Kinetic study of chemically activated chlorocyclopropane

Chlorocyclopropane has been produced by addition of CH₂(¹A₁) and CH₂(³B₁) to chloroethene. CH₂ was generated by the photolysis of ketene at 313 and 366 nm. Chlorocyclopropane was formed in a chemically activated state, had an energy content between 378 and 427 kJ/mol, and reacted in three parallel channels to 3-chloropropene, cis- and trans-1-chloropropene. As secondary reactions elimination of HCl from the chemically activated primary products occurred to form allene and propyne. The apparent rate constants for the isomerization and elimination reactions are reported. The results of RRKM calculations including distribution functions for the activated chlorocyclopropane and a stepladder model for the deactivation support the proposed reaction scheme.     

An investigation of nonequilibrium kinetic isotope effects in chemically activated vinyl radicals

New experimental data have been obtained for H + C₂H₂, D + C₂H₂, H + C₂D₂, and D + C₂D₂ at room temperature. Two previously described apparatus were used in order to measure the pressure dependence of the reactions. The absolute rate constants are compared to results from other laboratories. The present results and those of Payne and Stief are used to obtain the high-pressure limiting rate constant at room temperature. When the activation energy from the work of Payne and Stief is considered, it is shown that the A factor for H + C₂H₂ is too low by a factor of ～20. If a transmission coefficient is introduced which is constant for all isotopic variations, the pressure dependence can be explained in terms of the randomly energized radicals. RRKM theory is then invoked to explain the observed statistical nonequilibrium kinetic isotope effects.     

On the electronic and molecular structures of methyl chloride,cyclopropyl chloride and 1,1-dichlorocyclopropane

Ab initio SCF—MO calculations have been carried out on cyclopropyl chloride and 1,1-dichlorocyclopropane, to illuminate the effect of the substituent upon the cyclopropane ring. For cyclopropyl chloride, reasonable agreement is found with experimental findings. For 1,1-dichlorocyclopropane, excellent agreement is found with the results from a recent NMR study. As a means of studying the effect of a change in basis set, calculations are also carried out on methyl chloride.     

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.     

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.     

Isomerization of chemically activated secondary butyl radical

Photolytic decomposition of hydrogen sulfide followed by the addition of energized hydrogen atoms to cisbut-2-ene gives excited sec-butyl radicals. The radicals undergo a 1,3 hydrogen migration as is evidenced by the good agreement between the RRKM calculations and experiment. The contribution of a 1,2 hydrogen shift cannot be excluded. The threshold energies are 153.8 and 176.8 kJ mol^–1 for 1,3 and 1,2 isomerization, respectively.

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-2 .- . 1,3- , . , 1,2- . 153,8 176,8 / 1,3- 1,2-, .

     

Complexes of rhodium carbonylchloride and phosphorus(iii) cyanides in mediated electrochemical reduction of 2-carbomethoxy-2-methyl-1,1-dichlorocyclopropane

Electrochemical reduction of the complexes Rh(CO)ClL₂ [L = (EtO)₂PCN (1), Ph₂PCN (2)] and Rh₂(CO)₄L [L = P(CN)₃ (3), (4)] and their catalytic properties in electrochemical reduction of 2-carbomethoxy-2-methyl-1,1-dichloro-cyclopropane were studied. The catalytic electroreduction of a substrate at the reduction potentials of the central ion was developed for complexes2–4. This process is accelerated substantially for complexes2 and3 in the presence of anthracene.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1970–1972, October, 1995.     

Vibrational energy transfer from chemically activated 1,4-cyclohexadiene

The thermal isomerization of cis, anti, cis-tricyclo[3.1.0.0^2,4] hexane was used to produce highly vibrationally excited 1,4-cyclohexadiene. The competition between unimolecular decomposition of the energized diene (to benzene and hydrogen) and collisional stabilization was studied using the parent compound, SF₆, CO₂, N₂, and He as quenching gases. Quenching efficiencies decreased in the order given above. By applying RRKM theory to the isomerization and decomposition reactions, it was possible to calculate the step size in a stepladder model of the deactivation of cyclohexadiene. The step sizes 〈ΔE〉 deduced (at 528 K and in units of kJ/mol) were: parent compound and SF₆, 7; CO₂, 5; N₂, 4; He, 2. The study confirmed the utility of this unimolecular chemical activation system for energy transfer studies.     

Paramagnetic species on the surface of chemically activated silica

Active centers of SiO₂ formed during thermolysis of methoxylated silica surface has been studied by ESR. Several types of coordinatively unsaturated silicon atoms, which differ in location and number but show similar reactivity toward various adsorbates, have been found.

