Very low-pressure pyrolysis (VLPP) of hex-1-ene. Kinetics of the retro-ene decomposition of a mono-olefin

The thermal unimolecular decomposition of hex-1-ene has been investigated over the temperature range of 915–1153 K using the technique of very low-pressure pyrolysis (VLPP). The reaction proceeds via the competitive pathways of C₃?C₄ fission and retro-ene elimination, with the latter dominant at low temperatures and the former at high temperatures. This behavior results in an isokinetic temperature of 1035 K under VLPP conditions (both reactions in the unimolecular falloff regime). RRKM calculations, generalized to take into account two competing pathways, show that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters given by log k₁ (sec^?1) = (12.6 ± 0.2) -(57.7 ± 1.5)/θ for retro-ene reaction, and log k₂ (sec^?1) = (15.9 ± 0.2) - (70.8 ± 1.0)/θ for C-C fission, where θ = 2.303 RT kcal/mol. The A factors were assigned from the results of a recent shock-tube study of the decomposition in the high-pressure regime, and the activation energies were found by matching the RRKM calculations to the VLPP data. The parameters for C-C fission are consistent with the known thermochemistry of n-propyl and allyl radicals. A clear measure of the importance of the molecular pathway in the decomposition of a mono-olefin has been obtained.     

Kinetics of competitive pathways in the thermal unimolecular decomposition of hex-1-yne

The thermal unimolecular decomposition of hex-1-yne has been investigated over the temperature range of 903–1153 K using the technique of very low-pressure pyrolysis (VLPP). The reaction proceeds via the competitive pathways of C₃? C₄ fission and molecular retro-ene decomposition, with the latter being the major pathway under the experimental conditions. RRKM calculations, generalized to take into account two competing pathways, show that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters at 1100 K given by and where θ = 2.303 RT kcal/mol and the A factors were assigned from the results of recent shock-tube studies of hex-1-yne and related alkynes. The results for C? C fission are consistent with previous VLPP and shock-tube determinations of the propargyl resonance energy, and the parameters for the molecular pathway are consistent with systematic trends for the retro–ene decomposition of unsaturated hydrocarbons.     

Atomic mechanism of fracture of solid polymers

Macromolecular chain scission under mechanical stress has been studied by infrared spectroscopy. The dependence of accumulation of chemical bond scissions on temperature T and uniaxial tensile stress σ has been investigated. The rate constant K for bond dissociation under mechanical stress has been found to obey the modified Arrhenius equation: K = K₀ exp{ ? (E_A ? ασ)/RA}. The quantitative connection between the rate constant for bond dissociation and mechanical lifetime τ has been established. Analysis of the experimental data indicates that the strength and mechanical lifetime of polymers is determined by the kinetics of mechanochemical scission of the main chains of polymer molecules.     

Very low-pressure pyrolysis (VLPP) of methyl- and ethynyl-cyclopentanes and cyclohexanes

Studies of the kinetics of thermal unimolecular decomposition of methylcyclopentane, methylcyclohexane, ethynylcyclopentane, and ethynylcyclohexane have been carried out at temperatures in the range 861–1218 K using the technique of very low-pressure pyrolysis (VLPP). Multiple reaction pathways and secondary decomposition of primary products results in a complex array of reaction products. VLPP rate data (fall-off regime) were obtained for the overall decompositions and interpreted via the application of RRKM theory. The data for methylcyclopentane and methylcyclohexane were interpreted in terms of ring-opening bond fission pathways and bond fission to methyl and cycloalkyl radicals. By selecting Arrhenius parameters consistent with the analogous pathways in open-chain alkanes, a good fit to the overall decomposition is obtained. The data for ethynylcyclopentane and ethynylcyclohexane were interpreted in terms of ring-opening bond fission and alkyne to allene isomerization. The A factors for ring opening were based on known values for C-C fission in open-chain alkynes and the Arrhenius parameters for isomerization were chosen to be consistent with previously reported alkyne to allene isomerizations. The VLPP data are consistent with the following high-pressure rate expressions (at < T > = 1100 K) for the dominant primary reaction channel of ring opening adjacent to the substitutent group: where θ = 2.303RT kJ mol^?. Comparison of the activation energies for the ethynyl-cycloalkanes with those for the methyl-cycloalkanes shows that the effect of the ethynyl substituent is consistent with the propargyl resonance energy. This evidence supports the assumption of a biradical mechanism for ring opening in these cycloalkanes.     

