Insights into the reaction mechanism between propadienylidene and ethylene: a DFT study

The reaction mechanism between propadienylidene and ethylene has been systematically investigated employing the B3LYP/6-311++G** and MP2/cc-pVTZ levels of theory to better understand the reactivity of propadienylidene with unsaturated hydrocarbons. Geometry optimization, vibrational analysis, and energy property for the involved stationary points on the potential energy surface have been calculated. Two important initial reaction complexes characterized by three- and four-membered ring structures have been located firstly. After that, three different products possessing three-, four-, and five-membered ring characters have been obtained through three reaction pathways. In the first reaction pathway, a three-membered ring alkyne compound has been obtained. As for the second reaction pathway, it is a diffusion-controlled reaction, resulting in the formation of the four-membered ring conjugated diene compound. A five-membered conjugated diene compound has been obtained in the third reaction pathway, which is the most stable product in the available products thermodynamically. On the other hand, the second reaction pathway is the most favorable reaction to proceed kinetically.     

Steric impedence in the 1,3-photocycloaddition reaction between cyclopentene and anisole derivatives

The detrimental effect of increasing substitution on the olefin and aromatic partners for the title reaction is demonstrated; several transformations of the resulting photoadducts are described.     

Products and mechanism of the gas phase reaction between nitrate radical and arenes

Summary The reaction between the nitrate radical and ortho- and para-xylene and some deuterated analogues was studied in the gas phase in a 480 l reaction chamber. Sampling of the gas mixture and gas chromatography-mass spectrometry allowed to obtain kinetic isotope effect values in the range of 1.48–1.87. These were similar to the values of 1.25–1.67 observed in the oxidation of these substrates with cerium(IV) ammonium nitrate in acetonitrile. This suggests that the hydrogen abstraction mechanism commonly invoked for these reaction may compete with a single electron transfer and an addition of the nitrate radical to the aromatic compound in the rate-determining first reaction step. Lowering the concentration of dioxygen in the reaction gas mixture leads to an increase of the ratio nitrate ester/aromatic aldehyde.     

Theoretical study on the mechanism of the cycloaddition reaction between stannylene and ethylene or formaldehyde

The mechanism of the cycloaddition reaction of singlet stannylene and ethylene or formaldehyde has been studied by using density functional theory. The geometrical parameters, harmonic vibrational frequencies and energies of stationary points for potential energy surface are calculated by RB3LYP/3–21G* method. The results show that the two reaction processes are both two steps: (1) stannylene and ethylene or formaldehyde form an energy‐rich intermediate complex respectively, which is an exothermal reaction with no barrier; (2) two intermediate complexes isomerize to the product, respectively, with the barriers of these two reactions being 52.97 and 45.15 kJ/mol at RB3LYP/3–21G* level.     

The mechanism of the high-pressure free radical polymerization of ethylene

The reaction path of the free radical polymerization of ethylene is usually considered identical to the polymerization mechanism of other vinyl monomers. Available experimental data on the polymerization of ethylene, however, hardly fitted the well-established path of free radical polymerization. Obviously the mechanism of ethylene polymerization is more complex and not well understood. One reason for this, in our opinion, is insufficient knowledge of the physicochemical state of ethylene under high pressure. A model that described the behavior of ethylene under compression has been proposed. According to the model, and increase in pressure causes the formation of various supermolecular forms of ethylene, each accompanied by transition of the second order. By proposing a stereochemical shape for each supermolecular form calculation of activation volumes for each of these transitions was made. Good agreement was obtained when calculated volumes of activation were compared with corresponding experimental values in the literature.     

