Unimolecular reactions of vibrationally excited CF2ClCHFCH3 and CF2ClCHFCD3: evidence for the 1,2-FCl interchange pathway

Chemically activated CF2ClCHFCH3 and CF2ClCHFCD3 molecules were prepared with 94 kcal mol-1 of vibrational energy by the recombination of CF2ClCHF and CH3(CD3) radicals at room temperature. The unimolecular reaction pathways were 2,3-FH(FD) elimination, 1,2-ClF interchange and 1,2-ClH elimination; the interchange produces CF3CHClCH3(CF3CHClCD3) with 105 kcal mol-1 of vibrational energy. Rate constants for CF2ClCHFCH3 [CF2ClCHFCD3] were (3.1+/-0.4)x10(6) s-1 [(1.0+/-0.1)x10(6) s-1] for 2,3-FH [FD] loss, (1.5+/-0.2)x10(6) s-1 [(8.3+/-0.9)x10(5) s-1] for 1,2-ClF interchange, and (8.2+/-1.0)x10(5) s-1 [(5.3+/-0.6)x10(5) s-1] for 1,2-ClH [DCl] loss. These correspond to branching fractions of 0.55+/-0.06 [0.43+/-0.04] for 2,3-FH [FD] loss, 0.29+/-0.03 [0.35+/-0.04] for 1,2-ClF interchange, and 0.16+/-0.02 [0.22+/-0.02] for 1,2-ClH [ClD] loss. Kinetic-isotope effects were 3.0+/-0.6 for 2,3-FH [FD] loss, 1.6+/-0.3 for 1,2-ClH loss, and 1.8+/-0.4 for 1,2-ClF interchange. The CF3CHClCH3 (CF3CHClCD3) molecules formed by 1,2-FCl interchange react by loss of HCl [DCl] with rate constants of (5.6+/-0.9)x10(7) s-1 [(2.1+/-0.4)x10(7)] s-1 for an isotope effect of 2.7+/-0.4. Density functional theory was employed to calculate vibrational frequencies and moments of inertia for the molecules and for the transition-state structures. These results were used with RRKM theory to assign threshold energies from comparison of computed and experimental unimolecular rate constants. The threshold energy for ClF interchange is 57.5 kcal mol-1, and those for HF and HCl channels are 2-5 kcal mol-1 higher. Experiments with vibrationally excited CF2ClCF2CF3, CF2ClCF2CF2Cl, and CF2ClCF2Cl, which did not show evidence for ClF interchange, also are reported.     

Unimolecular elimination of HF and HCl from chemically activated CF3CFClCH2Cl

The unimolecular reactions of CF3CFClCH2Cl molecules formed with 87 kcal mol(-1) of vibrational energy by recombination of CF3CFCl and CH2Cl radicals at room temperature have been characterized by the chemical activation technique. The 2,3-ClH and 2,3-FH elimination reactions, which have rate constants of (2.5 +/- 0.8) x 10(4) and (0.38 +/- 0.11) x 10(4) s(-1), respectively, are the major reactions. The 2,3-FCl interchange reaction was not observed. The trans (or E)-isomers of CF3CFCHCl and CF3CClCHCl are favored over the cis (or Z)-isomers. Density functional theory at the B3PW91/6-31G(d',p') level was used to evaluate thermochemistry and structures of the molecule and transition states. This information was used to calculate statistical rate constants. Matching the calculated to the experimental rate constants for the trans-isomers gave threshold energies of 62 and 63 kcal mol(-1) for HCl and HF elimination, respectively. The threshold energy for FCl interchange must be 3-4 kcal mol(-1) higher than for HF elimination. The results for CF3CFClCH2Cl are compared to those from CF3CFClCH3; the remarkable reduction in rate constants for HCl and HF elimination upon substitution of one Cl atom for one H atom is a consequence of both a lower E and higher threshold energies for CF3CFClCH2Cl.     

