Propargyl Research Articles

ChemInform Abstract: Control of the Propargylic Radical Stabilization by Carbon-Chain Length in Manganese(III)-Mediated Reactions of 1-Alken-3-ynes. - A Facile Synthetic Way to Long-Chain 4-Acetoxy-5-alkynoic Acids.

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Fuel-structure dependence of benzene formation processes in premixed flames fueled by C6H12 isomers

The fuel-structure-dependent significance of various benzene formation pathways is analyzed using data from rich (ϕ=1.7) flames fueled by four C6H12 isomers: 1-hexene, cyclohexane, methylcyclopentane, and 3,3-dimethyl-1-butene. The isomer-resolved chemical compositions of the four premixed, laminar low-pressure flat flames are determined by flame-sampling molecular-beam mass spectrometry employing single-photon ionization by synchrotron generated vacuum-ultraviolet photons. Isomer-resolving photoionization efficiency curves and quantitative mole fraction profiles reveal the dominant fuel destruction pathways, the influence of different fuel consumption processes on the formation of commonly considered benzene precursors, and the contributions of several routes towards benzene formation. While propargyl and allyl radicals dominate benzene formation in the combustion of 1-hexene, contributions from reactions involving i-C4H5 and C5H5 radicals are revealed in the flames of 3,3-dimethyl-1-butene and methylcyclopentane, respectively. Close to the burner surface, successive dehydrogenation of the fuel is found to be important for the cyclohexane flame and to some smaller extent for the methylcyclopentane flame.

Radical Chemistry in the Thermal Decomposition of Anisole and Deuterated Anisoles: An Investigation of Aromatic Growth

The pyrolyses of anisole (C(6)H(5)OCH(3)), d(3)-anisole (C(6)H(5)OCD(3)), and d(8)-anisole (C(6)D(5)OCD(3)) have been studied using a hyperthermal tubular reactor and photoionization reflectron time-of-flight mass spectrometer. Gas exiting the reactor is subject to an immediate supersonic expansion after a residence time of approximately 65 mus. This allows the detection of highly reactive radical intermediates. Our results confirm that the first steps in the thermal decomposition of anisole are the loss of a methyl group to form phenoxy radical, followed by ejection of a CO to form cyclopentadienyl radical (c-C(5)H(5)); C(6)H(5)OCH(3) --> C(6)H(5)O + CH(3); C(6)H(5)O --> c-C(5)H(5) + CO. At high temperatures (T(wall) = 1200 degrees C - 1300 degrees C) the c-C(5)H(5) decomposes to propargyl radical (CH(2)CCH) and acetylene; c-C(5)H(5) --> CH(2)CCH + C(2)H(2). The formation of benzene and naphthalene is demonstrated with 1 + 1 resonance-enhanced multiphoton ionization. Propargyl radical recombination is a significant benzene formation channel. However, we show the majority of benzene is formed by a ring expansion reaction of methylcyclopentadiene (C(5)H(5)CH(3)) resulting from methyl radical addition to cyclopentadienyl radical; CH(3) + c-C(5)H(5) --> C(5)H(5)CH(3) --> C(6)H(6) + 2H. The naphthalene is generated from cyclopentadienyl radical recombination; 2c-C(5)H(5) --> C(5)H(5)-C(5)H(5) --> C(10)H(8) + 2H. The respective intermediate amu 79 and 129 species associated with these reactions are detected, confirming the stepwise nature of the decompositions. These reactions are verified by pyrolysis studies of cyclopentadiene (C(5)H(6)) and C(5)H(5)CH(3) obtained from rapid thermal dissociation of the respective dimer compounds, as well as pyrolysis studies of propargyl bromide (BrCH(2)CCH).

