Kinetics of C2H radical reactions with ethene, propene and 1-butene measured in a pulsed Laval nozzle apparatus at T=103 and 296 K

Ethynyl Radical Hydrogen Abstraction Energetics and Kinetics Utilizing High-Level Theory

The ethynyl radical, C2H, is found in a variety of different environments ranging from interstellar space and planetary atmospheres to playing an important role in the combustion of various alkynes under fuel-rich conditions. Hydrogen-atom abstraction reactions are common for the ethynyl radical in these contrasting environments. In this study, the C2H + HX → C2H2 + X, where HX = HNCO, trans-HONO, cis-HONO, C2H4, and CH3OH, reactions have been investigated at rigorously high levels of theory, including CCSD(T)-F12a/cc-pVTZ-F12. For the stationary points thus located, much higher levels of theory have been used, with basis sets as large as aug-cc-pV5Z and methods up to CCSDT(Q), and core correlation was also included. These molecules were chosen because they can be found in either interstellar or combustion environments. Various additive energy corrections have been included to converge the relative enthalpies of the stationary points to subchemical accuracy (≤0.5 kcal mol-1). Barriers predicted here (2.19 kcal mol-1 for the HNCO reaction and 0.47 kcal mol-1 for C2H4) are significantly lower than previous predictions. Reliable kinetics were acquired over a wide range of temperatures (50-5000 K), which may be useful for future experimental studies of these reactions.

Nucleation of Carbonaceous Nanoparticles in a Low Pressure Ar/C2H2 Plasma with C2H4 Impurity

Computer simulations of one-dimensional multi-fluid equations are used to study the effects of C2H4 addition on the plasma chemistry and formation of nanoparticles in a capacitively coupled Ar/C2H2 plasma. The results indicate that the most important effect is an enhanced production of hydrogen-rich species, especially C4H4. Regarding the effect of C2H4 inlet on different routes of nanoparticles nucleation, it was found that the nucleation via growth of neutral molecules is significantly suppressed, while the radicals and anions remain almost unaffected. As C2H4 participates in many cationic reactions, the concentrations of hydrogen-rich cations are largely enhanced, whereas those of carbon-rich cations are reduced. The numerical results also suggest that the main mechanism behind the production of C6H6 is the insertion of the C2H3 radical into C4H4. The concentrations of the precursors species of C6H6 are increased with an increase in the pressure but are reduced with the applied RF voltage.

Halide–propene complexes: validated DSD-PBEP86-D3BJ calculations and photoelectron spectroscopy

Anion photoelectron spectroscopy has been used to determine the electron binding energies of the X−⋯C3H6 (X = Cl, Br, I) complexes.

Mechanisms of the Ethynyl Radical Reaction with Molecular Oxygen

The ethynyl radical, ^•C₂H, is a key intermediate in the combustion of various alkynes. Once produced, the ethynyl radical will rapidly react with molecular oxygen to produce a variety of products. This research presents the first comprehensive high level theoretical study of the reaction of the ^•C₂H (²Σ⁺) radical with molecular oxygen (³Σ_g^-). Correlation methods as complete as CCSDT(Q) were used; basis sets as large as cc-pV6Z were adopted. Focal point analysis was employed to approach relative energies within the bounds of chemical accuracy (≤1 kcal mol^-1). Two dominate reaction pathways from the ethynyl peroxy radical include oxygen-oxygen cleavage from the ethynyl peroxy radical that is initially formed to produce HCCO (²A″) and O (³P) and an isomerization of the ethynyl peroxy radical to eventually yield HCO (²A') and CO (¹Σ⁺). The branching ratio between these two competitive reaction pathways was determined to be 1:1 at 298 K. Minor reaction pathways leading to the production of CO₂ (¹Σ_g⁺) and CH (²Π, ⁴Σ^-, ²Δ) were also characterized. The absence of CCO (³Σ^-) and OH (²Π) was explained in terms competition with more accessible reaction pathways.