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, . - , , , H₂, CO, O₂.

     

The rearrangement of chemically activated phenylmethylenes at 77K

The cocondensation of atomic carbon with o, m and p-tolualdehyde at 77K results in deoxygenation to the o, m and p-tolylmethylenes which possess sufficient energy to rearrange to benzocyclobutene and styrene.     

Decomposition of chemically activated dimethylcyclopropane molecules vibrationally excited to various extent

The photolyses of ketene (at 313 and 280 nm) and diazirine at 313 nm in the presence of cis-butene-2 were studied. Vibrational relaxation of chemically activated dimethylcyclopropane was shown to occur as a multistep process, and 17 ± 4 kJ mol^?1 was obtained for the average energy transferred per collision with butene-2 collider. Activated cis-dimethylcyclopropane is formed in the reaction of singlet methylene and cis-butene-2 with broad energy distribution which originates from the energy partitioning in the photolytic act. About 30% of the energy released in the photolysis of the methylene source is carried by singlet methylene as vibrational energy at the time of reaction, and this fraction was found to be practically independent of the radical source.     

Adsorption of a reactive dye on chemically modified activated carbons--influence of pH

The surface chemistry of a commercial activated carbon with a slightly basic nature was modified by appropriate treatments in order to obtain two additional samples, respectively with acidic and basic properties, without changing its textural parameters significantly. Different techniques (N2 adsorption at 77 K, temperature programmed desorption, and determination of acidity, basicity, and pH at the point of zero charge) were used to characterize the adsorbents. Kinetic and equilibrium adsorption data of a selected textile reactive dye (Rifafix Red 3BN, C.I. reactive red 241) on the mentioned materials were obtained at the pH values of 2, 7, and 12. The kinetic curves are fitted using the second-order model. The respective rate constants seem to diminish progressively with the initial concentration for the more diluted solutions tested, reaching a constant value at higher concentrations, which depends on the experimental system under consideration (adsorbent and pH). In general, the Langmuir model provides the best fit for the equilibrium data. The different uptakes obtained are discussed in relation to the surface chemical properties of the adsorbents. It is shown that the adsorption of the reactive (anionic) dye on the basic sample (prepared by thermal treatment under H2 flow at 700 degrees C) is favored. This conclusion is explained on the basis of the dispersive and electrostatic interactions involved. Moreover, it is also shown that the optimal adsorption condition for all the activated carbons tested corresponds to solution pH values not higher than the pH(pzc) of the adsorbents, which may be interpreted by taking into account the electrostatic forces present.     

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.     

Theory of chemically activated unimolecular reactions. Weak collisions and steady states

The prediction of decomposition/stabilization ratios for chemically activated unimolecular reactions is usually based on the assumption of steady state and irreversible deactivation. The levels below the activation energy E_o are treated as a sink. In the present work we assume that the reaction can be described by a weak-collision master equation. An exact analysis shows that one can expect three timescales: an initial transient, an intermediate steady state, and an asymptotic steady state. The intermediate steady state is well defined only if the eigenvalues of the corresponding thermal rate matrix can be separated into two groups one of which contains eigenvalues of substantially smaller magnitude. An efficient and accurate method for the estimation of intermediate steady-state populations and branching ratios is described. It avoids the irreversible deactivation assumption which is shown to lead to errors particularly for weak collisions. The new analysis is illustrated in a series of calculations for a model of the sec-butyl radical system. The numerical solutions are readily obtained by use of a recently developed equilibrium finite-basis-set method.     

The effect of ring substitution on the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals. A product and time-resolved kinetic study

[reaction: see text] A product and time-resolved kinetic study of the effect of ring substitution on the reactivity of 1,1-diarylalkoxyl radicals has been carried out. The radicals undergo an O-neophyl shift to give the isomeric 1-aryl-1-aryloxyalkyl radicals from which the corresponding aromatic ketones are formed. The rearrangement rate constants are influenced by ring substitution, increasing in the presence of electron-withdrawing substituents and decreasing in the presence of electron-donating ones. From the results of product and kinetic studies, the following migratory aptitudes have been obtained: 4-trifluoromethylphenyl > phenyl approximately = 4-methylphenyl > 4-methoxyphenyl. Excellent Hammett-type correlations between the sigma+ substituent constants and both the visible absorption band maxima and the rearrangement rate constants have been obtained. The experimental results indicate that the rearrangement is governed by electronic effects in the starting 1,1-diarylalkoxyl radicals, whereas the stability of the rearranged carbon-centered radical plays a minor role, in line with a reactant-like transition state, strongly supporting the hypothesis that the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals proceeds through a concerted mechanism.