Unimolecular reactions including ClF interchange of vibrationally excited CF2ClCHFCH2CH3 and CF2ClCHFCD2CD3

Vibrationally excited CF(2)ClCHFC(2)H(5)(CF(2)ClCHFC(2)D(5)) molecules were prepared in the gas phase at 300 K with approximately 93 kcal mol(-1) of energy by recombination of CF(2)ClCHF and C(2)H(5) or C(2)D(5) radicals. Three unimolecular reactions were observed. 1,2-ClF interchange converts CF(2)ClCHFC(2)H(5)(CF(2)ClCHFC(2)D(5)) into CF(3)CHClC(2)H(5)(CF(3)CHClC(2)D(5)), and subsequent 2,3-ClH (ClD) elimination gives CF(3)CH=CHCH(3) (CF(3)CH=CDCD(3)). 2,3-FH(FD) elimination gives cis- and trans-CF(2)ClCH=CHCH(3) (CF(2)ClCH=CDCD(3)), and 1,2-ClH elimination gives CF(2)=CFCH(2)CH(3) (CF(2)=CFCD(2)CD(3)). The experimental rate constants for CF(2)ClCHFC(2)H(5) (CF(2)ClCHFC(2)D(5)) were 1.3 x 10(4) (0.63 x 10(4)) s(-1) for 1,2-FCl interchange and 2.1 x 10(4) (0.61 x 10(4)) s(-1) with a trans/cis ratio of 3.7 for 2,3-FH(FD) elimination. The 1,2-ClH process was the least important with a branching fraction of only 0.08 +/- 0.04. The rate constants for 2,3-ClH (ClD) elimination from CF(3)CHClC(2)H(5) (CF(3)CHClC(2)D(5)) were 1.8 x 10(6) (0.49 x 10(6)) s(-1) with a trans/cis ratio of 2.4. Density functional theory was used to compute vibrational frequencies and structures needed to obtain rate constants from RRKM theory. Matching theoretical and experimental rate constants provides estimates of the threshold energies, E0, for the three reaction pathways; 1,2-FCl interchange has the lowest E0. The unimolecular reactions of CF(2)ClCHFC(2)H(5) are compared to those of CF(2)ClCHFCH(3). Both of these systems are compared to CH(3)CHFC(2)H(5) to illustrate the influence of a CF(2)Cl group on the E0 for FH elimination.     

Unimolecular reactions of chemically activated CF2BrCF2CH3 and CF2BrCF2CD3: evidence for 1,2-FBr interchange

Vibrationally excited CF2BrCF2CH3 and CF2BrCF2CD3 molecules were prepared with 96 kcal mol-1 energy at room temperature by the recombination of CF2BrCF2 and CH3 (CD3) radicals. The observed unimolecular reactions are 1,2-BrF interchange to give CF3CFBrCH3 (CD3) molecules and 2,3-FH (FD) elimination; the rate constants are 2.2 x 10(5) (1.5 x 10(5)) s(-1) and 2.0 x 105 (0.75 x 10(5)) s(-1), respectively. The CF3CFBrCH3 (CD3) molecules rapidly, relative to the reverse reaction, eliminate HBr or DBr to give the observed product CF3CF=CH2 (CD2). Density functional theory at the B3PW91/6-311+G(2d,p) level was used to obtain vibrational frequencies and moments of inertia of the molecule and transition states for subsequent calculations of statistical rate constants for CF2BrCF2CH3 and CF2BrCF2CD3. Matching experimental and calculated rate constants gave threshold energies of 62 and 66 kcal mol-1 for 1,2-BrF interchange and 2,3-FH elimination, respectively. The BrF interchange reaction is compared to ClF interchange from CF2ClCF2CH3 and CF2ClCHFCH3.     

Unimolecular reactions in the CF3CH2Cl ↔ CF2ClCH2F system: isomerization by interchange of Cl and F atoms

The recombination of CF(2)Cl and CH(2)F radicals was used to prepare CF(2)ClCH(2)F* molecules with 93 ± 2 kcal mol(-1) of vibrational energy in a room temperature bath gas. The observed unimolecular reactions in order of relative importance were: (1) 1,2-ClH elimination to give CF(2)═CHF, (2) isomerization to CF(3)CH(2)Cl by the interchange of F and Cl atoms and (3) 1,2-FH elimination to give E- and Z-CFCl═CHF. Since the isomerization reaction is 12 kcal mol(-1) exothermic, the CF(3)CH(2)Cl* molecules have 105 kcal mol(-1) of internal energy and they can eliminate HF to give CF(2)═CHCl, decompose by rupture of the C-Cl bond, or isomerize back to CF(2)ClCH(2)F. These data, which provide experimental rate constants, are combined with previously published results for chemically activated CF(3)CH(2)Cl* formed by the recombination of CF(3) and CH(2)Cl radicals to provide a comprehensive view of the CF(3)CH(2)Cl* ? CF(2)ClCH(2)F* unimolecular reaction system. The experimental rate constants are matched to calculated statistical rate constants to assign threshold energies for the observed reactions. The models for the molecules and transition states needed for the rate constant calculations were obtained from electronic structures calculated from density functional theory. The previously proposed explanation for the formation of CF(2)═CHF in thermal and infrared multiphoton excitation studies of CF(3)CH(2)Cl, which was 2,2-HCl elimination from CF(3)CH(2)Cl followed by migration of the F atom in CF(3)CH, should be replaced by the Cl/F interchange reaction followed by a conventional 1,2-ClH elimination from CF(2)ClCH(2)F. The unimolecular reactions are augmented by free-radical chemistry initiated by reactions of Cl and F atoms in the thermal decomposition of CF(3)CH(2)Cl and CF(2)ClCH(2)F.     

