Kinetic study of the reaction CH(X 2Π)+H2⇄CH2(X 3B1)+H in the temperature range 372 to 675 K

Studies on the Kinetics of the CH + H2 Reaction and Implications for the Reverse Reaction, 3CH2 + H

The reaction of CH radicals with H₂ has been studied by the use of laser flash photolysis, probing CH decays under pseudo-first-order conditions using laser-induced fluorescence (LIF) over the temperature range 298-748 K at pressures of ∼5-100 Torr. Careful data analysis was required to separate the CH LIF signal at ∼428 nm from broad background fluorescence, and this interference increased with temperature. We believe that this interference may have been the source of anomalous pressure behavior reported previously in the literature (Brownsword, R. A.; J. Chem. Phys. 1997, 106, 7662-7677). The rate coefficient k₁ shows complex behavior: at low pressures, the main route for the CH₃* formed from the insertion of CH into H₂ is the formation of ³CH₂ + H, and as the pressure is increased, CH₃* is increasingly stabilized to CH₃. The kinetic data on CH + H₂ have been combined with experimental shock tube data on methyl decomposition and literature thermochemistry within a master equation program to precisely determine the rate coefficient of the reverse reaction, ³CH₂ + H → CH + H₂. The resulting parametrization is k_CH2+H(T) = (1.69 ± 0.11) × 10^-10 × (T/298 K)^{(0.05±0.010)} cm³ molecule^-1 s^-1, where the errors are 1σ.

Treatment of pyridine in industrial liquid waste by atmospheric DC water plasma

Pyridine is a basic heterocyclic compound with high toxicity, widely found in liquid waste from industrial processes. The treatment of highly-concentrated pyridine was demonstrated using a novel mist-type water thermal plasma torch. Decomposition rate and TOC removal rate were more than 94% in all conditions, while the max energy efficiency reached about 23 g/kWh. With a high temperature of 5500-7500 K, more than 95% of carbon content in pyridine was converted into valuable gas products, while a little amount of formic acid and acetic acid were observed as liquid by-products. The production of hydrogen cyanide (HCN) during the thermal decomposition of pyridine was observed, which can be inhibited by increasing the input power. Based on the experimental results, detailed decomposition mechanisms in the high-temperature and the downstream region were discussed respectively. Water plasma shows significant potential in the treatment of non-biodegradable industrial liquid waste.

The CH(X2Π) + H2O reaction: two transition state kinetics

The reaction of ground state methylidyne (CH) with water vapor (H2O) is theoretically re-investigated using high-level coupled cluster computations in combination with semi-classical transition state theory (SCTST) and two-dimensional master equation simulations. Insertion of CH into a H-O bond of H2O over a submerged barrier via a well-skipping mechanism yielding solely H and CH2O is characterized. The reaction kinetics is effectively determined by the formation of a pre-reaction van der Waals complex (PRC, HC-OH2) and its subsequent isomerization to activated CH2OH in competition with PRC re-dissociation. The tunneling effects are found to be minor, while variational effects in the PRC → CH2OH step are negligible. The calculated rate coefficient k(T) is nearly pressure-independent, but strongly depends on temperature with pronounced down-up behavior: a high value of 2 × 10-10 cm3 s-1 at 50 K, followed by a fairly steep decrease down to 8 × 10-12 cm3 s-1 at 900 K, but increasing again to 5 × 10-11 cm3 s-1 at 3500 K. Over the T-range of this work, k(T) can be expressed as: k(T, P = 0) = 2.31 × 10-11 (T/300 K)-1.615 exp(-38.45/T) cm3 s-1 for T = 50-400 K k(T, P = 0) = 1.15 × 10-12 (T/300 K)0.8637 exp(892.6/T) cm3 s-1 for T = 400-1000 K k(T, P = 0) = 4.57 × 10-15 (T/300 K)3.375 exp(3477.4/T) cm3 s-1 for T = 1000-3500 K.

Optical and Mass Spectrometric Measurements of the CH4-CO2 Dry Reforming Process in a Low Pressure, Very High Density, and Purely Inductive Plasma

This paper presents a study of a CH₄-CO₂ plasma-reforming process carried out in a high power density (5-50 W/cm³), using toroidal transformer-coupled plasma, and operated at low pressure (0.2-0.7 Torr). Using the intermediate between a thermal and nonthermal plasma (electron density, n_e ≈ 3 × 10¹² cm^-3 and a maximum gas temperature of ∼4000-6000 K along the center line), the low-pressure study provides a unique set of conditions to investigate reaction mechanisms, where three-body reactions can be ignored. Reactive species in the plasma were identified by optical emission spectroscopy. End products of the reforming process were measured by mass spectrometry. Quite high conversions of CO₂ and CH₄ were found (90%), with selectivities for CO and H₂ of 80% at 300 sccm feed gas flow rate in a 0.5 Torr plasma, with a mole ratio CO₂-CH₄ of 1:1. A detailed reaction mechanism is presented, taking into account the combined detection of reactive intermediates in the plasma (H, O, CH, and C₂) and stable product downstream.

