1981-80-2Relevant academic research and scientific papers
Mechanism of thermal decomposition of allyltrichlorosilane with formation of three labile intermediates: dichlorosilylene, allyl radical, and atomic chlorine
Boganov,Promyslov,Krylova,Zaitseva,Egorov
, p. 1216 - 1224 (2017/02/05)
It is experimentally found that allyltrichlorosilane dissociates under vacuum pyrolysis (~10–2 Torr) at temperatures above 1100 K to form three labile intermediates: allyl radical, dichlorosilylene, and monoatomic chlorine. On the basis of experimental and theoretical data obtained, it is shown that the decomposition reaction proceeds in two steps. The first step is a typical reaction of homolytic decomposition to two radicals (C3H5 and SiCl3) at the weakest Si—C bond. Due to weakness of the Si—Cl bond in the SiCl3 radical, the energy of which is even somewhat lower than the dissociation energy of the Si—C bond in starting AllSiCl3, this radical undergoes further dissociation to SiCl2 and Cl, thus resulting in three intermediates of different classes of highly reactive species formed from AllSiCl3.
Homolytic dissociation of 1-substituted cyclohexa-2,5-diene-1-carboxylic acids: An EPR spectroscopic study of chain propagation
Jackson, Leon V.,Walton, John C.
, p. 1758 - 1764 (2007/10/03)
Hydrogen abstraction from 1-substituted cyclohexa-2,5-diene-1-carboxylic acids containing linear, branched and cyclic alkyl substituents, as well as allyl, propargyl (prop-2-ynyl), cyanomethyl and benzyl substituents, has been studied by EPR spectroscopy. For each carboxylic acid, EPR spectra of the corresponding cyclohexadienyl radicals were observed at lower temperatures, followed by spectra due to ejected carbon-centred radicals at higher temperatures. Rate constants, for release of the carbon-centred radicals from the cyclohexadienyl radicals, were determined from radical concentration measurements for the above range of substituents. The rate of cyclohexadienyl radical dissociation increased with branching in the 1-alkyl substituent and with electron delocalisation in the ejected carbon-centred radical; 3,5-and 2,6-dimethyl-substitution of the cyclohexadienyl ring led to reductions in the dissociation rate constants. Rate data for abstraction of bisallylic hydrogens from the cyclohexadienyl acids were also obtained for ethyl, n-propyl and isopropyl radicals. These results indicated a sharp drop in the rate of hydrogen abstraction as the degree of branching in the attacking radical increased. Small decreases in the hydrogen abstraction rate constants were observed for cyclohexadienes containing CO2R substituents.
Exploitation of aldoxime esters as radical precursors in preparative and EPR spectroscopic roles
McCarroll, Andrew J.,Walton, John C.
, p. 2399 - 2409 (2007/10/03)
Photolyses of aldoxime esters, containing a considerable range of alkyl groups, lead to cleavage of their N-O bonds and formation of aryliminyl and alkyl radicals. The process was found to be favoured by 4-methoxyacetophenone as a photosensitiser and by methoxy substituents in the aryl rings. 4-Nitro- and pentafluoro-substitutions of the aryl rings were, on the other hand, deleterious. The intermediate iminyl radicals, together with primary, secondary and tertiary alkyl radicals were characterised by 9 GHz EPR spectroscopy. Cyclopropyl, CF3, and CCl3 radicals were probably also formed, but were too reactive for direct EPR spectroscopic detection. Photosensitised reaction of benzophenone oxime O-nonanoyl ester produced the diphenylmethaniminoxyl, as well as the expected n-octyl and iminyl radicals. This indicated that O-C bond scission accompanied O-N scission for this ketoxime ester. At higher temperatures the C-centred radicals added to the starting oxime esters to produce alkoxyaminyl radicals that were also spectroscopically detected in some cases. No evidence for abstraction of the iminyl hydrogen by tertbutoxyl radicals was obtained. Instead, the t-BuO radicals added to the C=N double bonds of the oxime esters. Similarly, chlorine abstraction from alkylbenzohydroximoyl chlorides by trimethyltin radicals did not take place. Preparative scale experiments with oxime esters containing suitably unsaturated alkyl groups showed that good yields of cyclised products could be obtained in the presence of the photosensitiser. This process constitutes a general method by which carboxylic acids or acid chlorides can be converted into alkyl radicals and hence to cyclised derivatives.
