Reaction Of Singlet Methylene Research Articles

Predictive Theory for the Addition and Insertion Kinetics of 1CH2 Reacting with Unsaturated Hydrocarbons

The reactions of singlet methylene, (1)CH2, with unsaturated hydrocarbons are of considerable significance to the formation and growth of polycyclic aromatic hydrocarbons (PAHs). In this work, we employ high level ab initio transition state theory to predict the high pressure rate coefficient for singlet methylene reacting with acetylene (C2H2), ethylene (C2H4), propyne (CH3CCH), propene (CH3CHCH2), allene (CH2CCH2), 1,3-butadiene (CH2CHCHCH2), 2-butyne (CH3CCCH3), and benzene (C6H6). Both addition and insertion channels are found to contribute significantly to the kinetics, with the insertion kinetics of increasing importance for larger hydrocarbons due to the increasing number of CH bonds and increasingly attractive interactions. We treat the addition kinetics with direct CASPT2 based variable-reaction-coordinate transition state theory. One-dimensional corrections to the CASPT2 interaction energies are obtained from geometry relaxation calculations and CCSD(T)/CBS evaluations. The insertion kinetics is treated with traditional variational TST methods employing CCSD(T)/CBS energies obtained along the CASPT2/cc-pVTZ distinguished coordinate reaction paths. The overall rate constant and branching fractions are obtained from a multiple transition state model that accounts for the physical distinction between tight inner and loose outer transition states. The predicted rate constants, which cover the range from 200 to 2000 K, are found to be in excellent agreement with the available experimental data, with a maximum observed discrepancy of about 40%.

Kinetics and Product Branching Ratios of the Reaction of 1CH2 with H2 and D2

The reactions of singlet methylene (a(1)A1 (1)CH2) with hydrogen and deuterium have been studied by experimental and theoretical techniques. The rate coefficients for the removal of singlet methylene with H2 (k1) and D2 (k2) have been measured from 195 to 798 K and are essentially temperature-independent with values of k1 = (10.48 +/- 0.32) x 10(-11) cm(3) molecule(-1) s(-1) and k2 = (5.98 +/- 0.34) x 10(-11) cm(3) molecule(-1) s(-1), where the errors represent 2sigma, giving a ratio of k1/k2 = 1.75 +/- 0.11. In the reaction with H2, singlet methylene can be removed by reaction giving CH3 + H or deactivated to ground-state triplet methylene. Direct measurement of the H atom product showed that the fraction of relaxation decreased from 0.3 at 195 K to essentially zero at 398 K. For the reaction with deuterium, either H or D may be eliminated. Experimentally, the H:D ratio was determined to be 1.8 +/- 0.5 over the range 195-398 K. Theoretically, the reaction kinetics has been predicted with variable reaction coordinate transition state theory and with rigid-body trajectory simulations employing various high-level, ab initio-determined potential energy surfaces. The magnitudes of the calculated rate coefficients are in agreement with experiment, but the calculations show a significant negative temperature dependence that is not observed in the experimental results. The calculated and experimental H to D ratios from the reaction of singlet methylene with D2 are in good agreement, suggesting that the reaction proceeds entirely through the formation of a long-lived methane intermediate with a statistical distribution of energy.

Fast, scalable master equation solution algorithms. IV. Lanczos iteration with diffusion approximation preconditioned iterative inversion

In this paper we propose a second linearly scalable method for solving large master equations arising in the context of gas-phase reactive systems. The new method is based on the well-known shift-invert Lanczos iteration using the GMRES iteration preconditioned using the diffusion approximation to the master equation to provide the inverse of the master equation matrix. In this way we avoid the cubic scaling of traditional master equation solution methods while maintaining the speed of a partial spectral decomposition. The method is tested using a master equation modeling the formation of propargyl from the reaction of singlet methylene with acetylene, proceeding through long-lived isomerizing intermediates.

Fast, scalable master equation solution algorithms. III. Direct time propagation accelerated by a diffusion approximation preconditioned iterative solver

In this paper we propose a novel fast and linearly scalable method for solving master equations arising in the context of gas-phase reactive systems, based on an existent stiff ordinary differential equation integrator. The required solution of a linear system involving the Jacobian matrix is achieved using the GMRES iteration preconditioned using the diffusion approximation to the master equation. In this way we avoid the cubic scaling of traditional master equation solution methods and maintain the low temperature robustness of numerical integration. The method is tested using a master equation modelling the formation of propargyl from the reaction of singlet methylene with acetylene, proceeding through long lived isomerizing intermediates.

