The unimolecular decomposition and isomerization of chemically activated 1-methyl-2,2,3,3-tetrafluorocyclopropane

The reaction of CH₂(¹A₁) with 1,1,2,2-tetrafluorocyclopropane was studied at 300 K and at pressures between 9.0 and 365.0 torr. Chemically activated 1-methyl-2,2,3,3-tetrafluorocyclopropane was formed and two competitive reaction paths, namely decomposition and isomerization, were observed. By fitting the experimental results to calculated values from RRKM theory, we estimated the Arrhenius parameters for both reaction processess as well as the heat of formation of 1-methyl-2,2,3,3-tetrafluorocyclopropane. © John Wiley & Sons, Inc.     

Unimolecular decomposition of chemically activated methylallylether

The unimolecular decomposition of chemically activated methylallylether (MAE) formed by the cross combination of methoxymethyl and vinyl radicals was studied in the gas phase. The experimentally determined rate constant was found to be 1.11 × 10⁸ sec^?1 at 9.6°C for the decomposition of MAE into propene and formaldehyde. The decomposition of MAE via the six-center retro-“ene” type transition state is analyzed by using the RRKM unimolecular reaction theory. For the molecular parameter assignments of energized MAE, a model which contains one internal rotational mode is supported, and MAE decomposition is characterized by a tight complex model. The best agreement between experimental and theoretical results was found when a critical energy of 40.1 kcal/mol was used.     

The kinetics of thermal decomposition of 1-butyl-3-methylimidazolium hexafluorophosphate

It is demonstrated that 1-butyl-3-methylimidazolium hexafluorophosphate decomposes in a vacuum in the temperature interval of T=(410 to 505) K according to zero-order kinetics with the activation energy EA=68.0+/-2.8 kJ.mol-1.     

Comparison between experimental and calculated results on the decomposition of chemically activated alcohols

The life times of chemically activated alcohols have been determined using the high-pressure unimolecular rate parameters for thermal decomposition of alcohols from shocktube studies and RRKM calculations. They are compared with literature numbers (from insertion of 0(¹D) into hydrocarbons). It is suggested that in some cases singlet oxygen carries excess energy into the hydrocarbon. The consequences of such an assumption are explored and discrepancies with previously published conclusions discussed.     

The decomposition kinetics of mechanically activated alunite ore in air atmosphere by thermogravimetry

The thermal decompositions of both non-activated and mechanically activated alunite ore have been studied by thermogravimetry (TG). The ore was activated mechanically in an attritor for 15 min and amorphisation in the structure was studied by X-ray diffraction analysis. It can be verified that alunite decomposes in two steps, which are dehydration and desulphation. It was also established that the mechanical activation affected especially on the temperature range of dehydration reaction. The activation energies of dehydration and desulphation reactions have been calculated from the thermogravimetric data at heating rates of 5, 10, 15 and 20 K min⁻¹ involving isoconversional methods of Ozawa and Kissenger-Akahira-Sunose (KAS).     

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.     

Non-RRKM decomposition of chemically activated radicals: Reaction of fluorine atoms with tetraallyl germanium

Thermal F atoms react with gaseous (CH₂CHCH₂)₄Ge to produce CH₂CHF through dissociation of the (CH₂CHCH₂)₃-GeCH₂CHFCH₂^* radical. The rapid decomposition is attributed to non-RRKM behavior with excitation energy largely confined to the CH₂CHFCH₂ side group. The central atoms mass effect found in linear seven-atom trajectory calculations has not yet appeared in (CH₂CHCH₂)₄M with the replacement of Sn by Ge.     

Theoretical studies of decomposition kinetics of CF3CCl2O radical

The unimolecular decomposition reaction of CF₃CCl₂O radical has been investigated using theoretical methods. Two most important channels of decomposition occurring via C–C bond scission and Cl elimination have been considered during the present investigation. Ab initio quantum mechanical calculations are performed to get optimized structure and vibrational frequencies at DFT and MP2 levels of theory. Energetics are further refined by the application of a modified Gaussian-2 method, G2M(CC,MP2). The thermal rate constants for the decomposition reactions involved are evaluated using Canonical Transition State Theory (CTST) utilizing the ab initio data. Rate constants for C–C bond scission and Cl elimination are found to be 6.7 × 10⁶ and 1.1 × 10⁸ s^?1, respectively, at 298 K and 1 atm pressure with an energy barrier of 8.6 and 6.5 kcal/mol, respectively. These values suggest that Cl elimination is the dominant process during the decomposition of the CF₃CCl₂O radical. Transition states are searched on the potential energy surface of the decomposition reactions involved and are characterized by the existence of only one imaginary frequency (NIMAG = 1) during frequency calculation. The existence of transition states on the corresponding potential energy surface is further ascertained by performing intrinsic reaction coordinate (IRC) calculation.     