Pressure and temperature dependence of the reaction of vinyl radical with ethylene

This work reports measurements of absolute rate coefficients and Rice-Ramsperger-Kassel-Marcus (RRKM) master equation simulations of the C2H3+C2H4 reaction. Direct kinetic studies were performed over a temperature range of 300-700 K and pressures of 20 and 133 mbar. Vinyl radicals (H2C=CH) were generated by laser photolysis of vinyl iodide (C2H3I) at 266 nm, and time-resolved absorption spectroscopy was used to probe vinyl radicals through absorption at 423.2 nm. Measurements at 20 mbar are in good agreement with previous determinations at higher temperature. A weighted three-parameter Arrhenius fit to the experimental rate constant at 133 mbar, with the temperature exponent fixed, gives k=(7+/-1)x10(-14) cm3 molecule(-1) s(-1) (T/298 K)2 exp[-(1430+/-70) K/T]. RRKM master equation simulations, based on G3 calculations of stationary points on the C4H7 potential energy surface, were carried out to predict rate coefficients and product branching fractions. The predicted branching to 1-methylallyl product is relatively small under the conditions of the present experiments but increases as the pressure is lowered. Analysis of end products of 248 nm photolysis of vinyl iodide/ethylene mixtures at total pressures between 27 and 933 mbar provides no direct evidence for participation of 1-methylallyl.     

烯酮或乙烯与甲醛环加成协同反应机理的对比研究

本文研究了烯酮与甲醛, 乙烯与甲醛两个环加成反应的协同过程的过渡态(TS),用能量分解方法对两个过渡态中反应物间的相互作用能的本质做了剖析。通过对比,发现在乙烯与甲的过渡态中反应物的占有轨道间电子的交换排斥作用能比较大, 从而可以说明此反应比乙烯与烯酮间的协同过程难于进行。     

Theoretical study of mechanism of extraction reaction between silylene carbene and its derivatives and ethylene oxide

The mechanism of the oxide extraction reaction between singlet silylene carbene and its derivatives [X₂Si = C: (X = H, F, Cl, CH₃)] and ethylene oxide has been investigated with density functional theory, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated by B3LYP/6‐311G(d,p) method. From the potential energy profile, it can be predicted that the reaction pathway of this kind consists two steps, the first step is the two reactants firstly form an intermediate (INT) through a barrier‐free exothermic reaction; the second step is the INT then generates a product via a transition state (TS). This kind reaction has similar mechanism, when the silylene carbene and its derivatives [X₂Si = C: (X = H, F, Cl, CH₃)] and ethylene oxide close to each other, the shift of 2p lone electron pair of O in ethylene oxide to the 2p unoccupied orbital of C in X₂Si = C: gives a p → p donor–acceptor bond, thereby leading to the formation of INT. As the p → p donor–acceptor bond continues to strengthen (that is, the C? O bond continues to shorten), the INT generates product (P + C₂H₄) via TS. It is the substituent electronegativity, which mainly affects the extraction reactions. When the substituent electronegativity is greater, the energy barrier is lower, and the reaction rate is greater. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Novel reaction between cyclic formal and ethylene oxide

Novel reactions between trioxane and ethylene oxide were discovered, and three novel cyclic formals were isolated and identified. These novel cyclic compounds clarified the initiation mechanism of the copolymerization of trioxane and ethylene oxide. This type of reaction was not limited to the reaction between trioxane and ethylene oxide but was also generalized to the reaction between the cyclic formal and ethylene oxide. Although an NMR method for analyzing the ethylene oxide sequences of the acetal copolymer from trioxane and ethylene oxide has not yet been established, the three newly found novel cyclic compounds, composed of 1 mol of ethylene oxide and 1 mol of trioxane, 2 mol of ethylene oxide and 1 mol of trioxane, and 3 mol of ethylene oxide and 1 mol of trioxane, were useful for analyzing the ethylene oxide sequences. These compounds gave only one consecutive oxyethylene unit, two consecutive oxyethylene units, and three consecutive oxyethylene units in three consecutive oxymethylene units, respectively, and gave different ¹H NMR spectra for each oxyethylene unit. Considering these data, we synthesized three polymeric model compounds that had one consecutive oxyethylene sequence, two consecutive oxyethylene sequences, and three consecutive oxyethylene sequences in an oxymethylene main chain. By a linear combination of the ¹H NMR spectrum of each oxyethylene unit of the three polymeric model compounds, we succeeded in determining the ethylene oxide sequences by the ¹H NMR method for the copolymer from trioxane and ethylene oxide. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 520–533, 2004     

A comparison between ethylene dimerization and Diels-Alder reaction

The reaction pathways for the two reactions 2C₂H₄C₄H₈ and C₂H₄+C₄H₆ C₆H₁₀ were investigated. The transition state geometries and activation energies were determined with the SINDO method and compared. Both reactions were found concerted with different significance of diradical character at the transition state. Whereas the ethylene dimerization showed that a diradical will be encountered along the reaction pathway, we did not find one in the Diels-Alder reaction.     