Theoretical investigation of rotationally inelastic collisions of CH(X2Π) with molecular hydrogen

We report calculations of state-to-state cross sections for collision-induced rotational transitions of CH(X²Π) with molecular hydrogen. These calculations employed the diabatic matrix elements of the interaction potential determined by Dagdigian [J. Chem. Phys. 145, 114301 (2016)], which employed the multi-reference configuration-interaction method [MRCISD+Q(Davidson)]. Because of the presence of a deep well on the lower potential energy surface, the scattering calculations were carried out using the quantum statistical method of Manolopoulos and co-workers [Chem. Phys. Lett. 343, 356 (2001)]. The computed cross sections included contributions from direct scattering, as well as from the formation and decay of a collision complex. The magnitude of latter contribution was found to decrease significantly with increasing collision energy. Rotationally energy transfer rate constants were computed for this system since these are required for astrochemical modeling.

An Ab Initio Theoretical Study of the CH + H2 ⇌ Ch3 * ⇌ Ch2 + H Reactions

Ab initio calculations of the electronic wave function and associated potential energy of CH 3 at geometries appro priate for the title reaction described here allow charac terization of the reactants and the energetically most fa vorable routes that are followed during the reaction. The quantum mechanical description of the electronic mo tion at each molecular geometry involves the response of each electron to the average field of all other elec trons and the approximate correlated response of each electron to the instantaneous position of the other elec trons. The basic methodology for the computation of this wave function, using extensive orbital basis sets and large-scale configuration expansions, is described. This calculation is one of the largest ever attempted for the characterization of a polyatomic reaction path. However, the path description involves a fine balance of energy contributions that requires this level of sophistication. The calculated properties of the reactants, the interme diate CH3, and the reaction paths to form CH3 are pre sented. The computed energetics compare favorably to experimental results.

Capture and dissociation in the complex-forming CH(v = 0,1) + D2 → CHD + D, CD2 + H, CD + HD reactions and comparison with CH(v = 0,1) + H2

Rate coefficients for the CH(v = 0,1) + D(2) reaction have been determined for all possible channels (T: 200-1200 K), using the quasiclassical trajectory method and a suitable treatment of the zero point energy. Calculations have also been performed on the CH(v = 1) + H(2) reaction and the CH(v = 1) + D(2) → CH(v = 0) + D(2) process. Most of the results can be understood considering the key role played by the deep minimum of the potential energy surface (PES), the barrierless character of the PES, the energy of the reaction channels, and the kinematics. The good agreement found between theory and experiment for the rate coefficients of the capture process of CH(v = 0) + D(2), the total reactivity of CH(v = 1) + D(2), H(2), as well as the good agreement observed for the related CH(v = 0) + H(2) system (capture and abstraction), gives confidence on the theoretical rate coefficients obtained for the capture processes of CH(v = 1) + D(2), H(2), the individual reactive processes of CH(v = 1) + D(2), H(2), the abstraction and abstraction-exchange reactions for CH(v = 0) + D(2), and the inelastic process mentioned above, for which there are no experimental data available, and that can be useful in combustion chemistry and astrochemistry.

Using biogenic sulfur gases as remotely detectable biosignatures on anoxic planets

We used one-dimensional photochemical and radiative transfer models to study the potential of organic sulfur compounds (CS(2), OCS, CH(3)SH, CH(3)SCH(3), and CH(3)S(2)CH(3)) to act as remotely detectable biosignatures in anoxic exoplanetary atmospheres. Concentrations of organic sulfur gases were predicted for various biogenic sulfur fluxes into anoxic atmospheres and were found to increase with decreasing UV fluxes. Dimethyl sulfide (CH(3)SCH(3), or DMS) and dimethyl disulfide (CH(3)S(2)CH(3), or DMDS) concentrations could increase to remotely detectable levels, but only in cases of extremely low UV fluxes, which may occur in the habitable zone of an inactive M dwarf. The most detectable feature of organic sulfur gases is an indirect one that results from an increase in ethane (C(2)H(6)) over that which would be predicted based on the planet's methane (CH(4)) concentration. Thus, a characterization mission could detect these organic sulfur gases-and therefore the life that produces them-if it could sufficiently quantify the ethane and methane in the exoplanet's atmosphere.

Capture and dissociation in the complex-forming CH + H2 → CH2 + H, CH + H2 reactions

The rate coefficients for the capture process CH + H(2)→ CH(3) and the reactions CH + H(2)→ CH(2) + H (abstraction), CH + H(2) (exchange) have been calculated in the 200-800 K temperature range, using the quasiclassical trajectory (QCT) method and the most recent global potential energy surface. The reactions, which are of interest in combustion and in astrochemistry, proceed via the formation of long-lived CH(3) collision complexes, and the three H atoms become equivalent. QCT rate coefficients for capture are in quite good agreement with experiments. However, an important zero point energy (ZPE) leakage problem occurs in the QCT calculations for the abstraction, exchange and inelastic exit channels. To account for this issue, a pragmatic but accurate approach has been applied, leading to a good agreement with experimental abstraction rate coefficients. Exchange rate coefficients have also been calculated using this approach. Finally, calculations employing QCT capture/phase space theory (PST) models have been carried out, leading to similar values for the abstraction rate coefficients as the QCT and previous quantum mechanical capture/PST methods. This suggests that QCT capture/PST models are a good alternative to the QCT method for this and similar systems.