Unimolecular rate constants, kinetic isotope effects and threshold energies for FH and FD elimination from CF3CHFCH3 and CF3CHFCD3

The combination of CF(3)CHF and CH(3) or CD(3) radicals was used to prepare vibrationally excited CF(3)CHFCH(3) or CF(3)CHFCD(3) molecules with 97 kcal mol(-1) of internal energy. The experimental unimolecular rate constants were 3.7 x 10(6) s(-1) for 2,3-FH elimination from CF(3)CHFCH(3) and 1.3 x 10(6) s(-1) for 2,3-DF elimination from CF(3)CHFCD(3). Unimolecular rate constants for 1,2-FH elimination reaction were approximately 230 and 98 times smaller for CF(3)CHFCH(3) and CF(3)CHFCD(3), respectively, than the corresponding rate constants for 2,3-FH elimination. Density functional theory (DFT) was used to calculate the structures and vibrational frequencies of the molecules and transition states; this information was subsequently employed for calculations of RRKM rate constants. Comparison of the experimental and calculated rate constants gave a threshold energy of 73 +/- 2 kcal mol(-1) for the 1,2-FH elimination process and 60.5 +/- 1.5 kcal mol(-1) for the 2,3-FH elimination reaction from CF(3)CHFCH(3). The calculated kinetic-isotope effects agree with the experimental results. The experimentally derived threshold energies for 1,2-FH and 2,3-FH elimination reactions from several fluoropropanes and fluorochloropropanes are summarized and compared to those from DFT calculations.     

Unimolecular rate constants for HX or DX elimination (X = F, Cl) from chemically activated CF3CH2CH2Cl, C2H5CH2Cl, and C2D5CH2Cl: threshold energies for HF and HCl elimination

Vibrationally activated CF(3)CH(2)CH(2)Cl molecules were prepared with 94 kcal mol(-1) of vibrational energy by the combination of CF(3)CH(2) and CH(2)Cl radicals and with 101 kcal mol(-1) of energy by the combination of CF(3) and CH(2)CH(2)Cl radicals at room temperature. The unimolecular rate constants for elimination of HCl from CF(3)CH(2)CH(2)Cl were 1.2 x 10(7) and 0.24 x 10(7) s(-1) with 101 and 94 kcal mol(-1), respectively. The product branching ratio, k(HCl)/k(HF), was 80 +/- 25. Activated CH(3)CH(2)CH(2)Cl and CD(3)CD(2)CH(2)Cl molecules with 90 kcal mol(-1) of energy were prepared by recombination of C(2)H(5) (or C(2)D(5)) radicals with CH(2)Cl radicals. The unimolecular rate constant for HCl elimination was 8.7 x 10(7) s(-1), and the kinetic isotope effect was 4.0. Unified transition-state models obtained from density-functional theory calculations, with treatment of torsions as hindered internal rotors for the molecules and the transition states, were employed in the calculation of the RRKM rate constants for CF(3)CH(2)CH(2)Cl and CH(3)CH(2)CH(2)Cl. Fitting the calculated rate constants from RRKM theory to the experimental values provided threshold energies, E(0), of 58 and 71 kcal mol(-1) for the elimination of HCl or HF, respectively, from CF(3)CH(2)CH(2)Cl and 54 kcal mol(-1) for HCl elimination from CH(3)CH(2)CH(2)Cl. Using the hindered-rotor model, threshold energies for HF elimination also were reassigned from previously published chemical activation data for CF(3)CH(2)CH(3,) CF(3)CH(2)CF(3), CH(3)CH(2)CH(2)F, CH(3)CHFCH(3), and CH(3)CF(2)CH(3). In an appendix, the method used to assign threshold energies was tested and verified using the combined thermal and chemical activation data for C(2)H(5)Cl, C(2)H(5)F, and CH(3)CF(3).     