Dissociation of C3H3I and rates for C3H3 combination at high temperatures

The decomposition of propargyl iodide, C3H3I→C3H3+I, and the following combination of propargyl radical, C3H3+C3H3→C6H6, have been observed in the incident shock in mixtures of 1%, 2%, and 5% C3H3I in Kr, over 800–2000K, for nominal post-shock pressures of 20 and 130Torr. The iodide dissociates readily, the endothermic reaction producing an easily resolved positive density gradient whose kinetic modeling allows extraction of precise rates showing strong falloff. A simple Gorin-model RRKM treatment of the measurements results in a k∞=3.35×1018T−0.9exp(−22192/T)s−1. The initial positive gradient is immediately followed by a strong negative, showing exothermic reaction from propargyl combination. Modeling of this gradient with an assumed theoretical product distribution, provided by J.A. Miller, results in propargyl combination rates in excellent agreement with current theory and having a strong negative temperature dependence, the rate dropping near a factor of forty over 800–2000K. Possible errors associated with the need to assume a product distribution are discussed and it is suggested that this should not result in rate uncertainties for the C3H3 combination reaction greater than ±50%.

Calculated photoelectron spectra of isotopomers of the propargyl radical (H 2C 3H): An explicitly correlated coupled cluster study

Explicitly correlated coupled cluster theory at the (U)CCSD(T∗)-F12a level has been employed to study the vibrational structure of the first two bands of the low-temperature photoelectron spectra of four different isotopomers of the propargyl radical with C 2v symmetry (H 2C 3H, H 2C 3D, D 2C 3H, and D 2C 3D). A five-dimensional anharmonic model is employed to calculate the peak positions and relative intensities. While the first band of the PE spectra of all four isotopomers is dominated by the adiabatic peak, the second band shows a progression in the pseudoantisymmetric CC stretching vibration v 3 with relative intensities of 100:68:23:5:1 for n = 0–4 in the case of the most abundant isotopomer.

1,3-Steric Induction in Intermolecular Radical Reactions Mediated by a Co2(CO)6−Metal Core

Diastereoselectivity of propargyl coupling reactions can be controlled by using the bulkiness of a γ-substituent as a stereochemical tool. This 1,3-steric induction was observed with γ-t-Bu and γ-Me3Si groups, both favoring a d,l-configuration of the head-to-head coupling products, 3,4-disubstituted 1,5-alkadiynes (d,l- 95−100%). X-ray crystallography analysis suggests that the most favorable orientation of converging propargyl radicals is the one in which the bulky γ-substituents are positioned anti to each other. Overall, the synthetic strategy of employing a Me3Si auxiliary group involves five steps and affords, with 28−33% overall yields, pure d,l-3,4-diaryl-1,5-hexadiynes, otherwise hardly accessible.

Inter- and intramolecular isocarbon couplings of cobalt-complexed propargyl radicals: challenging the consensus

Intermolecular coupling reactions of Co 2(CO) 6-complexed, γ-ethyl propargyl radicals occurred with higher dl-diastereoselectivity than intramolecular cyclizations of isocarbon analogues (de inter−de intra≤52%). The observed phenomenon was interpreted in terms of a tandem action of two main determinants—sterics and CH/π coordination—with the latter being enabled by the methoxy groups located on the periphery of the aromatic nuclei (4-, 3,4-, 3,4,5-).

Modeling of 1,3-hexadiene, 2,4-hexadiene and 1,4-hexadiene-doped methane flames: Flame modeling, benzene and styrene formation

In this work, we have developed a detailed chemical kinetic model and reacting flow simulation for the hexadiene-doped 2-d methane diffusion flames studied experimentally by McEnally and Pfefferle. The GRI-Mech 2.11 methane oxidation and Lawrence Livermore butane oxidation mechanisms were used as the base mechanism to which hexadiene chemistry generated by Reaction Mechanism Generator (RMG) was added. Some important chemically activated pathways leading to aromatic species formation, including the reactions on C 5H 7, C 6H 10, C 6H 9, C 6H 7, C 8H 8 and C 8H 9 potential energy surfaces, are examined in great detail using quantum chemistry (CBS-QB3) and master equation analysis as implemented in Variflex. An efficient program to solve the doped methane diffusion flame was developed. The solver uses the method of lines to solve the species mass balance equation arising in the diffusion flame. It assumes that the temperature and velocity profiles of the doped flame are the same as those of the undoped flame. The mole fractions of various species as predicted by our model are compared to the experimentally measured mole fractions. The agreement between theory and experiments is quite good for most molecules. The added hexadiene dopants to the flame decompose to produce significant amount of cyclopentadienyl radical, which combines with methyl radical to produce benzene. We also show that styrene is formed primarily by recombination of cyclopentadienyl and propargyl radicals, a pathway which to our knowledge, has not been included in prior flame simulations.

