Rate Coefficients For Formation Research Articles

The temperature variation of the CH+ + H reaction rate coefficients: a puzzle finally understood?

CH+ was the first molecular ion identified in the interstellar medium and is found to be ubiquitous in interstellar clouds. However, its formation and destruction paths are not well understood, especially at low temperatures. A new theoretical approach based on the canonical variational transition state theory was used to study the H + CH+ reactive collisions. Rate coefficients for formation of C+ ions are calculated as a function of temperature. We considered the participation of a direct path and an indirect path in which the reactants should overcome an entropic barrier to form a van der Waals complex or pass through a CH2+ intermediate complex, respectively. We show that the contribution of both pathways to the formation of C+ has to be taken into account. The new reactive rate coefficients for the title reaction, complemented by reactive data for CH+/CH2+ in the H/H2/He mixture, have been used to simulate the corresponding kinetics experimentally measured using an Atomic Beam 22 Pole Trap apparatus at low temperature. A good agreement with the experimental findings was found at 50 K. At a lower temperature, the model overestimates the formation of C+. This shows that secondary reactions are not responsible for the weak C+ production in the experiments at such temperature. Then, we discuss the possible impact of non-adiabatic effects in the study of the H + CH+ reactive collisions and we found that such effects can be responsible for the decrease of the H + CH+ rate coefficients at low temperature. This study offers an explanation for the disagreement between H + CH+ theoretical and experimental rate coefficients which has been going on for 20 years and highlights the need for performing non-adiabatic studies for this simple chemical reaction.

H i in Molecular Clouds: Irradiation by FUV Plus Cosmic Rays

We extend the analytic theory presented by Sternberg et al. and Bialy & Sternberg for the production of atomic hydrogen (H i) via far-ultraviolet (FUV) photodissociation at the boundaries of dense interstellar molecular (H2) clouds to also include the effects of penetrating (low-energy) cosmic rays (CRs) for the growth of the total H i column densities. We compute the steady-state abundances of the H i and H2 in one-dimensional gas slabs in which the FUV photodissociation rates are reduced by depth-dependent H2 self-shielding and dust absorption and the CR ionization rates are either constant or reduced by transport effects. The solutions for the H i and H2 density profiles and the integrated H i columns depend primarily on the ratios I UV/Rn and ζ/Rn, where I UV is the intensity of the photodissociating FUV field, ζ is the H2 CR ionization rate, n is the hydrogen gas density, and R is the dust surface H2 formation rate coefficient. We present computations for a wide range of FUV field strengths, CR ionization rates, and dust-to-gas ratios. We develop analytic expressions for the growth of the H i column densities. For Galactic giant molecular clouds (GMCs) with multiphased (warm/cold) H i envelopes, the interior CR zones will dominate the production of the H i only if s−1, where M GMC is the GMC mass, and including attenuation of the CR fluxes. For most Galactic GMCs and conditions, FUV photodissociation dominates over CR ionization for the production of the H i column densities. Furthermore, the CRs do not affect the H i-to-H2 transition points.

The weakening effect of the inhibition of CO2 on the explosion of HCNG with the increase of hydrogen: Experimental and chemical kinetic research

To explore the inhibitory effect of CO2 on HCNG (Hydrogen enriched compressed natural gas) explosion and its chemical kinetic mechanism, the explosion pressure and flame of CO2/HCNG/Air (hydrogen ratio: 0–0.6, CO2 addition: 3 vol%, 6 vol%, and 9 vol%) were tested in a 20 L spherical explosion tank at normal temperature and normal pressure. The experiment results shown that: as the addition of CO2 increases, the inhibition effect on HCNG explosion flame and pressure becomes more obvious. The larger the H2 ratio, the more difficult it is to inhibit HCNG explosion flame and pressure. To explore the chemical kinetics reasons for the above laws, the chemical kinetic parameters (such as free radical formation rate and temperature sensitivity coefficient) of the explosion reaction under the above conditions were calculated by CHEMKIN-PRO. The results indicate that CO2 mainly acts as a stable third body in the reaction system, thereby slowing down the growth of explosion chain reactions. In addition, as the main product of the CH4 oxidation reaction, CO2 has a certain weakening effect on the CH4 oxidation reaction. As the proportion of H2 increases, the elementary reactions such as 3# and 84# that generate ·H radicals are promoted, increasing the concentration of ·H radicals in the reaction system. Therefore, the dominant role of 38# on the temperature rise of the reaction system is enhanced, and the contribution of the CH4 oxidation reaction to the temperature rise of the reaction system is weakened. This is the chemical kinetic reason that CO2 has a weak inhibitory effect on the energy release of the HCNG explosion reaction when the proportion of H2 is large. The research results can provide a theoretical basis for the development of HCNG explosion prevention and control technology.