Rate constants and kinetic isotope effects for unimolecular 1,2-HX or DX (X = F or Cl) elimination from chemically activated CF3CFClCH3-d0, -d1, -d2, and -d3

Chemically activated CF(3)CFClCH(3), CF(3)CFClCD(3), CF(3)CFClCH(2)D, and CF(3)CFClCHD(2) molecules with 94 kcal mol(-1) of internal energy were formed by the combination of CF(3)CFCl radicals with CH(3), CD(3), CH(2)D, and CHD(2) radicals, which were generated from UV photolysis of CF(3)CFClI and CH(3)I, CD(3)I, CH(2)DI, or CHD(2)I. The total (HF + HCl) elimination rate constants for CF(3)CFClCH(3) and CF(3)CFClCD(3) were 5.3 x 10(6) and 1.7 x 10(6) s(-1) with product branching ratios of 8.7 +/- 0.6 in favor of HCl (or DCl). The intermolecular kinetic isotope effects were 3.22 and 3.18 for the HCl and HF channels, respectively. The product branching ratios were 10.3 +/- 1.9 and 11.8 +/- 1.8 (10.8 +/- 3.8 and 11.6 +/- 1.7) for HCl/HF and DCl/DF, respectively, from CF(3)CFClCH(2)D (CF(3)CFClCHD(2)). The intramolecular kinetic-isotope effects (without correction for reaction path degeneracy) for HCl/DCl and HF/DF elimination from CF(3)CFClCH(2)D (CF(3)CFClCHD(2)) were 2.78 +/- 0.16 and 2.98 +/- 0.12 (0.82 +/- 0.04 and 0.91 +/- 0.03), respectively. Density function theory at the B3PW91/6-311+G(2d,p) and B3PW91/6-31G(d',p') levels was investigated, and the latter was chosen to calculate frequencies and moments of inertia for the molecules and transition states. Rate constants, branching ratios and kinetic-isotope effects then were calculated using RRKM theory with torsional motions treated as hindered internal rotations. Threshold energies for HF and HCl elimination from CF(3)CFClCH(3) were assigned as 61.3 +/- 1.5 and 58.5 +/- 1.5 kcal mol(-1), respectively. The threshold energy for Cl-F interchange was estimated as 67 kcal mol(-1). The difference between the transition states for HCl and HF elimination is discussed.     

Quantum chemistry study on cation structures of fluorinated and chlorinated germanes and their radicals

The structures and vibrational frequencies of cations of fluorinated and chlorinated germanes and radicals (GeHxXy+, X = F, Cl; x + y = 1-4) and protonated germanes are investigated theoretically at B3LYP/6-31+G(2df,p) level. For GeH2, GeHX, GeH2X, GeHX2, and germanes, the most stable cationic structures are largely distorted from their neutral ones and all can be viewed as ion complexes between a Ge-centered cation and a neutral atom or diatom. The ionization potentials, appearance energies, and proton affinities are obtained at Gaussian-3(CC) levels. Cations with the lowest energy (and adiabatic ionization potentials (in eV)) are Ge+-H2 (2B2, 8.94), Ge+-FH (9.42), Ge+-ClH (9.45), GeH3(+) (8.01), GeF+-H2 (7.71), GeCl+-H2 (8.01), GeF+-FH (7.69), GeCl+-ClH (7.80), GeH2(+)-H2 (10.45), GeH2(+)-FH (10.32), GeHF+-FH (10.64), GeF2(+)-FH (11.40), GeF4(+) (15.22), GeH2(+)-ClH (10.29), GeHCl+-ClH (10.33), GeCl2(+)-ClH (10.43), and GeCl4(+) (11.48). The most stable protonated germanes (and proton affinities (in kJ/mol, 0 K)) are GeH3(+)-H2 (658.3), GeH3(+)-FH (672.5), GeH2F+-FH (634.2), GeHF2(+)-FH (583.4), GeF3(+)-FH (516.3), GeH3(+)-ClH (672.7), GeH2Cl+-ClH (652.6), GeHCl2(+)-ClH (637.5), and GeCl3(+)-ClH (624.4), respectively. The G3 atomization energies of fluorinated Ge-species are found to be significantly different from G3X and G4 ones, and this may merit further investigation.     

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.     