Experimental study of the reaction between vinyl and methyl radicals in the gas phase. Temperature and pressure dependence of overall rate constants and product yields
Stoliarov, Stanislav I.,Knyazev, Vadim D.,Slagle, Irene R.
, p. 9687 - 9697 (2007/10/03)
The vinyl-methyl radicals are critical intermediates in hydrocarbon combustion systems with elementary reactions of CH3 and C2H3 influencing the rate and products of the overall combustion process. The vinyl-methyl cross-radical reaction was studied using laser photolysis/photoionization MS. C2H2 yields did not experience any pressure dependence at 310, 500, and 900 K. The C3H5 decreased with an increasing pressure, which was well resolved at 310 and 500 K. The overall C2H3 + CH3 rate constants and quantitative product yields were obtained in direct real-time experiments at 300-900 K and bath gas (He) density (3-12) x 1016 molecules/cc. The primary products of the C2H3 + CH3 reaction were propylene, acetylene, and allyl radicals. A mechanism consisting of two major routes was proposed, i.e., via direct abstraction of a hydrogen atom from the vinyl radical by the methyl radical resulting in the formation of acetylene and methane and via the formation of chemically activated propylene that can undergo collisional stabilization or further decomposition into allyl radical and hydrogen atom.
Polar effects on iodine atom abstraction by charged phenyl radicals
Heidbrink, Jenny L.,Thoen, Kami K.,Kenttaemaa, Hilkka I.
, p. 645 - 651 (2007/10/03)
The ability of differently substituted charged phenyl radicals (a class of distonic radical cations) to abstract an iodine atom from allyl iodide was systematically examined in the gas phase by using Fourier transform ion cyclotron resonance mass spectrometry. The reaction products and second-order reaction rate constants were determined for several radicals that differ by the type and/or number of substituents located in the ortho- and/or meta- position with respect to the radical site. All the radicals also carry a para-pyridinium group needed for mass spectrometric manipulation. These electron-deficient phenyl radicals react with allyl iodide by predominant iodine atom abstraction. The reaction is facilitated by the presence of neutral electron-withdrawing substituents, such as F, CF3, Cl, or CN. The extent of rate increase depends on the type and number of the substituents, as well as their location relative to the radical site. Based on molecular orbital calculations (PM3 and Becke3LYP/6-31G(d)+ZPVE), the indicated variations in the transition state energy are not related to enthalpic factors. Instead, the results are rationalized by polar effects arising from a variable contribution of a stabilizing charge transfer resonance structure to the transition state. A semiquantitative measure for the barrier-lowering effect of each substituent is provided by its influence on the electron affinity of the radical (the electron affinities were calculated by Becke3LYP/6-31+G(d) and AM1, which were found to produce similar values). Methyl substitution does not significantly affect the electron affinity, and accordingly, does not have a detectable effect on reactivity. Methyl groups located at ortho-positions are an exception, however. o-Methyl-substituted phenyl radicals undergo exothermic rearrangement to a benzyl radical in competition with iodine abstraction from allyl iodide.
Detailed kinetics of cyclopentadiene decomposition studied in a shock tube
Burcat, Alexander,Dvinyaninov, Michael
, p. 505 - 514 (2007/10/03)
Mixtures of cyclopentadiene diluted with argon were used to investigate its decomposition pattern in a single pulse shock tube. The temperatures ranged from 1080 to 1550 K and pressures behind the shock were between 1.7-9.6 atm. The cyclopentadiene concentrations ranged from 0.5 to 2%. Gas-chromatographic analysis was used to determine the product distribution The main products in order of abundance were acetylene, ethylene, methane, allene, propyne, butadiene, propylene, and benzene. The decomposition of cyclopentadiene was simulated with a kinetic scheme containing 44 species and 144 elementary reactions. This was later reduced to only 36 reactions The ring opening process of the cyclopentadienyl radical was found to be the crucial step in the mechanism. 1997 lohn Wiley and Sons, Inc.
