Isomerisation of CF2ClCH2Cl and CFCl2CH2F by Interchange of Cl and F Atoms with Analysis of the Unimolecular Reactions of Both Molecules

The recombination of CF₂Cl with CH₂Cl and CFCl₂ with CH₂F were employed to generate CF₂ClCH₂Cl* and CFCl₂CH₂F* molecules with 381 and 368 kJ mol^?1, respectively, of vibrational energy in a room‐temperature bath gas. The unimolecular reactions of these molecules, which include HCl elimination, HF elimination, and isomerisation by interchange of chlorine and fluorine atoms, were characterized. The three rate constants for CFCl₂CH₂F were 2.9×10⁷, 0.87×10⁷ and 0.04×10⁷ s^?1 for HCl elimination, isomerisation and HF elimination, respectively. The isomerisation reaction must be included to have a complete characterization of the unimolecular kinetics of CFCl₂CH₂F. The rate constants for HCl elimination and HF elimination from CF₂ClCH₂Cl were 14×10⁷and 0.37×10⁷ s^?1, respectively. Isomerisation that has a rate constant less than 0.08×10⁷ s^?1 is not important. These experimental rate constants were matched to calculated statistical rate constants to assign threshold energies, which are 264, 268, and 297 kJ mol^?1, respectively, for isomerisation, HCl elimination, and HF elimination for CFCl₂CH₂F and 314, 251, and 289 kJ mol^?1 in the same order for CF₂ClCH₂Cl. Density functional theory was used to evaluate the models that were needed for the statistical rate constants; the computational method was B3PW91/6‐31G(d′,p′). Threshold energies for the unimolecular reactions of CF₂ClCH₂Cl and CFCl₂CH₂F are compared to those for CF₂ClCH₃ and CFCl₂CH₃ to illustrate the elevation of threshold energies by F‐ or Cl‐atom substitution at the beta carbon atom (identified by C_H). The DFT calculations systematically underestimate the threshold energy for HCl elimination.     

A theoretical study of the condensation reactions of methyl cation with ammonia,water, hydrogen fluoride and hydrogen sulphide

Ab initio molecular orbital calculations have been used to study the condensation reactions of CH₃^? with NH₃, H₂O, HF and H₂S. Geometry optimization has been carried out at the Hartree—Fock (HF) level with the split-valence plus d-polarization 6-31G* basis set and improved relative energies obtained from calculations which employ the split-valence plus dp-polarization 6-31G** basis set with electron correlation incorporated via Moller—Plesset perturbation theory terminated at third order (MP3). Zero-point vibrational energies have also been determined and taken into account in deriving relative energies. The structures of the intermediates CH₃XH^? (X = NH₂, OH, F and SH) have been obtained and dissociation of these intermediates into CH₂X⁺ + H₂ on the one hand, and CH₃^? + HX on the other, has been examined. It is found that for those species for which the methyl condensation reaction is observed to have an appreciable rate (X = NH₂ and SH), the transition structure for hydrogen elimination from CH₃XH^? lies significantly lower in energy than the reactants CH₃^? + HX (by 75 and 70 kJ mol^?1 respectively). On the other hand, for those species for which the methyl condensation reaction is not observed (X = OH and F), the transition structure for H₂ elimination lies higher in energy than CH₃^? + HX (by 6 and 87 kJ mol^?1 respectively).     

Hydrogen abstraction by fluorine atoms: F + HX and F + DX (X = I,Br, Cl)

Absolute reaction rates for F + HX and F + DX (X = I, Br, Cl) have been obtained by monitoring the rise time of HF (DF) vibrational fluorescence following multiphoton dissociation of SF₆ in mixtures of HX (DX) and argon. The cross sections for reaction are, in units of 10^?16 cm², 4.37, 5.26, and 1.16 for HI, HBr, and HCl, respectively. The isotope effects k_HX/k_DX, are 1.29 ± 0.14, 1.29 ± 0.18, and 1.38 ± 0.29, respectively.     