Cis-trans isomerization of chemically activated 1-methylallyl radical and fate of the resulting 2-buten-1-peroxy radical

The cis-trans isomerization of chemically activated 1-methylallyl is investigated using RRKM/Master Equation methods for a range of pressures and temperatures. This system is a prototype for a large range of allylic radicals formed from highly exothermic (～35 kcal/mol) OH + alkene reactions. Energies, vibrational frequencies, anharmonic constants, and the torsional potential of the methyl group are computed with density functional theory for both isomers and the transition state connecting them. Chemically activated radicals are found to undergo rapid cis-trans isomerization leading to stabilization of significant amounts of both isomers. In addition, the thermal rate constant for trans → cis isomerization of 1-methylallyl is computed to be high enough to dominate reaction with O(2) in 10 atm of air at 700 K, so models of the chemistry of the (more abundant and more commonly studied) trans-alkenes may need to be modified to include the cis isomers of the corresponding allylic radicals. Addition of molecular oxygen to 1-methylallyl radical can form 2-butene-1-peroxy radical (CH(3)CH═CHCH(2)OO(?)), and quantum chemistry is used to thoroughly explore the possible unimolecular reactions of the cis and trans isomers of this radical. The cis isomer of the 2-butene-1-peroxy radical has the lowest barrier (via 1,6 H-shift) to further reaction, but this barrier appears to be too high to compete with loss of O(2).     

The reaction of hydrogen atoms with 1-butanethiol and thiolane the role of chemically activated 1-butanethiol

The reaction of 1-butanethiol with hydrogen atoms has been studied at temperatures of 295° and 576° K under the pressure of 660 Pa, using a conventional discharge-flow apparatus. The reaction products (besides hydrogen sulfide and methane) under the low conversion range (～10%) consisted mainly of n-butane, 1-butene, and propylene-propane, with the relative yields of 70, 25, and 5% at 295° K and 25, 50, and 10% at 576°K. Analysis of kinetic equations by numerical integration indicates that the following initial steps are consistent with the experimental results: where the following expressions have been derived for k₁ and k₂: The subsequent reaction of the butylthio radical with hydrogen atoms leads to the chemically activated 1-butanethiol which either stabilizes to 1-butanethiol or decomposes to 1-butene and hydrogen sulfide, depending on the experimental conditions. A similar analysis of the data on the thiolane-H system has yielded the following rate parameters for the initial step to form the 4-mercapto-1-butyl radical: .     

Tribochemistry and kinetics of Al2(SO4)3·xH2O decomposition

The thermal decomposition of tribochemically activated Al₂(SO₄)₃·xH₂O was studied by TG, DTA and EMF methods. For some of the intermediate solids, X-ray diffraction and IR-spectroscopy were applied to learn more about the reaction mechanism. Thermal and EMF studies confirmed that, even after mechanical activation of Al₂(SO₄)₃·xH₂O, Al₂O(SO₄)₂ is formed as an intermediate. Isothermal kinetic experiments demonstrated that the thermochemical sulphurization of inactivated Al₂(SO₄)₃·xH₂O has an activation energy of 102.2 kJ·mol^?1 in the temperature range 850–890 K. The activation energy for activated Al₂(SO₄)₃·xH₂O in the range 850–890 K is 55.0 kJ·mol^?1. The time of thermal decomposition is almost halved when Al₂(SO₄)₃·xH₂O is activated mechanically. The results permit conclusions concerning the efficiency of the tribochemical activation of Al₂(SO₄)₃·xH₂O and the chemical and kinetic mechanisms of the desulphurization process.     

Ab initio studies of alkyl radical reactions: Combination and disproportionation reactions of CH3 with C2H5, and the decomposition of chemically activated C3H8

This paper reports the first quantitative ab initio prediction of the disproportionation/combination ratio of alkyl+alkyl reactions using CH3+C2H5 as an example. The reaction has been investigated by the modified Gaussian-2 method with variational transition state or Rice-Ramsperger-Kassel-Marcus calculations for several channels producing (1) CH4+CH2CH2, (2) C3H8, (3) CH4CH3CH, (4) H2+CH3CHCH2, (5) H2+CH3CCH3, and (6) C2H6+CH2 by H-abstraction and association/decomposition mechanisms through singlet and triplet potential energy paths. Significantly, the disproportionation reaction (1) producing CH4+C2H4 was found to occur primarily by the lowest energy path via a loose hydrogen-bonding singlet molecular complex, H3CHC2H4, with a 3.5 kcal/mol binding energy and a small decomposition barrier (1.9 kcal/mol), instead of a direct H-abstraction process. Bimolecular reaction rate constants for the formation of the above products have been calculated in the temperature range 300-3000 K. At 1 atm, formation of C3H8 is dominant below 1200 K. Over 1200 K, the disproportionation reaction becomes competitive. The sum of products (3)-(6) accounts for less than 0.3% below 1500 K and it reaches around 1%-4% above 2000 K. The predicted rate constant for the disproportionation reaction with multiple reflections above the complex well, k1=5.04 x T(0.41) exp(429/T) at 200-600 K and k1=1.96 x 10(-20) T(2.45) exp(1470/T) cm3 molecule(-1) s(-1) at 600-3000 K, agrees closely with experimental values. Similarly, the predicted high-pressure rate constants for the combination reaction forming C3H8 and its reverse dissociation reaction in the temperature range 300-3000 K, k2(infinity)=2.41 x 10(-10) T(-0.34) exp(259/T) cm3 molecule(-1) s(-1) and k(-2)(infinity)=8.89 x 10(22) T(-1.67)exp(-46 037/T) s(-1), respectively, are also in good agreement with available experimental data.     