Energetics and mechanism of the decomposition of trifluoromethanol

The thermal instability of alpha-fluoroalcohols is generally attributed to a unimolecular 1,2-elimination of HF, but the barrier to intramolecular HF elimination from CF3OH is predicted to be 45.1 +/- 2 kcal/mol. The thermochemical parameters of trifluoromethanol were calculated using coupled-cluster theory (CCSD(T)) extrapolated to the complete basis set limit. High barriers of 42.9, 43.1, and 45.0 kcal/mol were predicted for the unimolecular decompositions of CH2FOH, CHF2OH, and CF3OH, respectively. These barriers are lowered substantially if cyclic H-bonded dimers of CF3OH with complexation energies of approximately 5 kcal/mol are involved. A six-membered ring dimer has an energy barrier of 28.7 kcal/mol and an eight-membered dimer has an energy barrier of 32.9 kcal/mol. Complexes of CF3OH with HF lead to strong H-bonded dimers, trimers and tetramers with complexation energies of approximately 6, 11, and 16 kcal/mol, respectively. The dimer, CH3OH:HF, and the trimers, CF3OH:2HF and (CH3OH)2:HF, have decomposition energy barriers of 26.7, 20.3, and 22.8 kcal/mol, respectively. The tetramer (CH3OH:HF)2 gives rise to elimination of two HF molecules with a barrier of 32.5 kcal/mol. Either CF3OH or HF can act as catalysts for HF-elimination via an H-transfer relay. Because HF is one of the decomposition products, the decomposition reactions become autocatalytic. If the energies due to complexation for the CF3OH-HF adducts are not dissipated, the effective barriers to HF elimination are lowered from approximately 20 to approximately 9 kcal/mol, which reconciles the computational results with the experimentally observed stabilities.     

Relative rate study of the kinetics, mechanism, and thermodynamics of the reaction of chlorine atoms with CF3CF═CH2 (HFO-1234yf) in 650-950 Torr of N2 or N2/O2 diluent at 296-462 K

The rate constant of the reaction Cl + CF(3)CF═CH(2) (k(1)) has been measured relative to several reference species using the relative rate technique with either gas chromatographic analysis with flame-ionization detection (GC/FID) or Fourier transform infrared (FTIR) analysis. Cl atoms were generated by UV irradiation of Cl(2)/CF(3)CF═CH(2)/reference/N(2)/O(2) mixtures. At 300-400 K in the presence of >20 Torr O(2), k(1) = 1.2 × 10(-11) e((+1100/RT)) cm(3) molecule(-1) s(-1). In N(2) diluent, k(1) has a sharp negative temperature coefficient resulting from the relatively small exothermicity of the following reactions: (1a) Cl + CF(3)CF═CH(2) ? CF(3)CFClCH(2)(?); (1b) Cl + CF(3)CF═CH(2) ? CF(3)CF(?)CH(2)Cl (reaction 1), which were determined in these experiments to be ～16.5 (±2.0) kcal mol(-1). This low exothermicity causes reaction 1 to become significantly reversible even at ambient temperature. The rate constant ratio for the reaction of the chloroalkyl radicals formed in reaction 1 with Cl(2) (k(2)) or O(2) (k(3)) was measured to be k(2)/k(3) = 0.4 e(-(3000/RT)) for 300-400 K. At 300 K, k(2)/k(3) = 0.0026. The reversibility of reaction 1 combined with the small value of k(2)/k(3) leads to a sensitive dependence of k(1) on the O(2) concentration. Products measured by GC/FID as a function of temperature are CF(3)CFClCH(2)Cl, CF(3)COF, and CH(2)Cl(2). The mechanism leading to these products is discussed. The rate constant for the reaction Cl + CF(3)CFClCH(2)Cl (k(11)) was measured as a function of temperature (300-462 K) at 760 Torr to be k(11) = 8.2 × 10(-12) e(-(4065/RT)) cm(3) molecule(-1) s(-1). Rate constants relative to CH(4) for the reactions of Cl with the reference compounds CH(3)Cl, CH(2)Cl(2), and CHCl(3) were measured at 470 K to resolve a literature discrepancy. (R = 1.986 cal K(-1) mol(-1)).     

Kinetic study of the reactions of Cl atoms with CF3CH2CH2OH, CF3CF2CH2OH, CHF2CF2CH2OH, and CF3CHFCF2CH2OH