Atomic data on inelastic processes in boron–hydrogen collisions with accounting for fine structure

ABSTRACT This paper reports the results of investigations of inelastic processes in B + H and B+ + H− collisions with accounting for the fine structure. The calculations of the cross-sections and rate coefficients for excitation, de-excitation, ion-pair formation, and mutual neutralization processes (240 in total) are performed by using the quantum asymptotic method, which is modified in this paper for accounting of the fine structure, and by the multichannel approach within the framework of the Born–Oppenheimer formalism. It is found that accounting for the fine structure may change the rate coefficients significantly, and the rates calculated with accounting for the fine structure cannot be obtained by a simple redistribution of the non-relativistic rates. The processes important for non-local thermodynamic equilibrium modelling of stellar atmospheres are designated.

Rate coefficients of the aluminium monoxide formation by radiative association

ABSTRACT Radiative association may be a key pathway for the formation of aluminium monoxide (AlO) in diffuse interstellar clouds, especially for the oxygen-rich asymptotic giant branch (AGB) stars. In this work, we investigated the radiative association of AlO by the collision of the aluminium and oxygen atoms in their electronic ground states, which is thought to be the most probable radiative association process. First, the potential energy curves for 12 electronic states and the dipole moments between these states were calculated by the state-of-the-art ab initio methodology. Then, the cross-sections and rate coefficients for the radiative association of AlO were computed for the temperatures in the range of 10–10 000 K. The total rate coefficients are of the order of 2.73 × 10−19–1.14 × 10−16 cm3 s−1, and the A2Π → X2Σ+, 12Δ → A2Π, 12Π → X2Σ+, and 22Σ− → A2Π transitions play a key role. The calculated results can be used to investigate the chemical evolution of dust formation in the photospheres of the oxygen-rich AGB stars.

Comprehensive Chemistry of HeH+ in the Early Universe*

Abstract The recent detection of the helium hydride ion (HeH+) in the planetary nebula NGC 7027 has inspired studies revisiting the dominant processes for its formation and destruction. Because radiative association is the primary formation mechanism for the helium hydride ion at all redshifts, and many early universe chemistry models rely on accurate results for calculations of destruction rates, we explicitly computed rovibrationally distinguished partial cross sections and rate coefficients of helium hydride formation via radiative association, and modeled the abundance of HeH+ in the early universe using recently available rate coefficients for all relevant formation and destruction reactions. The rate coefficients for the radiative association formation reaction of HeH+ and radiative charge transfer in He+ + H collisions were obtained using two distinct methods. Our newly calculated rate coefficients along with other recently reported He chemistry reactions and updated cosmological parameters were incorporated into a new calculation of the abundance of HeH+ for redshifts between z = 10 and 4000. We find that the abundance of the helium hydride ion is at least three orders of magnitude larger than previous predictions for redshifts near z = 20. Rovibrationally distinguished radiative association rate coefficients are given as a function of temperature, and the significance of stimulated radiative association for redshifts z > 400 is also shown.

Temperature dependence of the rate coefficient of formation of CN radical from C + NH

Rate coefficients of the complex-forming C + NH→H + CN reaction have been calculated at temperatures ranging from 5 K to 800 K using quasi-classical trajectories on a potential energy surface which accurately describes the attractive long-range interaction, along with results using two capture models. In contrast with the constant value recommended in astrochemical databases, a steep decrease of the rates has been found up to 150 K, and then they tends to be nearly constant. Such behavior is analyzed in terms of the rovibrational state-selected rate coefficients and cross sections singling out the role played by the rotational excitation of the initial diatom. The effect of the electronic degeneracy is also discussed.