Kinetics of Cl(2P) reactions with CF3CHCl2, CF3CHFCl,and CH3CFCl2

The rate coefficients for the removal of Cl atoms by reaction with three HCFCs, CF₃CHCl₂ (HCFC-123), CF₃CHFCl (HCFC-124), and CH₃CFCl₂ (HCFC 141b), were measured as a function of temperature between 276 and 397 K. CH₃CF₂Cl (HCFC-142b) was studied only at 298 K. The Arrhenius expressions obtained are: k₁ = (3.94 ± 0.84)× 10^?12 exp[?(1740 ± 100)/T] cm³ molecule^?1 s^?1 for CF₃CHCl₂ (HCFC 123); k₂ = (1.16 ± 0.41) × 10^?12 exp[?(1800 ± 150)/T] cm³ molecule^?1 s^?1 for CF₃CHFCl (HCFC 124); and k₃ = (1.6 ± 1.1) × 10^?12 exp[?(1800 ± 500)/T] cm³ molecule^?1 s^?1 for CH₃CFCl₂ (HCFC 141b). In case of HCFC 141b, non-Arrhenius behavior was observed at temperatures above ca. 350 K and is attributed to the thermal decomposition of CH₂CFCl₂ product into Cl + CH₂CFCl. In case of HCFC-142b, only an upper limit for the 298 K value of the rate coefficient was obtained. The atmospheric significance of these results are discussed. © 1993 John Wiley & Sons, Inc.     

The reaction of (trifluoromethyl)dialkylaminoboranes with HF,HCl and HBr. X-Ray structure investigation of the amineboranes (CF3)2B(X)NHMe2, X  F and OH

The reactions of CF₃B(NMe₂)₂ (I) and (CF₃)₂BNMe₂ (II) with HX (X  F, Cl and Br) have been investigated. Additions with preservation of the BC bonds to yield species with tetracoordinate boron, along with some BN cleavage, were observed. While I formed boronium salts CF₃B(X)(NHMe₂)₂⁺X⁻ with X  Cl and Br, CF₃BF₂ · NHMe₂ (V) was obtained with HF. On the other hand, reactions of II with HX yielded the 1 : 1 adducts (CF₃)₂B(X) · HNMe₂ in each case. Of these, the species with X  F (VI) and X  OH (IX) (obtained by hydrolysis) were examined by single crystal X-ray diffraction. Surprisingly, no difference was found between the average BC bond lengths of these borates (VI 1.612(8), IX 1.624(4) Å) and that of II. The implications of this observation for BCF₃ bonding are discussed.     

Rate constant for the reaction of the CFCl2 radical with oxygen in the pressure range 0.2–12 Torr at 298 K

The rate constant for the reaction of CFCl₂ with oxygen is measured in the pressure range 0.2–12 Torr using pulsed-laser photolysis and time-resolved mass spectrometry. CFCl₂ radicals are generated by photolysis of CFCl₃ at 193 nm. The reaction kinetics are recorded by monitoring the build-up of the CFCl₂O₂ radical concentration. The reaction is in its fall-off region, and the parameters of the relation for the treatment of the fall-off are for M = N₂: k(0) = (5.0 ± 0.8) × 10^?30 cm⁶ molecule^?2 s^?1. k(∞) = (6.0 ± 1.0) × 10^?12 cm³ molecule^?1 s^?1. This value of k(∞) is consistent with results obtained at low pressure taking F_c = 0.6, but the uncertainty in the high-pressure limit is much higher. The results are compared to measurements performed with CH₃ and CF₃. Estimates of the relative third-body efficiencies of He and N₂ are given for CFCl₂ and CF₃.     

On the kinetic stability of the SH3X species with X=F,Cl

Portions of the [S, H₃, X] (X=F, Cl) potential energy surfaces (PESs) were explored using the RHF, MP2, and QCISD(T) methods with emphasis on H₂ and HX eliminations, SH₃X→SHX+H₂ and SH₃X→SH₂+HX, respectively. Upon the halogen X substitution, the most favorable decomposition pathway of SH₄ went over to HX elimination, proceeding with a very low activation barrier of 6.9 (X=F) and 1.3 (X=Cl) kcal/mol. Moreover, the transition states (TSs) for H₂ elimination from SH₃X resembled the less favorable homopolar TS of SH₄. Upon the X=F substitution, the barrier to H₂ loss of SH₄ was calculated to increase from 19.5 to 21.5 kcal/mol. For X=Cl, only the indirect H₂ elimination path via the SH₂+HCl→SHCl+H₂ exchange was found. The hydrogen‐exchange reaction SH₂+HX→SH₂+HX was predicted to occur through formation of the hydrogen‐bonded complex XHSH₂ and with a relatively high barrier of 43.5 (X=F) and 38.5 (X=Cl) kcal/mol. FHSH₂ and ClHSH₂ were found to be the lowest energy species on the [S, H₃, F] and [S, H₃, Cl] PESs, lying 53.4 and 44.7 kcal/mol below SH₃F and SH₃Cl, respectively. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 37–43, 1999     