Very low-pressure pyrolysis. VII. The decomposition of methylhydrazine, 1,1-dimethylhydrazine, 1,2-dimethylhydrazine,and tetramethylhydrazine. Concerted deamination and dehydrogenation of methylhydrazine

The rate constants (k_uni) for the first-order disappearance of the title molecules have been determined under VLPP conditions. The k_uni are not the rate constants of ultimate interest since they reflect the fact that energy transfer competes with the chemical decomposition. Use of the Rice-Ramsperger-Kassel-(Marcus) [RRK(M)] theory allows the determination of the high-pressure rate constants (k_α), if the mode of decomposition is known. The heats of formation of the radicals NH₂, CH₃NH, and (CH₃)₂N are known. These values should be usable for prediction of the activation energy for N? N bond homolysis in the hydrazines. Measured rate constants for UDMH and TMH bear this out, but the rate constant for MMH does not. This and other evidence lead to the conclusion that MMH decomposes via molecular concerted elimination of NH₃ and H₂ not and by N? N bond scission. The following values are preferred from this work (θ = 2.303RT in kcal/mole). Mode of decomposition is N—N bond scission unless noted otherwise in parenthesis: .     

Competitive unimolecular reactions at low pressures. The pyrolysis of cyclobutyl chloride

The thermal decomposition of cyclobutyl chloride has been investigated over the temperature range of 892–1150 K using the technique of very low-pressure pyrolysis (VLPP). The reaction proceeds via two competitive unimolecular channels, one to yield ethylene and vinyl chloride and the other to yield 1,3-butadiene and hydrogen chloride, with the latter being the major reaction under the experimental conditions. With the usual assumption that gas-wall collisions are «strong,» RRKM calculations, generalized to take into account two competing pathways, show that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters given by log k₁(sec^?1) = (14.8 ± 0.3) ? (61.1 ± 1.0)/Θ for vinyl chloride formation and log k₂(sec^?1) = (13.6 ± 0.3) ? (55.7 ± 1.0)/Θ for 1,3-butadiene formation, where Θ = 2.303 RT kcal/mol. The A factors were assigned from previous high-pressure low-temperature data of other workers assuming a four-center transition state for 1,2-HCl elimination and a chlorine-bridged biradical transition state for vinyl chloride formation. The activation energies are in good agreement with the high-pressure results which were obtained with a conventional static system. The difference in critical energies is 4.6 kcal/mol.     

Photochemical reactions of high polymers. XIV. Syntheses and photochemical reactions of copolymers bearing O-acyloxime groups

Copolymers bearing pendant O-acyloxime groups were synthesized by two methods: copolymerizations of oxime acrylate (methyl β-naphthyl ketone oxime acrylate or benzophenone oxime acrylate) and styrene, condensation of acrylic acid—styrene copolymer with oximes (benzophenone oxime, p-nitrobenzophenone oxime, methyl β-naphthyl ketone oxime, benzalacetone oxime or 9-fluorenone oxime). The photochemical behavior of the O-acyloxime copolymers changed markedly with the irradiation conditions: irradiation of benzene solutions led to degradation in air and crosslinking under nitrogen, while irradiation of solid films in air resulted in simultaneous degradation and crosslinking. Photolysis of methyl β-naphthyl ketone oxime acetate, a model for the O-acyloxime copolymer, in benzene solution under nitrogen resulted in scission of the N? O bond. The same reaction was observed in the irradiation of the O-acyloxime copolymers. It is suggested that formation of free radicals on the polymer chains via scission of the N? O bond is followed by decarboxylation. In the absence of oxygen, crosslinking of the polymer by recombination of the free radicals competes with degradation via β-scission. In the presence of oxygen, autoxidative degradation predominates.     