Ionization Energy of Methylene Revisited: Improved Values for the Enthalpy of Formation of CH2 and the Bond Dissociation Energy of CH3 via Simultaneous Solution of the Local Thermochemical Network

The appearance energy of the CH2+ fragment from CH2CO has been carefully remeasured and fitted by a model curve, producing EA0(CH2+/CH2CO) = 13.743 ± 0.005 eV. This value can be sequentially propagated through selected thermochemical cycles to yield individual values for EI(CH2), D0(H−CH2), ΔH°f 0(CH2), and ΔH°f 0(CH2CO). A set of values with a statistically larger weight is produced by analyzing a local thermochemical network, which combines the present measurement with thirteen other experimental determinations from the literature and encompasses the enthalpies of formation of CH3, CH3+, CH2, CH2+, and CH2CO. The recommended simultaneously adjusted thermochemical values are: ΔH°f 0(CH3) = 35.86 ± 0.07 kcal/mol (35.05 ± 0.07 kcal/mol at 298 K), ΔH°f 0(CH3+) = 262.73 ± 0.06 kcal/mol (261.83 ± 0.06 kcal/mol at 298 K), ΔH°f 0(CH2) = 93.18 ± 0.20 kcal/mol (93.31 ± 0.20 kcal/mol at 298 K), ΔH°f 0(CH2+) = 332.92 ± 0.19 kcal/mol (333.04 ± 0.19 kcal/mol at 298 K), ΔH°f 0(CH2CO) = −11.10 ± 0.21 kcal/mol (−11.85 ± 0.21 kcal/mol at 298 K), as well as D0(H3C−H) = 103.42 ± 0.03 kcal/mol (104.99 ± 0.03 kcal/mol at 298 K), D0(H2C−H) = 108.95 ± 0.20 kcal/mol (110.35 ± 0.20 kcal/mol at 298 K), D0(H2CCO) = 77.08 ± 0.02 kcal/mol (78.73 ± 0.02 kcal/mol at 298 K), EI(CH3) = 9.3830 ± 0.0005 eV, and EI(CH2) = 10.3962 ± 0.0036 eV. These values are in excellent agreement with current and several previous experimental measurements. The recommended enthalpy of formation of CH2 implies that the reaction of singlet methylene with water is essentially thermoneutral (to within ±0.2 kcal/mol) at 0 and 298 K, and slightly endothermic (0.5 ± 0.2 kcal/mol) at 1000 K.

Facile formation of azines from reactions of singlet methylene and dimethylcarbene with precursor diazirines: theoretical explorations

The reactions of the singlet methylene (1a) and dimethylcarbene (1b), with their diazirine precursors, diazirine (2a), and dimethyldiazirine (2b), have been studied theoretically using ab initio and density functional theory. The reaction has no activation barriers for the parent system (1a + 2a) and proceeds via a reactive complex and a transition state with a small negative enthalpy of activation Δ Hnot =298 = -1.1 kcal mol-1, ΔSnot =298 = -34.4 cal mol-1 K-1, ΔG°298 = 9.2 kcal mol-1) for the dimethyl derivatives (1b + 2b). The formation of N-methylene diazirinium ylides (3a,b) is exothermic by 64-80 kcal mol-1. The isomer, 1,3-diazabicyclo[1.1.0]butane (4a), is more stable (5-12 kcal mol-1) than isomer 3a, but can neither be formed by direct thermal reaction of 1a with 2a nor undergo the direct rearrangement into formaldazine (5a). The rearrangement of ylides 3a,b into azines 5a,b proceeds by conrotatory C3-N1 ring opening. The predicted activation barrier of ca. 15 kcal mol-1 for the ring opening in ylide 3b is in excellent agreement with experimental data. The formation of pyridinium ylides from carbenes and pyridine is also studied.Key words: diazirinium ylide, ab initio MO (molecular orbital) theory, density functional theory, pyridinium ylide, CIS (singles configuration interaction) transition energies.