Kinetic study of the reaction of chlorine atoms with CF3I and the reactions of CF3 radicals with O2, Cl2 and NO at 296 K

The kinetics of the gas-phase reaction of Cl atoms with CF₃I have been studied relative to the reaction of Cl atoms with CH₄ over the temperature range 271–363 K. Using k(Cl + CH₄) = 9.6 × 10^?12 exp(?2680/RT) cm³ molecule^?1 s^?1, we derive k(Cl + CF₃I) = 6.25 × 10^?11 exp(?2970/RT) in which E_a has units of cal mol^?1. CF₃ radicals are produced from the reaction of Cl with CF₃I in a yield which was indistinguishable from 100%. Other relative rate constant ratios measured at 296 K during these experiments were k(Cl + C₂F₅I)/k(Cl + CF₃I) = 11.0 ± 0.6 and k(Cl + C₂F₅I)/k(Cl + C₂H₅Cl) = 0.49 ± 0.02. The reaction of CF₃ radicals with Cl₂ was studied relative to that with O₂ at pressures from 4 to 700 torr of N₂ diluent. By using the published absolute rate constants for k(CF₃ + O₂) at 1–10 torr to calibrate the pressure dependence of these relative rate constants, values of the low- and high-pressure limiting rate constants have been determined at 296 K using a Troe expression: k₀(CF₃ + O₂) = (4.8 ± 1.2) × 10^?29 cm⁶ molecule^?2 s^?1; k_∞(CF₃ + O₂) = (3.95 ± 0.25) × 10^?12 cm³ molecule^?1 s^?1; F_c = 0.46. The value of the rate constant k(CF₃ + Cl₂) was determined to be (3.5 ± 0.4) × 10^?14 cm³ molecule^?1 s^?1 at 296 K. The reaction of Cl atoms with CF₃I is a convenient way to prepare CF₃ radicals for laboratory study. © 1995 John Wiley & Sons, Inc.     

Substituent effects on the disproportionation-combination rate constant ratios for gas-phase halocarbon radicals. Part III: Reactions of CF3 + CF3CH2 CHCF3 and CF3 CH2CHCF3 + CF3CH2 CHCF3

Disproportionation/combination rate constant ratios, k_d/k_c, for the reactive collision between CF₃CH₂CHX + CF₃ radicals and between CF₃CH₂CHX + CF₃CH₂CHX radicals have been measured for X = CF₃. The k_d/k_c = 0.066 ± 0.013 when H is transferred to the CF₃ radical and 0.125 ± 0.025 for H transfer to the CF₃CH₂CHCF₃ radical. Comparison of these results with previous work shows that X = CF₃ increases the k_c/k_c' s relative to X = Cl or H. The effect of the CF₃ substituent on the disproportionation rate is discussed. © 1996 John Wiley & Sons, Inc.     

Reaction of F atoms with dichloromethane by modulated molecular beam mass spectrometry

The reaction of atomic fluorine with dichloromethane has been studied by the diffusion cloud in a flow technique. Fluorine atoms were generated through F₂ dissociation in a high-frequency discharge. The reaction products were detected mass spectrometrically, applying the technique of focusing the paramagnetic component of the molecular beam in an inhomogeneous magnetic field to detect radical species. Cl atoms and CHCl₂ and CF₃ free radicals have been identified among the reaction products. The initial step was shown to be hydrogen atom abstraction. The room temperature rate constant of this reaction was found to be k₀ = (1.51 ± 0.28) X 10^?11 cm³/s. The rate constant of the secondary reaction of fluorine atoms with dichloromethyl radicals, which is suggested to produce mainly HCl, was evaluated as 3 X 10^?10 cm³/s.     

Rate constants for the gas-phase reactions of Cl atoms with a series of hydrofluorocarbons and hydrochlorofluorocarbons at 298 ± 2 K

A relative rate method has been used to determine rate constants for the gas-phase reactions of a series of hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) at 298 ± 2 K and atmospheric pressure of air. Based on a rate constant for the reaction of the Cl atom with CH₄ of (1.0 ± 0.2) ? 10^?13 cm³ molecule^?1 s^?1 at 298 K, the following Cl atom reaction rate constants (in units of 10^?15 cm³ molecule^?1 s^?1) were obtained: CH₃F, 340 ± 70; CH₃CHF₂, 240 ± 50; CH₂FCl, 110 ± 25; CHFCl₂, 21 ± 4; CHCl₂CF₃, 14 ± 3; CHFClCF₃, 2.7 ± 0.6; CH₃CFCl₂, 2.4 ± 0.5; CHF₂Cl, 2.0 ± 0.4; CH₂FCF₃, 1.6 ± 0.3; CH₃CF₂Cl, 0.37 ± 0.08; and CHF₂CF₃, 0.24 ± 0.05. These Cl atom reaction rate constants are compared with literature data and with the corresponding OH radical reaction rate constants. © John Wiley & Sons, Inc.     