Kinetics of thermal depolymerization of trimethylsiloxy end-blocked polydimethylsiloxane and polydimethylsiloxane-N-phenylsilazane copolymer

The thermal degradation of Me₃SiO end-blocked polydimethylsiloxane (eb-PDMS) and polydimethylsiloxane-N-phenylsilazane (eb-PDMS–NPhSz) copolymer was studied. For both polymers relative degree of polymerization (DP /DP ₀) as a function of conversion (1 – W/W₀) data were obtained. For eb-PDMS the results were consistent with a mechanism involving a rate determining random siloxane bond cleavage initiation step followed by a rapid and complete depropagation of the active fragments evolving volatile cyclic oligomers. Rate constants (k) for initiation were obtained at four temperatures from plots of DP ^?1 vs. time. An Arrhenius activation energy of approximately 80 kcal/mol was determined and is consistent with a SiOSi scission transition state. The degradation of eb-PDMS–NPhSz appears to follow the same depolymerization process evolving cyclic oligomers. Although DP /DP ₀ vs. C data suggest a random cleavage–complete depolymerization mechanism, an Arrhenius plot suggests a more complex degradation mechanism. The role of impurities as degradation catalysts is discussed.     

Thermochemical kinetics of nitrogen compounds. V. The unimolecular thermal decomposition of diallylamine in the gas phase

The kinetics of the thermal decomposition of diallylamine to propylene and prop-2-enaldimine have been studied in the gas phase in presence of an excess of methylamine over the temperature range of 532.7 to 615.6°K, using a static reaction system. Methylamine reacted with the unstable primary product prop-2-enaldimine, forming the thermally stable N-methyl prop-2-enaldimine. First-order rate constants, based on the internal standard technique, fit the Arrhenius relationship log k(s^?1) = (11.04 ± 0.13) ? (37.11 ± 0.33 kcal/mole)/2.303 RT. They were independent on the initial total pressure (46–340 torr), the initial pressure of diallylamine (9.2–65 torr), or methylamine as well as the conversion attained. Despite an apparent surface sensitivity, the reaction is essentially homogeneous in nature as demonstrated by experiments carried out in a packed reaction vessel. The observed activation parameters for the title reaction together with those observed earlier for triallylamine and allylcyclohexylamine are consistent with the proposed concerted reaction mechanism involving a cyclic 6-center transition state. The observed substituent effects suggest a nonsynchronous mode of bond breaking and bond formation.     

Competitive paths for methanol decomposition on Pt(111)

Periodic, self-consistent, Density Functional Theory (PW91-GGA) calculations are used to study competitive paths for the decomposition of methanol on Pt(111). Pathways proceeding through initial C-H and C-O bond scission events in methanol are considered, and the results are compared to data for a pathway proceeding through an initial O-H scission event [Greeley et al. J. Am. Chem. Soc. 2002, 124, 7193]. The DFT results suggest that methanol decomposition via CH(2)OH and either formaldehyde or HCOH intermediates is an energetically feasible pathway; O-H scission to CH(3)O, followed by sequential dehydrogenation, may be another realistic route. Microkinetic modeling based on the first-principles results shows that, under realistic reaction conditions, C-H scission in methanol is the initial decomposition step with the highest net rate. The elementary steps of all reaction pathways (with the exception of C-O scission) follow a linear correlation between the transition state and final state energies. Simulated HREELS spectra of the intermediates show good agreement with available experimental data, and HREELS spectra of experimentally elusive reaction intermediates are predicted.     

The thermal unimolecular decomposition of bromocyclobutane

The thermal unimolecular decomposition of bromocyclobutane has been investigated over the temperature range of 791–1224 K using the technique of very low-pressure pyrolysis (VLPP). HBr elimination is the sole mode of decomposition under the experimental conditions. No evidence could be found for the ring-cleavage pathway to ethylene and vinyl bromide. Assuming a four-center transition state and an Arrhenius A factor the same as that for HCl elimination from chlorocyclobutane, RRKM calculations show that the experimental unimolecular rate constants are consistent with the Arrhenius expression where θ = 2.303RT kcal/mol. The activation energy is higher than that for the open-chain analog, 2? bromobutane. This finding is consistent with the results for the corresponding chloro and iodo compounds.     