Infrared Spectrum of the Propargyl Peroxyl Radical, HC≡C—CH2OO X̃ 2A′′

When the propargyl radical, HCCCH(2), and O(2) are codeposited onto a cold argon matrix, a chemical reaction ensues; infrared absorption spectra reveal the formation of the propargyl peroxyl radical: HC=C-CH(2) X (2)B(1) + O(2) --> trans-HC=C-CH(2)OO X (2)A. We do not observe the isomeric adduct, CH(2)=C=CHOO X (2)A''. The propargyl radicals are produced by a hyperthermal nozzle while a second nozzle alternately fires bursts of O(2)/Ar at the 20 K matrix. The absorption spectra of the radicals are measured using a Fourier transform infrared spectrometer. We observe 13 of the 18 fundamental infrared bands of the propargyl peroxyl radical in an Ar matrix at 20 K. The experimental frequencies (cm(-1)) of trans-HC[triple bond]C-CH(2)OO X (2)A'' are assigned. The a' modes are nu(1) = 3326, nu(2) = 2960, nu(3) = 2148, nu(4) = 1440, nu(5) = 1338, nu(6) = 1127, nu(7) = 982, nu(8) = 928, nu(9) = 684, and nu(10) = 499 cm(-1), while the a'' modes are nu(14) = 1218, nu(15) = 972, and nu(16) = 637 cm(-1). Linear dichroism spectra were measured with photo-oriented HCCCH(2)OO radical samples to establish the experimental polarizations of several vibrational bands. The experimental frequencies (nu) for the propargyl peroxyl radical are compared to the anharmonic frequencies (upsilon) resulting from electronic structure calculations. We have used CBS-QB3 electronic structure calculations to estimate the peroxyl bond energies: DeltaH(298)(trans-HC[triple bond]CCH(2)-OO --> CH(2)CCH X (2)B(1) + O(2)) = 19 +/- 1 kcal mol(-1) and DeltaH(298)(trans-CH(2)=C=CH-OO --> CH(2)CCH X (2)B(1) + O(2)) = 21 +/- 1 kcal mol(-1). The experimental thermochemistry for C(3)H(3) reacting with oxygen has been reanalyzed as Delta(rxn)H(298)(HCCCH(2) + O(2) --> CH(2)=C=O + HCO) = -83 +/- 3 kcal mol(-1); Delta(rxn)H(298)(HCCCH(2) + O(2) --> CH(3)CO + CO) = -111 +/- 3 kcal mol(-1); Delta(rxn)H(298)(HCCCH(2) + O(2) --> CH(2)CHO + CO) = -106 +/- 4 kcal mol(-1); Delta(rxn)H(298)(HCCCH(2) + O(2) --> HCHO + HCCO) = -67 +/- 4 kcal mol(-1); Delta(rxn)H(298)(HCCCH(2) + O(2) --> CH(2)CH + CO(2)) = -105 +/- 3 kcal mol(-1).

The C7H5 Fulvenallenyl Radical as a Combustion Intermediate: Potential New Pathways to Two- and Three-Ring PAHs

This study proposes the existence of a new, highly resonantly stabilized free radical, fulvenallenyl (C7H5). Fulvenallenyl forms from dissociation or abstraction of the weak H atoms in the C7H6 compounds fulvenallene and 1-ethynylcyclopentadiene, which were recently identified as intermediates in the combustion of aromatic fuels and in sooting flames. The fulvenallenyl radical shares properties of the propargyl and cyclopentadienyl radicals, and like these species, we propose that fulvenallenyl has a significant resonance stabilization energy. This resonance energy is lost upon further reaction, making the fulvenallenyl radical resistant to oxidation and thermal decomposition. As with the resonantly stabilized radicals propargyl and cyclopentadienyl, fulvenallenyl is expected to react with other radicals in molecular weight growth reactions. Several pathways are proposed for self-reaction and cross-reactions of fulvenallenyl that directly produce two- and three-ring PAHs like naphthalene, diphenyl, and phenanthrene. Rate constants are calculated for H atom abstraction from fulvenallene and 1-ethynylcyclopentadiene by the common radicals H, OH, and CH3, to facilitate the inclusion of fulvenallenyl in kinetic models.