Negative ion formation in CF2Cl2, CF3Cl and CFCl3 following low energy (0—10 eV) impact with near monoenergetic electrons

Negative ion formation in CF₂Cl₂, CF₃Cl and CFCl₃ under low-energy electron impact has been investigated using a trochoidal monochromat The ions observed are F^?, Cl^?, FCl^?, Cl₂^?, CFCl₂^? from CF₂Cl₂; F^?, Cl^?, FCl^?, CF₂Cl Quoting available thermochemical data, it can be shown that most of the observed negative ions arise from dissociative attachment processes. Appearance The extremely high yield of Cl^? in CFCl₃, which is observed at ε = 0.0 eV, will be discussed with regard to the lifetime of this molecule i     

The chain mechanism of ozone destruction by CFCl3 in the 214 nm photolysis of its mixtures with oxygen

The effect of CFCl₃ (0.025–0.200 mbar) addition on the formation of ozone in 214 nm photolysis of oxygen (800–2000 mbar) was investigated. Kinetic analysis of the drastic reduction in ozone formation in the presence of CFCl₃ shows that it proceeds by a chain mechanism with a chain length of 5.07 ± 0.21(2σ). This chain length is independent of CFCl₃ and O₂ pressures as well as incident light intensity and the mechanism of the chain reaction is governed by the Cl generating reactions of ClO radicals. A mechanism based only on the self reaction of these radicals: ClO + ClO → Cl₂ + O₂ (7), Cl + ClO₂ (8), and Cl + OClO (9), followed by fast decomposition of ClO₂ into Cl and O₂, predicts a chain length which is considerably lower than the observed value. Incorporation of the reaction CFCl₂O₂ + ClO → CFCl₂O + ClO₂ (11) in the mechanism satisfactorily accounts for the observed chain length. A lower limit of 3 × 10^?12 cm³ molecule^?1 s^?1 for k₁₁ is estimated.     

Reaction of trihalogenmethanesulfenyl chlorides,acetates and trifluoroacetates with norbornene

The reaction of trihalogenomethanesulfenyl chlorides, acetates and trifluoroacetates, RSX R = CF₃, CF₂Cl, CFCl₂, CCl₃; X = Cl, CH₃C(O)O, CF₃C(O)O with norbornene proceeded with skeletal rearrangement. According to the reactants used, 2-endo-X- 3-exo-(SR)-norbornanes, 3-(SR)-nortricylanes, 2-exo-X-7-syn- and -anti-(SR)-norbornanes were formed. The factors influencing the reaction course are discussed.     

Competitive Activation of C?H and C?X Bonds in Reactions of Pt+ with CH3X (X=F,Cl): Experiment and Theory

The reactions of Pt⁺ with CH₃X (X=F, Cl) are studied experimentally by employing an inductively coupled plasma/selected‐ion flow tube tandem mass spectrometer and theoretically by density functional theory. Dehydrogenation and HX elimination are found to be the primary reaction channels in the remarkably different ratios of 95:5 and 60:40 in the fast reactions of Pt⁺ with CH₃F and CH₃Cl, respectively. The observed kinetics are consistent with quantum chemistry calculations, which indicate that both channels in the reaction with CH₃F are exothermic with ground‐state Pt⁺(²D), but that HF elimination is prohibited kinetically because of a transition state that lies above the reactant entrance. The observed HF‐elimination channel is attributed to a slow reaction of CH₃F with excited‐state Pt⁺(⁴F) for which calculations predict a small barrier. The calculations also show that both the HCl‐elimination and dehydrogenation channels observed with CH₃Cl are thermodynamically and kinetically allowed, although the state‐specific product distributions could not be ascertained experimentally. Further CH₃F addition is observed with the primary products to produce PtCH₂⁺(CH₃F)_1,2 and PtCHF⁺(CH₃F)_1,2. With CH₃Cl, sequential HCl elimination is observed with PtCH₂⁺ to form PtC_nH_2n⁺ with n=2, 3, which then add CH₃Cl sequentially to form PtC₂H₄⁺(CH₃Cl)_1–3 and PtC₃H₆⁺(CH₃Cl)_1,2. Also, sequential addition is observed for PtCHCl⁺ to form PtCHCl⁺(CH₃Cl)_1,2.     