Communication: On the first ionization threshold of the C2H radical

The slow photoelectron spectrum of the ethynyl radical has been recorded for the first time by using the DESIRS beamline of the SOLEIL synchrotron facility. Ethynyl was generated using a microwave discharge flow tube. The observation of the X⁺Π3←XΣ+2 transition allowed the first direct measurement of the adiabatic ionization threshold of this radical (E_I = 11.641(5) eV). The experimental results are supported by ab initio calculations. Our preliminary investigation of the cationic ground state potential energy surfaces predicts a non-negligible Renner-Teller effect which has not been discussed previously.

Abstraction and addition kinetics of C2H radicals with CH4, C2H6, C3H8, C2H4, and C3H6: CVT/SCT/ISPE and hybrid meta-DFT methods

Rate coefficients for the reactions of C₂H radicals with methane (k₁), ethane (k₂), propane (k₃), ethylene (k₄), and propylene (k₅) were computed using canonical variational transition state theory (CVT) coupled with hybrid-meta density functional theory (DFT) over a wide range of temperatures from 150 to 5000 K.

The role of isovalency in the reactions of the cyano (CN), boron monoxide (BO), silicon nitride (SiN), and ethynyl (C2H) radicals with unsaturated hydrocarbons acetylene (C2H2) and ethylene (C2H4)

The classification of chemical reactions based on shared characteristics is at the heart of the chemical sciences, and is well exemplified by Langmuir's concept of isovalency, in which ‘two molecular entities with the same number of valence electrons have similar chemistries’.

Bimolecular rate constant and product branching ratio measurements for the reaction of C2H with ethene and propene at 79 K

The reactions of the ethynyl radical (C(2)H) with ethene (C(2)H(4)) and propene (C(3)H(6)) are studied under low temperature conditions (79 K) in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by 193 nm photolysis of acetylene (C(2)H(2)) and the reactions are studied in nitrogen as a carrier gas. Reaction products are sampled and subsequently photoionized by the tunable vacuum ultraviolet radiation of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. The product ions are detected mass selectively and time-resolved by a quadrupole mass spectrometer. Bimolecular rate coefficients are determined under pseudo-first-order conditions, yielding values in good agreement with previous measurements. Photoionization spectra are measured by scanning the ALS photon energy while detecting the ionized reaction products. Analysis of the photoionization spectra yields-for the first time-low temperature isomer resolved product branching ratios. The reaction between C(2)H and ethene is found to proceed by H-loss and yields 100% vinylacetylene. The reaction between C(2)H and propene results in (85 ± 10)% C(4)H(4) (m/z = 52) via CH(3)-loss and (15 ± 10)% C(5)H(6) (m/z = 66) by H-loss. The C(4)H(4) channel is found to consist of 100% vinylacetylene. For the C(5)H(6) channel, analysis of the photoionization spectrum reveals that (62 ± 16)% is in the form of 4-penten-1-yne, (27 ± 8)% is in the form of cis- and trans-3-penten-1-yne and (11 ± 10)% is in the form of 2-methyl-1-buten-3-yne.

Airfoil sampling of a pulsed Laval beam with tunable vacuum ultraviolet synchrotron ionization quadrupole mass spectrometry: application to low-temperature kinetics and product detection

A new pulsed Laval nozzle apparatus with vacuum ultraviolet (VUV) synchrotron photoionization quadrupole mass spectrometry is constructed to study low-temperature radical-neutral chemical reactions of importance for modeling the atmosphere of Titan and the outer planets. A design for the sampling geometry of a pulsed Laval nozzle expansion has been developed that operates successfully for the determination of rate coefficients by time-resolved mass spectrometry. The new concept employs airfoil sampling of the collimated expansion with excellent sampling throughput. Time-resolved profiles of the high Mach number gas flow obtained by photoionization signals show that perturbation of the collimated expansion by the airfoil is negligible. The reaction of C(2)H with C(2)H(2) is studied at 70 K as a proof-of-principle result for both low-temperature rate coefficient measurements and product identification based on the photoionization spectrum of the reaction product versus VUV photon energy. This approach can be used to provide new insights into reaction mechanisms occurring at kinetic rates close to the collision-determined limit.