Cobalt-Complexed Propargyl Cations: Generation under Neutral Conditions and Spontaneous, High-Temperature Conversion to Propargyl Radicals

A novel method for the generation of Co2(CO)6-complexed propargyl cations under neutral conditions is developed. The optimized experimental protocol involves treatment of the respective Co2(CO)6-co...

Synthesis of 1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hepta-2,5-dienylmethyl haloacetates

Hexachlorobicyclo[2.2.1]hepta-2,5-dienylmethyl haloacetates were prepared by [4+2]-cycloaddition of hexachlorocyclopentadiene to the corresponding propargyl haloacetates.

Substituent effect on electron affinity, gas‐phase basicity, and structure of monosubstituted propargyl radicals and their anions: a theoretical study

AbstractThe substituent effect of electron‐withdrawing groups on electron affinity and gas‐phase basicity has been investigated for substituted propargyl radicals and their corresponding anions. It is shown that when a hydrogen of the α‐CH2 group or acetylenic CH in the propargyl system is substituted by an electron‐withdrawing substituent, electron affinity increases, whereas gas‐phase basicity decreases. The calculated electron affinities are 0.95 eV (CHCCH2•), 1.15 eV (CHCCHF•), 1.38 eV (CHCCHCl•), 1.48 eV (CHCCHBr•) for the isomers with terminal CH and 1.66 eV (CFCCH2•), 1.70 eV (CClCCH2•), 1.86 eV (CBrCCH2•) for the isomers with terminal CX at B3LYP level. The calculated gas‐phase basicities for their anions are 378.4 kcal/mol (CHCCH2:−), 371.6 kcal/mol (CHCCHF:−), 365.1 kcal/mol (CHCCHCl:−), 363.5 kcal/mol (CHCCHBr:−) for the isomers with terminal CH and 362.6 kcal/mol (CFCCH2:−), 360.4 kcal/mol (CClCCH2:−), 356.3 kcal/mol (CBrCCH2:−) for the isomers with terminal CX at B3LYP level. It is concluded that the larger the magnitude of electron‐withdrawing, the greater is the electron affinity of radical and the smaller is the gas‐phase basicity of its anion. This tendency of the electron affinities and gas‐phase bacisities is greater in isomers with the terminal CX than isomers with the terminal CH. Copyright © 2009 John Wiley & Sons, Ltd.

High-Accuracy Extrapolated ab Initio Thermochemistry of the Propargyl Radical and the Singlet C3H2 Carbenes

Total atomization energies and enthalpies of formation (0 and 298.15 K) are evaluated using the high-accuracy extrapolated ab initio thermochemistry (HEAT) scheme for the two stable singlet C(3)H(2) carbenes [cyclopropenylidene (c-C(3)H(2)) and propadienylidene (vinylidencarbene, l-C(3)H(2))], as well as for the 2-propynyl (propargyl, C(3)H(3)) radical. In the case of propargyl, the HEAT protocol predicts an enthalpy of formation of 354.9 +/- 1.0 kJ mol(-1) for 0 K; corresponding values of 498.1 +/- 1.0 and 555.6 +/- 1.0 kJ mol(-1) are estimated for c-C(3)H(2) and l-C(3)H(2). Additional consideration of temperature corrections leads to estimates of 352.2 +/- 1.0, 497.1 +/- 1.0, and 556.7 +/- 1.0 kJ mol(-1) for the heats of formation at 298.15 K of the propargyl radical, c-C(3)H(2), and l-C(3)H(2), respectively. Potential strategies for simplifying the HEAT protocol are also investigated and shown to have negligible impact on accuracy.