Alternating stereospecific copolymerization of cyclopentene and ethylene with constrained geometry catalysts

The stereoselective copolymerization of cyclopentene (cP) and ethylene (E) to generate highly alternating polymers with isotactic cis 1,2-cyclopentene enchainment is reported.     

On the mechanism of a double ring-closing metathesis reaction

A detailed study of the steps involved in the double ring-closing metathesis reaction of 2 to 3 has been carried out. Both the selectivity and mechanism were affected by choice of catalyst.     

Radio frequency magnetic field effects on a radical recombination reaction: a diagnostic test for the radical pair mechanism

The photoinduced electron-transfer reaction of chrysene with isomers of dicyanobenzene is used to demonstrate the sensitivity of a radical recombination reaction to the orientation and frequency (5-50 MHz) of a approximately 300 muT radio frequency magnetic field in the presence of a 0-4 mT static magnetic field. The recombination yield is detected via the fluorescence of the exciplex formed exclusively from the electronic singlet state of the radical ion pair Chr*+/DCB*-. Magnetic field effects are simulated using a modified version of the gamma-COMPUTE algorithm, devised for the simulation of magic angle spinning NMR spectra of powdered samples. The response of a chemical or biological system to simultaneously applied radio frequency and static or extremely low-frequency magnetic fields could form the basis for a diagnostic test for the operation of the radical pair mechanism that would not require prior knowledge of the nature and properties of the radical reaction.     

The abnormal finkelstein reaction. A sequential ionic-free radical reaction mechanism

The reaction of alkyl halides with sodium iodide in acetone (Finkelstein Reaction) may yield coupled or rearranged products $\text{via}$ cationic intermediates.     

Chain transfer mechanism and kinetics in radical telomerization of ethylene with bromalkanes

Rate constants for chain transfer via bromoalkanes of various structure are estimated for primary, secondary and tertiary alkyl radicals and isokinetic dependences are shown to exist for their activation parameters. Isokinetic temperatures are calculated.

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Relation between lyoluminescence and radical reaction rate constants

Light emission during the dissolving of irradiated sugars (lyoluminescence, LL) allows the estimation of absorbed dose. The use of 1-mannose as LL substance and the correlation between the concentration of paramagnetic centres and LL yield in the presence of [Fe(CN)₆]^4? and CNS^? anions demonstrated the possibility to measure relative rate constants of the reaction of mannose peroxy-radicals with different substances soluble in water.     

Complex mechanism of the gas phase reaction between formic acid and hydroxyl radical. Proton coupled electron transfer versus radical hydrogen abstraction mechanisms

The gas phase reaction between formic acid and hydroxyl radical has been investigated with high level quantum mechanical calculations using DFT-B3LYP, MP2, CASSCF, QCISD, and CCSD(T) theoretical approaches in connection with the 6-311+G(2df,2p) and aug-cc-pVTZ basis sets. The reaction has a very complex mechanism involving several elementary processes, which begin with the formation of a reactant complex before the hydrogen abstraction by hydroxyl radical. The results obtained in this investigation explain the unexpected experimental fact that hydroxyl radical extracts predominantly the acidic hydrogen of formic acid. This is due to a mechanism involving a proton coupled electron-transfer process. The calculations show also that the abstraction of formyl hydrogen has an increased contribution at higher temperatures, which is due to a conventional hydrogen abstraction radical type mechanism. The overall rate constant computed at 298 K is 6.24 x 10(-13) cm3 molecules(-1) s(-1), and compares quite well with the range from 3.2 +/- 1 to 4.9 +/- 1.2 x 10(-13) cm3 molecules(-1) s(-1), reported experimentally.