CN:N υ=1 :N υ=2 :N υ=3 =0.38:0.42:0.20 for F+ CH 3NC:N υ=1 :N υ=2 :N υ=3 :N υ=4 :=0.46:0.32:0.17:0.05;for F+CH 3 SCH 3:N υ=1 :N υ=2 :N υ=3 :=0.36:0.39:0.25;for F+ CH 3SSCH 3:N υ=1 :N υ=2 :N υ=3 :N υ=4 :=0.43:0.35:0.19:0.04 Rotational distributions are drawn up and discussed for F + CH 3CN and F + CH 3SCH 3. There is strong evidence for the existence of two microscopic channels in all four systems investigated.

Computation of electron affinities of O and F atoms, and energy profile of F–H2 reaction by density functional theory and ab initio methods

The validity of hybrid and nonlocal DFT methods are tested on examples of systems which are difficult to model by way of quantum chemistry techniques. The electron affinities for the oxygen and fluorine atoms were calculated. The exothermicity, the barrier for the fluorine atom reaction with the hydrogen molecule, and the energy of the H–F bond and its distance were computed with DFT methods, as well as, with ROHF, MPn, and QCISD(T) ab initio methods. The computations were performed by using various basis sets, with 6-311++G(3df,3pd) as the largest. The obtained results are compared with the experimental values. The results of the Becke3LYP hybrid method is in qualitative agreement with experimental results and in the majority of the cases reassembles the high cost QCISD(T) calculation results. Considering the modest computational cost for DFT methods, Becke3LYP/6-311+G(2d,2p) is suggested as the standard theory model for computation, and Becke3LYP/6-311++G(3df,3pd) as the model for generating highly accurate results. They should be applicable to relatively sizable chemical systems.

Kinetics of the Reactions of Acetonitrile with Chlorine and Fluorine Atoms

The rate coefficients for the reactions of chlorine and fluorine atoms with acetonitrile have been measured using relative and direct methods. In the case of chlorine atoms the rate coefficient k1 was measured between 274 and 345 K using competitive chlorination and at 296 K using laser flash photolysis with atomic resonance fluorescence. The rate coefficient measured at ambient temperature (296 ± 2 K) is (1.15 ± 0.20) × 10-14 cm3 molecule-1 s-1, independent of pressure between 5 and 700 Torr (uncertainties are 2 standard deviations throughout). This result is a factor of 6 higher than the currently accepted value. The results from the three independent determinations reported here yield the Arrhenius expression k1 = (1.6 ± 0.2) × 10-11 exp[−(2140 ± 200)/T] cm3 molecule-1 s-1. Product studies show that the reaction of Cl atoms with CH3CN proceeds predominantly, if not exclusively, by hydrogen abstraction at 296 K. The rate coefficient for the reaction of fluorine atoms with acetonitrile was measured using...

Reactions of Fluorine Atoms with Self-Assembled Monolayers of Alkanethiolates

Self-assembled monolayers (SAMs) of methyl- and vinyl-terminated n-alkanethiolates on gold have been exposed at 305 K to beams of atomic fluorine in an ultrahigh-vacuum environment. In situ X-ray photoelectron spectroscopy of the resulting monolayers indicates that the incoming fluorine atoms penetrate the close-packed chains and react to form mono- and difluorinated methylene groups without fluorination of the gold substrate and with little or no loss of carbon atoms. No reaction was observed with molecular fluorine. After a cumulative dose of ∼680 fluorine atoms per carbon chain, approximately seven of the 20 chain carbon atoms have been fluorinated. The fluorination most likely occurs by a mechanism in which an incoming fluorine atom abstracts a chain methylene hydrogen atom, creating a radical site for subsequent addition of fluorine atom. Kinetic modeling of the fluorine atom uptake indicates that the cross sections for hydrogen abstraction and fluorine addition are somewhat smaller than their gas phase counterparts, suggesting that steric crowding limits the overall fluorine atom uptake. No significant differences between the reactivities of the two differently terminated SAMs could be discerned. Our results indicate that C 20 -alkanethiolate SAMs are effective coatings for preventing oxidation of metallic surfaces by gas phase reactants.