The reaction kinetics of chlorine atoms with a series of partially fluorinated straight-chain alcohols, CF(3)CH(2)CH(2)OH (1), CF(3)CF(2)CH(2)OH (2), CHF(2)CF(2)CH(2)OH (3), and CF(3)CHFCF(2)CH(2)OH (4), were studied in the gas phase over the temperature range of 273-363 K by using very low-pressure reactor mass spectrometry. The absolute rate coefficients were given by the expressions (in cm(3) molecule(-1) s(-1)): k(1) = (4.42 +/- 0.48) x 10(-11) exp(-255 +/- 20/T); k(1)(303) = (1.90 +/- 0.17) x 10(-11), k(2) = (2.23 +/- 0.31) x 10(-11) exp(-1065 +/- 106/ T); k(2)(303) = (6.78 +/- 0.63) x 10(-13), k(3) = (8.51 +/- 0.62) x 10(-12) exp(-681 +/- 72/T); k(3)(303) = (9.00 +/- 0.82) x 10(-13) and k(4) = (6.18 +/- 0.84) x 10(-12) exp(-736 +/- 42/T); k(4)(303) = (5.36 +/- 0.51) x 10(-13). The quoted 2sigma uncertainties include the systematic errors. All title reactions proceed via a hydrogen atom metathesis mechanism leading to HCl. Moreover, the oxidation of the primarily produced radicals was investigated, and the end products were the corresponding aldehydes (R(F)-CHO; R(F) = -CH(2)CF(3), -CF(2)CF(3), -CF(2)CHF(2), and -CF(2)CHFCF(3)), providing a strong experimental indication that the primary reactions proceed mainly via the abstraction of a methylenic hydrogen adjacent to a hydroxyl group. Finally, the bond strengths and ionization potentials for the title compounds were determined by density functional theory calculations, which also suggest that the alpha-methylenic hydrogen is mainly under abstraction by Cl atoms. The correlation of room-temperature rate coefficients with ionization potentials for a set of 27 molecules, comprising fluorinated C2-C5 ethers and C2-C4 alcohols, is good with an average deviation of a factor of 2, and is given by the expression log(k) (in cm(3) molecule(-1) s(-1)) = (5.8 +/- 1.4) - (1.56 +/- 0.13) x (ionization potential (in eV)).     

Selected Ion Flow Tube Study of the Gas-Phase Reactions of CF(+), CF(2)(+), CF(3)(+), and C(2)F(4)(+) with C(2)H(4), C(2)H(3)F, CH(2)CF(2), and C(2)HF(3)

We study how the degree of fluorine substitution for hydrogen atoms in ethene affects its reactivity in the gas phase. The reactions of a series of small fluorocarbon cations (CF(+), CF(2)(+), CF(3)(+), and C(2)F(4)(+)) with ethene (C(2)H(4)), monofluoroethene (C(2)H(3)F), 1,1-difluoroethene (CH(2)CF(2)), and trifluoroethene (C(2)HF(3)) have been studied in a selected ion flow tube. Rate coefficients and product cations with their branching ratios were determined at 298 K. Because the recombination energy of CF(2)(+) exceeds the ionization energy of all four substituted ethenes, the reactions of this ion produce predominantly the products of nondissociative charge transfer. With their lower recombination energies, charge transfer in the reactions of CF(+), CF(3)(+), and C(2)F(4)(+) is always endothermic, so products can only be produced by reactions in which bonds form and break within a complex. The trends observed in the results of the reactions of CF(+) and CF(3)(+) may partially be explained by the changing value of the dipole moment of the three fluoroethenes, where the cation preferentially attacks the more nucleophilic part of the molecule. Reactions of CF(3)(+) and C(2)F(4)(+) are significantly slower than those of CF(+) and CF(2)(+), with adducts being formed with the former cations. The reactions of C(2)F(4)(+) with the four neutral titled molecules are complex, giving a range of products. All can be characterized by a common first step in the mechanism in which a four-carbon chain intermediate is formed. Thereafter, arrow-pushing mechanisms as used by organic chemists can explain a number of the different products. Using the stationary electron convention, an upper limit for Δ(f)H°(298)(C(3)F(2)H(3)(+), with structure CF(2)═CH-CH(2)(+)) of 628 kJ mol(-1) and a lower limit for Δ(f)H°(298)(C(2)F(2)H(+), with structure CF(2)═CH(+)) of 845 kJ mol(-1) are determined.     

Theoretical studies of the reactions of CF3CHCLOCHF2/CF3CHFOCHF2 with OH radical and Cl atom and their product radicals with OH

The mechanisms and dynamics studies of the OH radical and Cl atom with CF(3)CHClOCHF(2) and CF(3)CHFOCHF(2) have been carried out theoretically. The geometries and frequencies of all the stationary points are optimized at the B3LYP/6-311G(d,p) level, and the energy profiles are further refined by interpolated single-point energies (ISPE) method at the G3(MP2) level of theory. For each reaction, two H-abstraction channels are found and four products (CF(3)CHFOCF(2), CF(3)CFOCHF(2), and CF(3)CHClOCF(2), CF(3)CClOCHF(2)) are produced during the above processes. The rate constants for the CF(3)CHClOCHF(2)/CF(3)CHFOCHF(2) + OH/Cl reactions are calculated by canonical variational transition-state theory (CVT) within 200-2000 K, and the small-curvature tunneling is included. The total rate constants calculated from the sum of the individual rate constants and the branching ratios are in good agreement with the experimental data. The Arrhenius expressions for the reactions are obtained. Our calculation shows that the substitution of Cl by F decreases the reactivity of CF(3)CHClOCHF(2) toward OH and Cl. In addition, the mechanisms of subsequent reactions of product radicals and OH radical are further investigated at the G3(MP2)//B3LYP/6-311G(d,p) level, and the main products are predicted in the this article.     