Investigating conformation dependence and nonadiabatic effects in the photodissociation of allyl chloride at 193 nm
Myers,Kitchen,Hu,Butler
, p. 5446 - 5456 (2007/10/03)
The experiments presented here investigate the competing photodissociation pathways for allyl chloride upon excitation of the nominally ππ*(C=C) transition at 193 nm. The measured photofragment velocity distributions evidence C-Cl bond fission and HCl elimination. The recoil kinetic energy distribution for the HCl products is bimodal, indicating two primary processes for HCl elimination. The experimental measurements show C-Cl bond fission dominates, giving an absolute branching ratio of HCl:C-Cl=0.12±0.03 when the parent molecule is expanded through a nozzle at 200°C. The branching ratio depends on the nozzle temperature; at 475°C, the absolute branching ratio measured is HCl:C-Cl=0.24±0.03. We analyze the experimental results along with Supporting ab initio calculations and earlier photodissociation studies of vinyl chloride in order to examine the potential influence of nonadiabaticity along the C-Cl fission reaction coordinate and its dependence on molecular conformation.
Radical-Stabilization-Energy - the MMEVBH Force Field
Roth, Wolfgang R.,Staemmler, Volker,Neumann, Martin,Schmuck, Carsten
, p. 1061 - 1118 (2007/10/02)
Making use of the VB method of Malrieu et al. a force field has been developed, which allows to calculate heats of formation of hydrocarbons (conjugated and non-conjugated olefins, radicals and diradicals) with high accuracy.With this method radical stabilization energies (RSE) for a great number of delocalized radicals are calculated and compared with experimental values, derived from shock-tube measurements of dissociation energies or from rotational barriers of substituted olefins.A detailed analysis of the RSE with respect to structure, substituents, strain, and aromaticity is presented. - Key Words: Resonance energy / Heats of formation / Single pulse shock tube / Intrisic rotational barrier
Ion-Molecule Reactions of CF3+ with Simple Unsaturated Aliphatic Hydrocarbons at Near-Thermal Energy
Tsuji, Masaharu,Aizawa, Masato,Nishimura, Yukio
, p. 3195 - 3200 (2007/10/02)
Ion-molecule reactions of CF3+ with C2H2, C2H4, and C3H6 have been studied at near-thermal energy (0.05 eV) by using an ion beam apparatus.Initial product ion distributions and reaction rate constants were determined and compared with previous beam and selected ion flow tube (SIFT) data.The CF3+/C2H2 reaction produces exclusively the electrophilic adduct C3H2F3+ ion.For C2H4 and C3H6, hydride abstraction and electrophilic addition followed by HF elimination or fluoride transfer occur in parallel.The branching ratios of the former and latter reactions are 0.29 +/- 0.04:0.71 +/- 0.06 for the CF3+/C2H4 reaction and 0.07 +/- 0.02:0.93 +/- 0.07 for the CF3+/C3H6 reaction.On the basis of theoretical calculations of potential energies for the CF3+/C2H2 and CF3+/C2H4 systems, the lack of the HF elmination channel in the CF3+/C2H2 reaction, whereas the lack of the initial adduct ion in the CF3+/C2H4 reaction, is attributed to the different stability of the intermediate adduct ions for HF elimination.The reaction rate constants were 0.45 x 1E-9, 1.3 x 1E-9, and 1.6 x 1E-9 cm3 s-1 for C2H2, C2H4, and C3H6, respectively, which correspond to 46percent, 120percent, and 130percent of calculated rate constants from Langevin theory or a parametrized trajectory model.Although there are significant discrepancies in the product ion distributions between the present beam experiment and the previous beam data, the product ion distributions and the reaction rate constants obtained here are in reasonable agreement with the previous SIFT data.