The Critical Role of Reductive Steps in the Nickel-Catalyzed Hydrogenolysis and Hydrolysis of Aryl Ether C−O Bonds

The hydrogenolysis of the aromatic C−O bond in aryl ethers catalyzed by Ni was studied in decalin and water. Observations of a significant kinetic isotope effect (k_H/k_D=5.7) for the reactions of diphenyl ether under H₂ and D₂ atmosphere and a positive dependence of the rate on H₂ chemical potential in decalin indicate that addition of H to the aromatic ring is involved in the rate-limiting step. All kinetic evidence points to the fact that H addition occurs concerted with C−O bond scission. DFT calculations also suggest a route consistent with these observations involving hydrogen atom addition to the ipso position of the phenyl ring concerted with C−O scission. Hydrogenolysis initiated by H addition in water is more selective (ca. 75 %) than reactions in decalin (ca. 30 %).     

The Critical Role of Reductive Steps in the Nickel‐Catalyzed Hydrogenolysis and Hydrolysis of Aryl Ether C−O Bonds

The hydrogenolysis of the aromatic C?O bond in aryl ethers catalyzed by Ni was studied in decalin and water. Observations of a significant kinetic isotope effect (k_H/k_D=5.7) for the reactions of diphenyl ether under H₂ and D₂ atmosphere and a positive dependence of the rate on H₂ chemical potential in decalin indicate that addition of H to the aromatic ring is involved in the rate‐limiting step. All kinetic evidence points to the fact that H addition occurs concerted with C?O bond scission. DFT calculations also suggest a route consistent with these observations involving hydrogen atom addition to the ipso position of the phenyl ring concerted with C?O scission. Hydrogenolysis initiated by H addition in water is more selective (ca. 75 %) than reactions in decalin (ca. 30 %).     

Poly-α-ester degradation studies. VI. Thermal degradation of poly(3-pentylidene carboxylate)

The thermal degradation of poly(3-pentylidene carboxylate) has been studied kinetically over the temperature range 200–300°C using thermogravimetry, gas evolution analysis, and rheogoniometry together with isolation and analysis of the reaction products. The observed behavior is completely different from that previously reported for poly(isopropylidene carboxylate) and poly(methylene carboxylate). Whereas in the latter cases the decomposition occurs by a first-order intramolecular ester interchange process characterized by an activation energy in the region of 27 kcal mole^?1, poly(3-pentylidene carboxylate) decomposition occurs by random chain scission superimposed on a first-order hydrogen abstraction process. The activation energy associated with this decomposition reaction is in the region of 47 kcal mole^?1, and the major degradation products are cis- and trans-2-ethyl crotonic acid.     

[Cu(TO)2(H2O)4](PA)2的制备、结构表征和热分解机理研究

[Cu（TO）2（H2O）4]（PA）2 was prepared by the reaction of aqueous 1,2,4-triazol-5-one （TO） solution with the solution of copper picrate Cu（PA）2 and characterized by elemental analysis, FT IR and X-ray powder diffraction analysis. The title complex has been studied by means of TG-DTG and DSC under conditions of linear temperature increase. The thermal decomposition residues were examined by FT IR analysis. Thermal decomposition mechanism of the title complex was proposed. In the temperature range of 30-680 ℃, the thermal decomposition process was composed of four major stages. The first stage was an endothermic process with the loss of four coordination water molecules. Since the dehydration product was unstable, when it was heated, it would be decomposed much more easily. The second stage was composed of an acute endothermic process and a continued strong exothermic process and the main decomposed residues were CuCO3, Cu（NCO）2 and polymers during this stage. The third stage was a sharp exothermic process, which resulted from the decomposition of the polymer. After the forth stage, the final decomposed residues were certainly copper oxide. The Arrhenius parameters have been also studied on the dehydration process and the first-step exothermic decomposition of [Cu（TO）2（H2O）4]（PA）2 using Kissinger＇s method and Ozawa-Doyle＇s method. The results using both methods were consistent with each other. The Arrhenius equation can be expressed as in k=24.0-179.8 × 10^3/RT for the dehydration process and in k= 16.7-206.0 × 10^3/RT for the first-step exothermic decomposition, on the basis of the average of Ea and In A through the two methods.     