Facile formation of azines from reactions of singlet methylene and dimethylcarbene with precursor diazirines: theoretical explorations

The reactions of the singlet methylene (1a) and dimethylcarbene (1b), with their diazirine precursors, diazirine (2a), and dimethyldiazirine (2b), have been studied theoretically using ab initio and density functional theory. The reaction has no activation barriers for the parent system (1a + 2a) and proceeds via a reactive complex and a transition state with a small negative enthalpy of activation Δ Hnot =298 = -1.1 kcal mol-1, ΔSnot =298 = -34.4 cal mol-1 K-1, ΔG°298 = 9.2 kcal mol-1) for the dimethyl derivatives (1b + 2b). The formation of N-methylene diazirinium ylides (3a,b) is exothermic by 64-80 kcal mol-1. The isomer, 1,3-diazabicyclo[1.1.0]butane (4a), is more stable (5-12 kcal mol-1) than isomer 3a, but can neither be formed by direct thermal reaction of 1a with 2a nor undergo the direct rearrangement into formaldazine (5a). The rearrangement of ylides 3a,b into azines 5a,b proceeds by conrotatory C3-N1 ring opening. The predicted activation barrier of ca. 15 kcal mol-1 for the ring opening in ylide 3b is in excellent agreement with experimental data. The formation of pyridinium ylides from carbenes and pyridine is also studied.Key words: diazirinium ylide, ab initio MO (molecular orbital) theory, density functional theory, pyridinium ylide, CIS (singles configuration interaction) transition energies.

ChemInform Abstract: Stereodirecting Effects in the Characterization of Ylide Intermediates in Reactions of Singlet Methylene with an Allylic Ether and an Allylic Chloride.

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Stereodirecting Effects in the Characterization of Ylide Intermediates in Reactions of Singlet Methylene with an Allylic Ether and an Allylic Chloride

The stereodirecting effects of substrate substituents were examined in addition of 1:CH2 to the double bonds of 3-methoxycyclohexene (3a), 3-chlorocyclohexene (3b), and 3-methylcyclohexene (3c) in pentane solution at temperatures from 23°C to −50°C. A preference for addition syn to the directing substituent was observed in reactions of 3a and 3b and was attributed to a reversible interaction of carbene and substituent. In the reaction with 3a this interaction was not obviously ylide formation. © 1997 Elsevier Science Ltd.

Propargyl from the Reaction of Singlet Methylene with Acetylene

The technique of infrared kinetic spectroscopy has been used to study the production of propargyl radical from the reaction of singlet methylene with acetylene. The rate constant for this product channel was determined to be (3.5 ± 0.7) × 10-10 cm3 molecule-1 s-1 at 295 K, measured relative to the known rate for 1CH2 with H2 or CH4. Methylene was produced in the singlet state by excimer laser photolysis of ketene at 308 nm in the presence of acetylene and either H2 or CH4. Reaction of 1CH2 with acetylene produces propargyl, and reaction of 1CH2 with either H2 or CH4 produces CH3. The intensity of a propargyl infrared absorption line was compared with that of a methyl infrared absorption line, and the rate of formation of propargyl was determined from the ratio of these two intensities and the known rate of reaction of singlet methylene with H2 (or CH4) to produce CH3. The relative peak infrared absorption cross sections of methyl and propargyl were calibrated under the conditions of the experiment by photolyzing crotyl bromide at 193 nm to produce methyl and propargyl in equal concentrations.

Identification of C5H8isomers through reactions of singlet methylene, CH2(ã1A1), with unsaturated hydrocarbons using capillary gas chromatography-mass spectrometry

Kovats retention indices of C5H8 isomers were determined by capillary gas chromatography with mass spectral detection on a capillary column combination (OV-1 and SE-54 stationary phases) and on a PoraPLOT Q molecular sieve column, at room temperature and in the temperature range 160–180 °C, respectively. Commercially unavailable isomers were identified as products of the reactions for singlet methylene with buta-1,3-diene, but-1-yne, but-2-yne and methylenecyclopropane with kinetics in the high-pressure regime. In the identification, helpful aids were (i) comparison of retention times with authentic samples (standards); (ii) using the spectral library and computer search facilities of a gas chromatographic-mass spectrometric instrument; (iii) applying chemical evidence, analogies and literature data (e.g., product ratios); and (iv) utilising boiling-point correlations (elution order). On these slightly polar columns, the main factor affecting the retention is the boiling-point, although the correlation between retention indices and boiling-points is weak for prediction purposes.

A solvent effect in reactions of singlet methylene

The effect of several solvents on the selectivity of singlet methylene ( 1:CH 2) was investigated. It was found that product ratios from reactions in pentane, ethyl ether and cyclohexene solutions were identical; however, product ratios from reactions carried out in benzene were slightly different.