Thermal Decomposition of Furan Generates Propargyl Radicals

The thermal decomposition of furan has been studied by a 1 mm x 2 cm tubular silicon carbide reactor, C(4)H(4)O + Delta --> products. Unlike previous studies, these experiments are able to identify the initial furan decomposition products. Furan is entrained in either He or Ar carrier gas and is passed through a heated (1600 K) SiC tubular reactor. Furan decomposes during transit through the tubular reactor (approximately 65 micros) and exits to a vacuum chamber. Within one nozzle diameter of leaving the nozzle, the gases cool to less than 50 K, and all reactions cease. The resultant molecular beam is interrogated by photoionization mass spectroscopy as well as infrared spectroscopy. Earlier G2(MP2) electronic structure calculations predicted that furan will thermally decompose to acetylene, ketene, carbon monoxide, and propyne at lower temperatures. At higher temperatures, these calculations forecast that propargyl radical could result. We observe all of these species (see Scheme 1). As the pressure in the tubular reactor is raised, the photoionization mass spectra show clear evidence for the formation of aromatic hydrocarbons.

An experimental and theoretical study of toluene pyrolysis with tunable synchrotron VUV photoionization and molecular-beam mass spectrometry

An experimental study of toluene pyrolysis (1.24 vol.% toluene in argon) was performed at low pressure (1.33 kPa) in the temperature range of 1200–1800 K. The pyrolysis process was detected with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS). Species up to m/ z = 202 (C 16H 10), containing many radicals (CH 3, C 3H 3, C 5H 3, C 5H 5, C 7H 5, C 7H 7, C 9H 7, C 11H 7 and C 13H 9) and isomers, such as C 3H 4 (propyne and allene), C 4H 4 (vinylacetylene and 1,2,3-butatriene), C 5H 5 (cyclopentadienyl radical and pent-1-en-4-yn-3-yl radical), C 6H 4 (3-hexene-1,5-diyne and benzyne), C 6H 6 (benzene and fulvene), C 7H 8 (toluene and 5-methylene-1,3-cyclohexadiene) and so on, were identified from near-threshold measurements of photoionization mass spectra, and the mole fraction profiles of the pyrolysis products were evaluated from measurements of temperature scan. Experimental results indicate that the reaction C 7H 8 → C 7H 7 and the subsequent reactions are dominant at comparatively low temperature (<1440 K), while the reaction C 7H 8 → C 6H 5 and subsequent reactions gradually become competitive and important with increasing temperature. Furthermore the barriers of the decomposition pathways of toluene and benzyl radical determined by quantum mechanical calculation are in good agreement with the initial formation temperatures of the species. Based on the mole fractions and formation temperatures of the detected pyrolysis species, a simple reaction network is deduced. At relatively high temperatures, H-abstraction is prevalent and the mole fraction of C 2H 2 is so high that many aromatics are formed through the hydrogen-abstraction/C 2H 2-addition (HACA) mechanism. Moreover the reactions of benzyl with toluene/benzyl/phenyl/propargyl radicals to directly produce larger aromatics should play an influential role in PAH formation. Meanwhile the five-member-ring recombination mechanism also plays an indispensable role in the aromatics growth, as cyclopentadienyl radical (C 5H 5) was determined to be a major product of the decomposition of toluene.

Formation of H-atoms in the Pyrolysis of 1,3-butadiene and 2-butyne: A Shock Tube and Modelling Study

Abstract For investigating the pyrolysis of 1,3-butadiene (1,3-C4H6) and 2-butyne (2-C4H6), reactive gas mixtures highly diluted with argon as bath gas were prepared. The experiments were carried out in a high purity shock tube device over a temperature range of about 1500–1800 K at total pressures between 1.2 and 1.9 bar. The time-dependent formation of H-atoms was measured behind reflected shock waves by using the very sensitive method of atomic resonance absorption spectrometry (ARAS). A detailed chemical kinetic reaction mechanism consisting of 33 elementary reactions and 26 species was used to model the experimentally obtained H-atom profiles. From kinetic modelling, with help of sensitivity and reaction flux analysis, it was concluded that reaction R 1 2-C4H6 ↔ 2-C4H5 + H is crucial for the observed formation of H-atoms during the thermal decomposition of both investigated species and within the investigated range of temperatures and pressures. Moreover, at temperatures above about 1650 K, the decay of propargyl radicals (C3H3) turns out to contribute significantly to the amount of produced H-atoms. The following rate expressions were obtained for three reactions (R 1–R 3) – among them the isomerisation from 2-butyne to 1,3-butadiene – important with respect to the formation of H-atoms within the investigated parameter range. The uncertainties are estimated to be ±30%: 2-C4H6 ↔ 2-C4H5 + H (R 1) 2-C4H6 ↔ 1,2-C4H6 (R 2) 1,3-C4H6 ↔ C2H2 + C2H4 (R 3) k 1=3.8⋅1015exp(-44871K/T)s-1 k 2=6.9⋅1013exp(-32496K/T)s-1 k 3=7.0⋅1012exp(-33768K/T)s-1