Elementary reactions and their role in gas-phase prebiotic chemistry

The formation of complex organic molecules in a reactor filled with gaseous mixtures possibly reproducing the primitive terrestrial atmosphere and ocean demonstrated more than 50 years ago that inorganic synthesis of prebiotic molecules is possible, provided that some form of energy is provided to the system. After that groundbreaking experiment, gas-phase prebiotic molecules have been observed in a wide variety of extraterrestrial objects (including interstellar clouds, comets and planetary atmospheres) where the physical conditions vary widely. A thorough characterization of the chemical evolution of those objects relies on a multi-disciplinary approach: 1) observations allow us to identify the molecules and their number densities as they are nowadays; 2) the chemistry which lies behind their formation starting from atoms and simple molecules is accounted for by complex reaction networks; 3) for a realistic modeling of such networks, a number of experimental parameters are needed and, therefore, the relevant molecular processes should be fully characterized in laboratory experiments. A survey of the available literature reveals, however, that much information is still lacking if it is true that only a small percentage of the elementary reactions considered in the models have been characterized in laboratory experiments. New experimental approaches to characterize the relevant elementary reactions in laboratory are presented and the implications of the results are discussed.

A crossed molecular beams study of the reaction of the ethynyl radical (C2H(X2Sigma+)) with allene (H2CCCH2(X1A1))

The crossed beams reaction of ground state ethynyl radicals, C(2)H(X(2)Sigma(+)), with allene, H(2)CCCH(2)(X(1)A(1)), was conducted under single collision conditions at a collision energy of 22.0 +/- 0.4 kJ mol(-1). The center-of-mass functions were combined with earlier ab initio calculations and revealed that the reaction was barrier-less, proceeded via indirect reaction dynamics through an addition of the ethynyl radical to the terminal carbon atom of the allene molecule, and was terminated by atomic hydrogen emission via a tight exit transition state to form the ethynylallene product. The overall reaction was found to be exoergic by 93 +/- 15 kJ mol(-1). Since the reaction is barrier-less, exoergic, and all transition states involved are located below the energy level of the separated reactants, the formation of ethynylallene is predicted to take place in low temperature atmospheres of planets and their satellites such as Titan and also in cold molecular clouds via the neutral-neutral reaction of ethynyl radicals with allene. Implications to interstellar chemistry and a comparison with the chemistry of the isoelectronic cyano radical, CN(X(2)Sigma(+)), are also presented.

A study of the hydrogen abstraction reactions of C2H radical with CH3CN, C2H5CN, and C3H7CN by dual-level generalized transition state theory

The hydrogen abstraction reactions C2H + CH3CN --> products (R1), C2H + CH3CH2CN --> products (R2), and C2H + CH3CH2CH2CN --> products (R3) have been investigated by dual-level generalized transition state theory. Optimized geometries and frequencies of all the stationary points and extra points along the minimum-energy path (MEP) are performed at the BH&H-LYP and MP2 methods with the 6-311G(d, p) basis set, and the energy profiles are further refined at the MC-QCISD level of theory. The rate constants are evaluated using canonical variational transition state theory (CVT) with a small-curvature tunneling correction (SCT) over a wide temperature range 104-2000 K. The calculated CVT/SCT rate constants are in good agreement with the available experimental values. Our calculations show that for reaction R2, the alpha-hydrogen abstraction channel and beta-hydrogen abstraction channel are competitive over the whole temperature range. For reaction R3, the gamma-hydrogen abstraction channel is preferred at lower temperatures, while the contribution of beta-hydrogen abstraction will become more significant with a temperature increase. The branching ratio to the alpha-hydrogen abstraction channel is found negligible over the whole temperature range.