Experimental Study of the Formation of Organosulfates from α-Pinene Oxidation. 2. Time Evolution and Effect of Particle Acidity.

The present work is an extensive laboratory study of organosulfate (OS) formation from the reaction of α-pinene oxidation products or proxies with acidified ammonium sulfate aerosols in three different acidity conditions ((NH4)2SO4 0.06 M; (NH4)2SO4/H2SO4 0.06 M/0.005 M; (NH4)2SO4/H2SO4 0.03 M/0.05 M). The kinetics of the reactions of α-pinene, α-pinene oxide, isopinocampheol, pinanediol, and myrtenal with ammonium sulfate particles were studied using a quasi-static reactor. The reaction of α-pinene oxide with the highly acidic ammonium sulfate particles was determined to be 7, 10, 21, and 24 times faster than for isopinocampheol, α-pinene, pinanedial, and myrtenal, respectively, for an OS precursor concentration of 1 ppm and after 1 h reaction time. The effective rate coefficients for OS formation from α-pinene oxide were determined to be 2 orders of magnitude higher in highly acidic conditions than for the two other acidity conditions. For α-pinene oxide reactions with highly acidic ammonium sulfate particles, OS formation was observed to increase linearly with (i) the time of reaction up to 400 min (r2 > 0.95) and (ii) α-pinene oxide gas-phase concentration. However, OS formation from α-pinene oxide reactions with slightly acidic or pure ammonium sulfate particles was limited, with a plateau ([OS]max = 0.62 ± 0.03 μg) reached after around 15-20 min. Organosulfate dimers (m/z 401 and m/z 481) were detected not only with highly acidic particles but also with slightly acidic and pure ammonium sulfate particles, indicating that oligomerization processes do not require strong acidity conditions. Dehydration products of organosulfates (m/z 231 and m/z 383) were observed only under highly acidic conditions, indicating the key role of H2SO4 on the dehydration of organosulfates and the formation of olefins in the atmosphere. Finally, this kinetic study was completed with simulation chamber experiments in which the mass concentration of organosulfates was shown to depend on the available sulfate amount present in the particle phase (r2 = 0.96). In conclusion, this relative comparison between five organosulfate precursors shows that epoxide was the most efficient reactant to form organosulfates via heterogeneous gas-particle reactions and illustrates how gas-particle reactions may play an important role in OS formation and hence in the atmospheric fate of organic carbon. The kinetic data presented in this work provide strong support to organosulfate formation mechanisms proposed in part 1 ( J. Phys. Chem. A 2016 , 120 , 7909 - 7923 ).

Thermalized Epoxide Formation in the Atmosphere.

Epoxide formation was established a decade ago as a possible reaction pathway for β-hydroperoxy alkyl radicals in the atmosphere. This epoxide-forming pathway required excess energy to compete with O2 addition, as the thermal reaction rate coefficient is many orders of magnitude too slow. However, recently, a thermal epoxide-forming reaction was discovered in the ISOPOOH + OH oxidation pathway. Here, we computationally investigate the effect of substituents on the epoxide formation rate coefficient of a series of substituted β-hydroperoxy alkyl radicals. We find that the thermal reaction is likely to be competitive with O2 addition when the alkyl radical carbon has a OH group, which is able to form a hydrogen bond to a substituent on the other carbon atom in the epoxide ring being formed. Reactants fulfilling these requirements can be formed in the OH-initiated oxidation of many biogenic hydrocarbons. Further, we find that β-OOR alkyl radicals react similarly to β-OOH alkyl radicals, making epoxide formation a possible decomposition pathway in the oxidation of ROOR peroxides. GEOS-Chem modeling shows that the total annual production of isoprene dihydroxy hydroperoxy epoxide is 23 Tg, making it by far the most abundant C5-tetrafunctional species from isoprene oxidation.