A comparative study of the formation of aromatics in rich methane flames doped by unsaturated compounds

For a better modeling of the importance of the different channels leading to the first aromatic ring, we have compared the structures of laminar rich premixed methane flames doped with several unsaturated hydrocarbons: allene and propyne, because they are precursors of propargyl radicals which are well known as having an important role in forming benzene, 1,3-butadiene to put in evidence a possible production of benzene due to reactions of C 4 compounds, and, finally, cyclopentene which is a source of cyclopentadienyl methylene radicals which in turn are expected to easily isomerizes to give benzene. These flames have been stabilized on a burner at a pressure of 6.7 kPa (50 Torr) using argon as dilutant, for equivalence ratios ( ϕ) from 1.55 to 1.79. A unique mechanism, including the formation and decomposition of benzene and toluene, has been used to model the oxidation of allene, propyne, 1,3-butadiene and cyclopentene. The main reaction pathways of aromatics formation have been derived from reaction rate and sensitivity analyses and have been compared for the three types of additives. These combined analyses and comparisons can only been performed when a unique mechanism is available for all the studied additives.

Ultraviolet Photodissociation Dynamics of the Propargyl Radical

Ultraviolet (UV) photodissociation dynamics of jet-cooled propargyl (C(3)H(3)) radical is studied in the photolysis wavelength region of 230 to 250 nm with high-n Rydberg atom time-of-flight (HRTOF) and resonance enhanced multiphoton ionization (REMPI) techniques. In this wavelength region, the photofragment yield (PFY) spectra of the H + C(3)H(2) product channel are obtained by using propargyl chloride, allene, and propyne as precursors of the C(3)H(3) radicals, and they have a broad peak centered near 240 nm and are in good agreement with the previous UV absorption spectrum of C(3)H(3) by Fahr et al. The H + C(3)H(2) product translational energy distributions, P(E(T))'s, are obtained from all three precursors and are essentially the same. The P(E(T)) distributions peak at approximately 5 kcal/mol, and the fraction of average translational energy in the total excess energy, f(T), is approximately 0.3. The H-atom product angular distribution is isotropic, with the anisotropy parameter beta approximately 0. The dissociation mechanism is consistent with internal conversion of the electronically excited propargyl followed by unimolecular decomposition on the ground state. Our study supports the previously observed UV absorption spectrum of propargyl near 240 nm by Fahr et al. and is in general agreement with the results in the UV photodissociation of propargyl by the groups of Chen and Neumark, but disagrees with the recent theoretical calculations by Eisfeld.

Photodissociation of the propargyl and propynyl (C3D3) radicals at 248 and 193 nm

The photodissociation of perdeuterated propargyl (D(2)CCCD) and propynyl (D(3)CCC) radicals was investigated using fast beam photofragment translational spectroscopy. Radicals were produced from their respective anions by photodetachment at 540 and 450 nm (below and above the electron affinity of propynyl). The radicals were then photodissociated at 248 or 193 nm. The recoiling photofragments were detected in coincidence with a time- and position-sensitive detector. Three channels were observed: D(2) loss, CD+C(2)D(2), and CD(3)+C(2). Observation of the D loss channel was incompatible with this experiment and was not attempted. Our translational energy distributions for D(2) loss peaked at nonzero translational energy, consistent with ground state dissociation over small (<1 eV) exit barriers with respect to separated products. Translational energy distributions for the two heavy channels peaked near zero kinetic energy, indicating dissociation on the ground state in the absence of exit barriers.