Far infrared spectra of methyl nitrate and methyl-d3 nitrate

The far infrared spectra from 300 to 50 cm⁻¹ of methyl nitrate, CH₃ONO₂, and methyl-d₃ nitrate, CD₃NO₂, have been recorded at a resolution of 0.12 cm⁻¹. The fundamental methyl torsional mode has been observed at 204.5 cm⁻¹ (154.2 cm⁻¹ for CD₃ONO₂) with two excited states falling to lower frequencies which gives a V₃ barrier of 980 ± 40 cm⁻¹ (2.80 ± 0.11 kcal/mol). The NO₂ torsion (methoxy) has been observed with the 1 ← 0 transition being at 133.7 cm⁻¹ (119.5 cm⁻¹ for CD₃ONO₂) and eight successive excited states falling to lower frequencies. From these data the twofold barrier to internal rotation has been calculated to be 2650 ± 75 cm⁻¹ (7.69 ± 0.21 kcal/mol).     

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.     

Mass spectra of vinyltrimethylsilane and vinyltri(methyl-d3) silane

This paper reports the mass spectrum of vinyltrimethylsilane over the range of electron-impact energy of 9 to 70 eV as determined with a quadrupole mass spectrometer. Less extensive data obtained with magnetic spectrometers are included for comparison as well as the spectrum of vinyl-tri(methyl-d₃)silane. Plots of ion intensity as a function of electron energy, nominal appearance potentials and metastable ion transitions are given. The primary ionization results in an electron deficiency on the silicon atom, followed by loss of either ·CH₃, C₂H₄ or ·C₂H₃ to give fragments of mass 85, 73 or 72, respectively. The primary fragment of mass 85 loses C₂H₂ to give the mass 59 ion. The primary fragment of mass 73 loses successively C₂H₄ and H₂ to give first the mass 45 ion and then the mass 43 ion. At 15 eV, the relative intensities of the ions are 85 > 72 > 100 > 59 > 73. At 70 eV, the relative intensities are 59 > 85 > 43 > 45 > 73 > 72 > 100.     

On the nonstatistical character of chemically activated CHF2Cl* and CHF3* decomposition

The RRKM theory and the quantum variation of RRK theory are used to calculate the decomposition rate constants of chemically activated CHF₂Cl^* and CHF₃^*. The calculated values of the rate constants are compared with experimental data. It is shown that the decompositions of CHF₃^* and CHF₂Cl^* proceed much faster than they should in terms of statistical theory.     

Kinetics of chemically activated ethane

Chemically activated ethane, with an excitation energy of 114.9 ± 2 kcal/mole, was formed by reaction with methane of excited singlet methylene radicals produced by the 4358 Å photolysis of diazomethane. A decomposition rate constant of (4.6 ± 1.2) × 10⁹ sec^?1 was measured for the chemically activated ethane. This result agrees, via RRKM theory, with most other chemically activated ethane data, and the result predicts, via RRKM and absolute rate theory for E₀ = 85.8 kcal/mole, E* = 114.9 kcal/mole, and k_E = 4.6 × 101 sec^?1, a thermal A-factor at 600°K of 10^16.6±0.2 sec^?1, in approximate agreement with the more recent experimental values. Combining 2 kcal/mole uncertainties in E₀ and E* with the uncertainty in our rate constant yields an A-factor range of 10^16.6±0.7 sec^?1. It is emphasized that this large uncertainty in the A-factor results from an improbable combination of uncertainty limits for the various parameters. These decomposition results predict, via absolute rate theory (with E₀(recombination) = 0) and statistical thermodynamic equilibrium constants, methyl radical recombination rates at 25°C of between 4.4 × 10⁸ to 3.1 × 10⁹ l.-mole^?1-sec^?1, which are 60 to 8 times lower, respectively, than the apparently quite reliable experimental value. A value of E₀(recombination) greater than zero offers no improvement, and a value less than zero would be quite unusual. Activated complexes consistent with the experimental recombination rate and E₀(recombination) = 0 greatly overestimate the experimental chemical activation and high pressure thermal decomposition rate data. Absolute rate theory as it is applied here in a straightforward way has failed in this case, or a significant amount of internally consistent data are in serious error. Some corrections to our previous calculations for higher alkanes are discussed in Appendix II.     

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.