Unimolecular Rate Constants for the HF and HCl Elimination Reactions from Chemically Activated CF2ClSH

Chemically activated CF₃SH, CFCl₂SH, and CF₂ClSH were formed through combination of SH and CF₃, CFCl₂, and CF₂Cl radicals, respectively. The SH radical was prepared by abstraction of an H‐atom from H₂S by the halocarbon radical produced during photolysis of (CF₃)₂C=O, (CFCl₂)₂C=O, or (CF₂Cl)₂C=O. 1,2‐HX (X = F, Cl) elimination reactions were observed from CF₃SH, CFCl₂SH, and CF₂ClSH with products detected by GC‐MS. The combination reaction of CF₂Cl radicals with SH radicals prepared CF₂ClSH molecules with approximately 318 kJ/mol of internal energy. The experimental rate constants for elimination of HCl and HF from CF₂ClSH were 3 ± 3 × 10¹⁰ and 2 ± 1 × 10⁹ s^?1, respectively. Comparison to Rice–Ramsperger–Kassel–Marcus (RRKM) calculated rate constants assigned the threshold energies as 171 ± 12 and 205 ± 12 kJ/mol for the unimolecular elimination of HCl and HF, respectively. Theoretical calculations using the B3PW91, MP2, and M062X methods with the 6311+G(2d,p) and 6‐31G(d',p') basis sets established that for a specific method the threshold energies differ by only 4 kJ/mol between the two different basis sets. There was wide variation among the three methods, but the M062X approach appeared to give threshold energies closest to the experimental values. Chemically activated CF₃SH and CFCl₂SH were also prepared with about 318 kcal mol^?1 of internal energy, and the HX (X = F, Cl) elimination reactions were observed. Only HCl loss was detected from CFCl₂SH, but the rate was too fast to measure with our kinetic method; however, based on our detection limit the HF elimination channel is at least 50 times slower.     

Theoretical study of the reactions CF3CH2OCHF2 + OH/Cl and its product radicals and parent ether(CH3CH2OCH3) with OH

A dual-level direct dynamic method is employed to study the reaction mechanisms of CF3CH2OCHF2 (HFE-245fa2; HFE-245mf) with the OH radicals and Cl atoms. Two hydrogen abstraction channels and two displacement processes are found for each reaction. For further study, the reaction mechanisms of its products (CF3CH2OCF2 and CF3CHOCHF2) and parent ether CH3CH2OCH3 with OH radical are investigated theoretically. The geometries and frequencies of all the stationary points and the minimum energy paths (MEPs) are calculated at the B3LYP/6-311G(d,p) level. The energetic information along the MEPs is further refined at the G3(MP2) level of theory. For reactions CF3CH2OCHF2 + OH/Cl, the calculation indicates that the hydrogen abstraction from --CH2-- group is the dominant reaction channel, and the displacement processes may be negligible because of the high barriers. The standard enthalpies of formation for the reactant CF3CH2OCHF2, and two products CF3CH2OCHF2 and CF3CHOCHF2 are evaluated via group-balanced isodesmic reactions. The rate constants of reactions CF3CH2OCHF2 + OH/Cl and CH3CH2OCH3 + OH are estimated by using the variational transition state theory over a wide range of temperature (200-2000 K). The agreement between the theoretical and experimental rate constants is good in the measured temperature range. From the comparison between the rate constants of the reactions CF3CH2OCHF2 and CH3CH2OCH3 with OH, it is shown that the fluorine substitution decreases the reactivity of the C--H bond.     