Very low-pressure pyrolysis (VLPP) of alkyl cyanides. II. n-propyl cyanide and isobutyl cyanide. The heat of formation and stabilization energy of the cyanomethyl radical

The very low-pressure pyrolysis (VLPP) technique has been used to study the pyrolysis of n-propyl cyanide over the temperature range of 1090–1250°K. Decomposition proceeds via two pathways, C₂? C₃ bond fission and C₃? C₄ bond fission, with the former accounting for >90% of the overall decomposition. Application of unimolecular reaction rate theory shows that the experimental unimolecular rate constants for C₂? C₃ fission are consistent with the high-pressure Arrhenius parameters given by where θ=2.303RT kcal/mole. The activation energy leads to DH₂₉₈⁰[C₂H₅? CH₂CN]=76.9±1.7 kcal/mole and ΔH(?H₂CN, g)=58.5±2.2 kcal/mole. The stabilization energy of the cyanomethyl radical has been found to be 5.1±2.6 kcal/mole, which is the same as the value for the α-cyanoethyl radical. This result suggests that DH[CH₂(CN)? H] ～ 93 kcal/mole, which is considerably higher than previously reported. The value obtained for ΔH_?⁰(?H₂CN) should be usable for prediction of the activation energy for C₂? C₃ fission in primary alkyl cyanides, and this has been confirmed by a study of the VLPP of isobutyl cyanide over the temperature range of 1011–1123°K. The decomposition reactions parallel those for n-propyl cyanide, and the experimental data for C₂? C₃ fission are compatible with the Arrhenius expression A significant finding of this work is that HCN elimination from either compound is practically nonexistent under the experimental conditions. Decomposition of the radical, CH₃CHCH₂CN, generated by C₃? C₄ fission in isobutyl cyanide, yields vinyl cyanide and not the expected product, crotonitrile. This may be explained by a radical isomerization involving either a 1,2-CN shift or a 1,2-H shift.     

Photodecomposition of polystyrene on long-wave ultraviolet irradiation: A possible mechanism of initiation of photooxidation

The long-wave (λ < 3000 Å) photo-oxidation of polystyrene in solution at 25°C has been studied osmometrically. Two types of chain scission have been observed: a purely photo process which occurs completely independently of oxygen and which is attributed to fission of photolabile groups in the polymer, and another process associated with random photolyses of the products of oxidation Scavenger experiments with ¹³¹I₂ have shown that approximately two iodine atoms are incorporated per chain scission when photolysis is carried out in solution (benzene, hexafluorobenzene, methylene chloride) under high vacuum conditions in the presence of ¹³¹I₂. No iodine incorporation or chain scission occurs when ionically prepared polystyrenes are treated similarly. The nature of the photolabile bond has been discussed, and there is some evidence for a peroxidic linkage arising from oxygen copolymerization in the chains. It is suggested that fission of the photolabile groups contributes to the initiation of the long-wave photooxidation of the polymer.     

The kinetics and mechanism of the shock induced gas phase decomposition of ethylsilane

The decomposition kinetics of ethylsilane under shock tube conditions (P_T ca. 3100 torr, T ? 1080–1245 K), both in the absence and presence of silylene trapping agents (butadiene and acetylene) are reported. Arrhenius parameters under maximum butadiene inhibition are: log k(C₂H₅SiH₃) = 15.14-64,769 ± 1433 cal/2.303 RT; log k(C₂H₅SiD₃) = 15.29-66,206 ± 1414/2.303 RT. The uninhibited reaction is subject to silylene induced decomposition (63% lowest T -- 24% highest T). Major reaction products are ethylene and hydrogen, consistent with two dominant primary dissociation reactions: C₂H₅SiD₃ → C₂H₅SiD + D₂, ? ? 0.66; C₂H₅SiD₃ → CH₃CH = SiD₂ + HD, ? ? 0.30. Minor products suggest several other less important primary processes: alkane elimination, ? ?0.02, and free-radical production via simple bond fission, ? ?0.02. An upper limit for the activation energy of the decomposition, C₂H₅SiH → C₂H₄ + SiH₂, of E < 30 ± 4 kcal is established, and speculations on the mechanism of this decomposition (concerted or stepwise) with conclusions in favor of the stepwise path are made. Computer modeling studies for the reaction both in the absence and presence of butadiene are shown to be in good agreement with the experimental observations.     