Photoinduced and thermal reactions of molecular σ radical cations of some branched alkanes in low-temperature matrices: An ESR study

The photoinduced and thermal reactions of σ radical cations of methyl-substituted propanes and butanes in the condensed phases have been characterized by ESR. Photoelimination of CH₄ giving olefinic π radical cations commonly takes place except for H₂ elimination from isobutane radical cations. The thermal reactions of methyl-substituted butane radical cations in CFCl₂CF₂Cl were found to be deprotonation from the CH₃ group to give primary alkyl radicals.     

Studies on the influence of vibrational excitation of CF2Cl* radicals on the rate of their reaction with HCl

A method is proposed for studying the influence of vibrational excitation of radicals on their reactivity in bimolecular reactions. Investigations of the reaction CF₂Cl + HCl → CF₂ HCl + Cl by this method show for the first time that this reaction is accelerated by vibrational excitation of CF₂Cl* radicals. Under the experimental conditions, it was found that k^*₁/k₁ ? 6.0.     

Kinetic study of gas-phase reactions with CF2Cl2 and CFCl3. Analysis using the transition state theory (TST) of the chlorine atom transfer reactions by CF3 and CH3 radicals from halomethanes

The following gas-phase reactions: were studied by the competitive method with CF₃I as the source of radicals. The kinetic parameters obtained in the temperature range 533–613 K and 503–613 K respectively for chlorine atom transfer reactions are given by: where θ = 2.303 RT (cal mol^?1). The Arrhenius A values were calculated for seven chlorine atom transfer reactions (CF₂Cl₂, CFCl₃, CCl₄ with CF₃ radicals; CF₃Cl, CF₂Cl₂, CFCl₃ and CCl₄ with CH₃ radicals) by using the thermochemical kinetic version of the Transition State Theory (TST).     

Bond dissociation energies and radical heats of formation in CH3Cl,CH2Cl2, CH3Br,CH2Br2, CH2FCl,and CHFCl2

An analysis of thermochemical and kinetic data on the bromination of the halomethanes CH_4–nX_n (X = F, Cl, Br; n = 1–3), the two chlorofluoromethanes, CH₂FCl and CHFCl₂, and CH₄, shows that the recently reported heats of formation of the radicals CH₂Cl, CHCl₂, CHBr₂, and CFCl₂, and the C? H bond dissociation energies in the matching halomethanes are not compatible with the activation energies for the corresponding reverse reactions. From the observed trends in CH₄ and the other halomethanes, the following revised ΔH°_f,298 (R) values have been derived: ΔH°_f(CH₂Cl) = 29.1 ± 1.0, ΔH°_f(CHCl₂) = 23.5 ± 1.2, ΔH_f(CH₂Br) = 40.4 ± 1.0, ΔH°_f(CHBr₂) = 45.0 ± 2.2, and ΔH°_f(CFCl₂) = ?21.3 ± 2.4 kcal mol^?1. The previously unavailable radical heat of formation, ΔH°_f(CHFCl) = ?14.5 ± 2.4 kcal mol^?1 has also been deduced. These values are used with the heats of formation of the parent compounds from the literature to evaluate C? H and C? X bond dissociation energies in CH₃Cl, CH₂Cl₂, CH₃Br, CH₂Br₂, CH₂FCl, and CHFCl₂.     

Rates of processes initiated by pulsed laser production of F atoms in the presence of HCl,CH4, and CF3H

Time-resolved vibrational chemiluminescence from HF has been recorded following the production of F atoms by the pulsed laser photolysis (λ = 266 nm) of F₂ in the presence of HCl, CH₄, and CF₃H. In the first two cases, experiments have been conducted by observing emission from HF(ν = 3) at four temperatures from 295 to 139 K. Rate constants have been determined over this range of temperature for the reactions of F atoms with HCl and CH₄ and of CH₃ radicals with F₂, and for the relaxation of HF(ν = 3) by HCl and CH₄. The reaction of F atoms with CF₃H is slower than those with HCl and CH₄ and measurements on the emission from HF(ν = 2) have been used to infer rate constants for reaction and relaxation only at 295 K. © 1994 John Wiley & Sons, Inc.