Study of pi halogen bonds in complexes C2H(4-n)Fn-ClF (n = 0-2)

The pi-halogen bond may be considered, in a broad sense, essentially a pi-hydrogen bond. Using the counterpoise-corrected potential energy surface method (interaction energy optimization), the stationary structures of the C2H(4-n)Fn-ClF (n = 0-2) complexes with all real frequencies have been obtained at the MP2/aug-cc-pVDZ level. For C2H(4-n)Fn-ClF (n = 0-2), the pi-halogen bond has a long distance and is elongated by the F substituent effect. The pi-halogen bond length order is 2.661 A for C2H4-ClF < 2.745 A for C2H3F-ClF < 2.766 A for g-C2H2F2-ClF < 2.8076 A for trans-C2H2F2-ClF < 2.8079 A for cis-C2H2F2-ClF. For three complexes C2H3F-ClF, g-C2H2F2-ClF, and cis-C2H2F2-ClF, the pi-halogen bonds are further shifted and sloped by the F substituent effect. The F substituent effect reduces also the interaction energy of the pi-halogen bond. The interaction energies are respectively -3.7 for C2H4-ClF, -2.8 for C2H3F-ClF, -2.3 for g-C2H2F2-ClF, -1.9 for cis-C2H2F2-ClF, and -1.8 kcal/mol for trans-C2H2F2-ClF, at the CCSD(T)/aug-cc-pVDZ level. The electron correlation contribution of the interaction energy is large for C2H(4-n)Fn-ClF (n = 0-2), which shows that the stabilities of the pi-halogen bond systems results primarily from the dispersion interaction. In the double F substituent systems, the interaction energy of the pi-halogen bond structure with a longer interaction distance is larger than that of the corresponding pi-hydrogen bond structure with a shorter interaction distance. This may be because there are the large electron correlation contributions of the interaction energy, and a secondary interaction between lone pairs of Cl atom and some atoms (H, C) with positive charges in the pi-halogen bond structure.     

Kinetic mechanism of the hydrogen abstraction reactions of the chlorine atoms with CH3CF2Cl and CH3CFCl2: a dual level direct dynamics study

The mechanisms of the reactions: CH(3)CFCl(2) + Cl (R1) and CH(3)CF(2)Cl + Cl (R2) are studied over a wide temperature range (200-3000 K) using the dual-level direct dynamics method. The minimum energy path calculation is carried out at the MP2/6-311G(d,p) and B3LYP/6-311G(d,p) levels, and energetic information is further refined by the G3(MP2) theory. The H-abstraction from the out-of-plane for (R1) is the major reaction channel, while the in-plane H-abstraction is the predominant route of (R2). The canonical variational transition-state theory (CVT) with the small-curvature tunneling (SCT) correction method is used to calculate the rate constants. Using group-balanced isodesmic reactions and hydrogenation reactions as working chemical reactions, the standard enthalpies of formation for CH(3)CFCl(2), CH(3)CF(2)Cl, CH(2)CFCl(2), and CH(2)CF(2)Cl are evaluated at the CCSD(T)/6-311 + G(3df,2p)//MP2/6-311G(d,p) level of theory. The results indicate that the substitution of fluorine atom for the chlorine atom leads to a decrease in the C-H bond reactivity with a small increase in reaction enthalpies. Also, for all reaction pathways the variational effect is small and the SCT effect is only important in the lower temperature range on the rate constants.     

Origin of the SN2 benzylic effect

The S N2 identity exchange reactions of the fluoride ion with benzyl fluoride and 10 para-substituted derivatives (RC6H 4CH 2F, R = CH3, OH, OCH 3, NH2, F, Cl, CCH, CN, COF, and NO2) have been investigated by both rigorous ab initio methods and carefully calibrated density functional theory. Groundbreaking focal-point computations were executed for the C6H5CH 2F + F (-) and C 6H 5CH2Cl + Cl (-) SN2 reactions at the highest possible levels of electronic structure theory, employing complete basis set (CBS) extrapolations of aug-cc-pV XZ (X = 2-5) Hartree-Fock and MP2 energies, and including higher-order electron correlation via CCSD/aug-cc-pVQZ and CCSD(T)/aug-cc-pVTZ coupled cluster wave functions. Strong linear dependences are found between the computed electrostatic potential at the reaction-center carbon atom and the effective SN2 activation energies within the series of para-substituted benzyl fluorides. An activation strain energy decomposition indicates that the SN2 reactivity of these benzylic compounds is governed by the intrinsic electrostatic interaction between the reacting fragments. The delocalization of nucleophilic charge into the aromatic ring in the SN2 transition states is quite limited and should not be considered the origin of benzylic acceleration of SN2 reactions. Our rigorous focal-point computations validate the benzylic effect by establishing SN2 barriers for (F (-), Cl (-)) identity exchange in (C6H5CH2F, C6H 5CH2Cl) that are lower than those of (CH3F, CH3Cl) by (3.8, 1.6) kcal mol (-1), in order.