Kinetics and mechanism of the thermal decomposition of cyclopentyl cyanide

The thermal decomposition of cyclopentyl cyanide has been investigated in the temperature range of 905–1143 K using both conventional stirred-flow reactor and very low-pressure pyrolysis (VLPP) techniques. The results from both techniques are consistent. The main primary processes are HCN elimination to form cyclopentene: and ring fragmentation to form vinyl cyanide plus propylene and ethylene plus cyanopropenes: Under the experimental conditions cyclopentene undergoes further decomposition to cyclopentadiene plus hydrogen. There is evidence for conversion of some of the reactant to a solid residue, presumably polymer. From the stirred-flow reactor results the following Arrhenius expressions were obtained: log k₁(s^?1) = (12.8 ± 0.3) ? (65.6 ± 1.3)/θ and log k₂(s^?1) = (16.0 ± 0.3) ? (80.0 ± 1.1)/θ, where θ = 2.303RT kcal/mol. Application of RRKM theory shows that the VLPP experimental rate constants are consistent with high-pressure Arrhenius parameters given by log k₁(s^?1) = (12.8 ± 0.3) ? (67.8 ± 2.5)/θ for HCN elimination, and log k₄(s^?1) = (16.3 ± 0.3) ? (80.1 ± 2.0)/θ for the sum of the ring fragmentation pathways. The rate parameters for HCN elimination are in good agreement with previous VLPP studies of alkyl cyanides and with theoretical predictions. The difference in activation energies for the ring opening of cyclopentane and cyclopentyl cyanide is reasonably close to the established value for the cyano stabilization energy. This supports the assumption of a biradical mechanism.     

Revised mechanism of carboxylation of ribulose‐1,5‐biphosphate by rubisco from large scale quantum chemical calculations

Here, we describe a computational approach for studying enzymes that catalyze complex multi‐step reactions and apply it to Ribulose 1,5‐bisphosphate carboxylase–oxygenase (Rubisco), the enzyme that fixes atmospheric carbon dioxide within photosynthesis. In the 5‐step carboxylase reaction, the substrate Ribulose‐1,5‐bisphosphate (RuBP) first binds Rubisco and undergoes enolization before binding the second substrate, CO₂. Hydration of the RuBP.CO₂ complex is followed by C C bond scission and stereospecific protonation. However, details of the roles and protonation states of active‐site residues, and sources of protons and water, remain highly speculative. Large‐scale computations on active‐site models provide a means to better understand this complex chemical mechanism. The computational protocol comprises a combination of hybrid semi‐empirical quantum mechanics and molecular mechanics within constrained molecular dynamics simulations, together with constrained gradient minimization calculations using density functional theory. Alternative pathways for hydration of the RuBP.CO₂ complex and associated active‐site protonation networks and proton and water sources were investigated. The main findings from analysis of the resulting energetics advocate major revision to existing mechanisms such that: hydration takes place anti to the CO₂; both hydration and C C bond scission require early protonation of CO₂ in the RuBP.CO₂ complex; C C bond scission and stereospecific protonation reactions are concerted and, effectively, there is only one stable intermediate, the C3‐gemdiolate complex. Our main conclusions for interpreting enzyme kinetic results are that the gemdiolate may represent the elusive Michaelis–Menten‐like complex corresponding to the empirical K_m (=K_c) with turnover to product via bond scission concerted with stereospecific protonation consistent with the observed catalytic rate. © 2018 Wiley Periodicals, Inc.