Low temperature NH(X 3sigma-) radical reactions with NO, saturated, and unsaturated hydrocarbons studied in a pulsed supersonic laval nozzle flow reactor between 53 and 188 K

The reactions of ground-state imidogen radicals (NH(X 3sigma-)) with NO and select saturated and unsaturated hydrocarbons have been measured in a pulsed supersonic expansion Laval nozzle flow reactor in the temperature range 53-188 K. The rate coefficients for the NH + NO system display negative temperature dependence in the temperature regime currently investigated and a global temperature-dependent fit is best represented in a modified power law functional form, with k1(NH + NO) = (4.11 +/- 0.31) x 10(-11) x (T/300)(-0.30+/-0.17) x exp(77+/-21/T) cm3/s. The reactions of NH with ethylene, acetylene, propene, and diacetylene were measured over the temperature range 53-135 K. In addition, the reactions of NH with methane and ethane were also measured at 53 K, for reasons discussed later. The temperature dependence of the reactions of NH with the unsaturated hydrocarbons are fit using power law expressions, k(T) = A(T/300)(-n), and are as follows: k4 = (2.3 +/- 1.2) x 10(-12) x (T/300)(-1.09+/-0.33) cm3/s, k5 = (4.5 +/- 0.3) x 10(-12) x (T/300)(-1.07+/-0.04) cm3/s, k6 = (5.6 +/- 1.9) x 10(-12) x (T/300)(-1.23+/-0.21) cm3/s, and k7 = (7.4 +/- 1.8) x 10(-12) x (T/300)(-1.23+/-0.15) cm3/s for ethylene, acetylene, propene, and diacetylene, respectively. The rate for NH + ethane at 53 K is measured to be k3 = (6.8 +/- 1.7) x 10(-12) cm3/s, while that for methane at the same temperature represents an upper bound of k2 < or = (1.1 +/- 4.3) x 10(-12) cm3/s, as this is at the limits of measurement with our current technique. The behavior of these systems throughout the temperature range explored indicates that these reactions occur over a potential energy surface without an appreciable barrier through a complex formation mechanism. Implications for chemistry in low temperature environments where these species are found are briefly discussed.

Kinetic parameters for gas-phase reactions: experimental and theoretical challenges

This article aims to illustrate the added value provided to experimental kinetics investigations by complementary theoretical kinetics studies, using as examples (i) reactions of two major hydrocarbon flame radicals, HCCO and C(2)H, and (ii) reactions of several oxygenated organic compounds with hydroxyl radicals of interest to atmospheric chemistry. The first part, on HCCO and C(2)H kinetics, does not attempt to give an extensive literature review, but rather addresses some major experimental techniques, mainly specific ones, that have allowed a great part of the available reactivity databases on these two species to be established. For several key reactions, it is shown how potential energy surfaces and statistical rate predictions based thereon have provided insight into the molecular mechanisms and have allowed estimates of product distributions as well as reliable extrapolations of experimental rate coefficients and branching ratios to higher temperatures. The second part addresses current issues in atmospheric chemistry relating mainly to hydroxyl radical reactions with oxygenated organics, and focuses on the experimental characterization of the often unusual temperature dependence of their rate coefficients and on the theoretical rationalization thereof, through the formation of hydrogen-bonded pre-reactive complexes and resulting tunnelling-enhanced H-abstraction. Finally, the development of general structure-activity relationships for OH reactions with organics, H-abstractions as well as OH-additions for unsaturated compounds, is briefly discussed.

Kinetics in Cold Laval Nozzle Expansions: From Atmospheric Chemistry to Oxidation of Biomolecules in the Gas Phase

New developments and recent applications of pulsed and miniaturised Laval nozzle technology allowing many gas-phase molecular processes to be studied at very low temperatures are highlighted. In the present Minireview we focus on molecular energy transfer and reactions of molecular radicals (e.g. OH) with neutral molecules. We show that with the combination of pulsed laser photolysis and sensitive laser-induced fluorescence detection a large number of fast reactions of radicals with more or less complex neutral molecules can be measured in Laval nozzle expansions nowadays. It is also demonstrated that collisional energy transfer of neutral molecules can be measured via kinetically controlled selective fluorescence (KCSF) excitation down to 58 Kelvin. Finally, we show that even the primary steps in the oxidation of biomolecules or biomolecular building blocks initiated by OH radicals can be followed at low temperatures. The temperature dependence of the measured rate constants is the key for an understanding of the underlying molecular mechanisms and the Laval nozzle expansion provides a unique environment for these measurements. The experimental finding that many reactions between radicals and neutral species can be rapid at low temperatures are discussed in terms of pre-reactive complexes formed in the overall complex forming bimolecular reactions.