Quantum-chemical calculation of the reaction of singlet methylene and its alkyl derivatives with the simplest saturated hydrocarbons and the hydrogen molecule

1. According to MINDO/3 data, the insertion of singlet methylene into the C-H bond of methane and ethane is characterized by an early transition state and by a low activation barrier (∿2 kcal/mole), while the reaction of methylcarbene1CHCH3 with methane is less exothermic and has a later transition state. 2. In the reaction of singlet carbenes with the H2 molecule the successive introduction of alkyl substituents at the carbene center also reduces the exothermic nature of insertion in the series methylene > methylcarbene, ethylcarbene > dimethylcarbene and displaces the transition state toward the final product. 3. According to the calculations and to analysis of the published data, the insertion reactions of singlet carbenes and the reverse processes of carbene fragmentation of alkanes are most sensitive to the introduction of the substituent at the reacting or forming carbene center.

A theoretical insight into the insertion reaction of singlet methylene to the hydrogen molecule

The least-motion and non-least-motion energy profiles for the insertion reaction of singlet methylene into the hydrogen molecule have been calculated using the 6-31G* basis set and introducing the correlation energy with the Møller–Plesset perturbation theory and with the MC-SCF technique. From the results obtained the effect of geometry optimization including electron correlation on the shape of the energy surface is discussed. The building up of a bidimensional biconfigurational energy surface has permitted one to interpret the opening of the methylene angle during the least-motion process as a direct consequence of the change in the electronic configuration. The TC-SCF/6-31G* second order transition structure for this forbidden process has been directly located using gradient minimization methods.

Decomposition of chemically activated dimethylcyclopropane molecules vibrationally excited to various extent

AbstractThe 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.

Reaction of singlet methylene with methylenecyclopropane. Part 2.—High energy reaction pathways for chemically activated insertion products

The reaction of singlet methylene, formed by the photolysis of keten (λ≃ 313 nm) with methylenecyclopropane has been investigated over the pressure range, 1–300 Torr, but predominantly at the lower end. The products are ethylene, propylene, acetylene, allene, spiropentane, methylenecyclobutane, methylmethylenecyclopropane (MMCP), ethylidenecyclopropane (ECP), isoprene and various penta-1,2-, penta-1,3- and penta-1,4-dienes. Studies of the pressure dependences of acetylene, isoprene and the pentadienes indicate that they originate from the chemically activated insertion products MMCP and ECP. Detailed consideration of the pressure dependences of the acetylene and isoprene yields in conjunction with RRKM calculations, provides support for multistep collisional deactivation in this system. A stepladder model with a deactivation step size of ≃ 7.2 kcal mol–1 fits the data.Further RRKM calculations are used together with this model to provide approximate values for the critical energies of the pentadiene forming pathways.

The Reaction of Singlet Methylene with Methane

Abstract Cyclopropane in an excess of methane was photolyzed at 1470 and 1634 Å. The decomposition rate constants of the resultant vibrationally excited ethane into methyl radicals were found to be 6.0×109 s−1 and 5.0×109 s−1 at 1470 and 1634 Å respectively. The following procedure was used in treating the results obtained in our and the other researchers’ systems. The statistical calculation was made to estimate the internal energy of singlet methylene, \barεi which is initially acquired in the primary decomposition process. The RRKM theory was then used to estimate the internal energy of the excited ethane. According to these calculations, the fraction of excess photon energy carried into ethane decreases as the energy, \barεi increases.

Reactions of singlet methylene with butadiene. High energy isomerizations of vinylcyclopropane

Reactions of singlet methylene with butadiene. High energy isomerizations of vinylcyclopropane

Reaction of singlet methylene with methylenecyclopropane. Part 1.—Evidence for multistep collisional deactivation of chemically activated spiropentane

The reaction of singlet methylene, formed by the 325 nm photolysis of ketene, with methylene-cyclopropane has been investigated over a wide range of pressures. The products are ethylene, allene, spiropentane, methylenecyclobutane, methylmethylenecyclopropane, ethylidenecyclopropane and isoprene. Their formation was interpreted in terms of decomposition, rearrangement and partial stabilisation of the initially formed products of addition and insertion. Studies of the pressure dependence of the product distribution, despite its complexity lead to the unequivocal conclusion that chemically activated spiropentane molecules can not be characterised by a unique lifetime. RRKM calculations used in conjunction with a step-ladder model for collisional deactivation give a fit to the data for a deactivation step size of 7.20 kcal mol–1 at an energy of 105.5 kcal mol–1 for the initially formed spiropentane. This result is consistent with a figure of 101 kcal mol–1 for the heat of formation of methylene, although uncertainties, particularly in the entropy of activation for spiropentane isomerisation, mean that a lower value cannot be ruled out.

Reaction of singlet methylene with cyclopropane

Reaction of singlet methylene with cyclopropane