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1.
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 CH). The DFT calculations systematically underestimate the threshold energy for HCl elimination.  相似文献   

2.
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).  相似文献   

3.
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 kHX/kDX, are 1.29 ± 0.14, 1.29 ± 0.18, and 1.38 ± 0.29, respectively.  相似文献   

4.
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.  相似文献   

5.
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.  相似文献   

6.
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 Fc = 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.  相似文献   

7.
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  相似文献   

8.
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 Ea 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; Fc = 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.  相似文献   

9.
Disproportionation/combination rate constant ratios, kd/kc, for the reactive collision between CF3CH2CHX + CF3 radicals and between CF3CH2CHX + CF3CH2CHX radicals have been measured for X = CF3. The kd/kc = 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 kc/kc' s relative to X = Cl or H. The effect of the CF3 substituent on the disproportionation rate is discussed. © 1996 John Wiley & Sons, Inc.  相似文献   

10.
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.  相似文献   

11.
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.  相似文献   

12.
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  相似文献   

13.
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.  相似文献   

14.
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.  相似文献   

15.
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 PtCnH2n+ 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.  相似文献   

16.
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.  相似文献   

17.
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.  相似文献   

18.
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).  相似文献   

19.
An analysis of thermochemical and kinetic data on the bromination of the halomethanes CH4–nXn (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, ΔHf(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.  相似文献   

20.
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.  相似文献   

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