Formation of CO+ by radiative association

ABSTRACT We theoretically estimate formation rate coefficients for CO+ through the radiative association of C+(2P) with O(3P). In 1989, Petuchowski et al. claimed radiative association to be the most important route for CO+ formation in SN 1987A. In 1990, Dalgarno, Du and You challenged this claim. Therefore, in this study, we improve previous estimates of the radiative association rate coefficients for forming CO+ from C+(2P) and O(3P). To do this, we perform quantum mechanically based perturbation theory calculations as well as semiclassical calculations, which are combined with Breit–Wigner theory in order to add the effect of shape resonances. We explicitly include four electronic transitions. The required potential energy and transition dipole-moment curves are obtained through large basis set multireference configuration interaction electronic structure calculations. We report cross-sections and from these we obtain rate coefficients in the range of 10 –10 000 K, finding that the CO+ formation rate coefficient is larger than the previous estimate by Dalgarno et al. Still our results support their claim that in SN 1987A, CO is mainly formed through radiative association and not through the charge transfer reaction CO+ + O → CO + O+ as earlier suggested by Petuchowski et al.

Catalytic consequences of the identity of surface reactive intermediates during direct hydrogen peroxide formation on Pd particles

H2O2 formation directly from H2 and O2 on Pd particles in water has proven to proceed via two different kinetic regimes defined by the operating H2/O2 ratios using kinetics, isotopic experiments and variation of particle sizes under conditions of kinetic control. These kinetic regimes exhibit different rate equations, kinetic isotope effects and structure sensitivity, while a transition among regimes occurs as the surface reactive intermediates changing from nearly uncovered to saturated with chemisorbed hydrogen (H*) with the increasing of H2/O2 ratio. On nearly bare Pd surfaces, irreversible H2 dissociation limits the rates of H2O2 formation and H2 conversion, thus, rates increase proportionally with H2 pressure but independent of O2 pressure. On H* saturated Pd surfaces, H2 dissociation approaches to chemical equilibrium and the H* mediated HOOH* formation step becomes the sole kinetically relevant step, therefore, rates become insensitive to H2 pressure but are linearly proportional to O2 pressure. The first-order rate coefficients for H2O2 formation become a single-valued function of the operating H2/O2 ratio.

Investigation on the influence of non-thermal plasma on reaction degree of wood gasification in a drop tube reactor

The influence of non-thermal plasma (NTP) on entrained-flow wood gasification at atmospheric pressure was investigated in experiments performed at a drop tube reactor with an inner diameter of 0.1 m. Three particle size categories covering diameters between 125 and 700 µm were used for testing. NTP and thermal gasification were compared applying similar experimental conditions in order to study the contribution of radicals, excited species, and charged particles to the gasification reaction. In case of NTP gasification about 1 kW of electric power was supplied to the preheated steam, whereas in case of thermal gasification the preheated steam was superheated applying about the same heat rate. Due to thermalization the temperature flow-down of the NTP zone approached the temperature after thermal superheating. The syngas-based carbon conversion degree was calculated from the ratio of the carbon content of the gaseous products (CO, CO2 and CH4) to that one of the original feedstock. Results show that NTP enhances syngas production for at least 20% and carbon conversion for 27% compared to thermal gasification. Kinetic rate coefficients for syngas formation as a function of temperature were obtained by fitting the syngas based carbon conversion degree assuming a first order reaction and temperature dependency described by the Arrhenius equation. For NTP gasification reaction rate coefficients increased by 33% as compared to thermal gasification were achieved.

Differentiation between O2 and NO2 impact on PtOx formation in a diesel oxidation catalyst

NO oxidation activity of platinum-based catalysts undergoes reversible changes during operation in oxidative atmosphere typical for Diesel engine exhaust gas due to platinum oxides (PtOx) formation and reduction. In recent years, these changes were studied mostly with the mixtures containing both O2 and NO2 so that it was difficult to quantify individual impacts of the oxidants on PtOx formation. In this paper, Pt/Al2O3 catalyst deactivation and PtOx formation with O2 and NO2 was examined separately by the means of transient isothermal experiments at 150, 175 and 200 °C with alternating deactivation and probe periods. The largest extent of deactivation was detected at 175 °C where the NO conversion in the isothermal experiment dropped from 85% (reduced catalyst) down to 10% (steady-state PtOx level) over 4 h. The experimental data were then utilized for parameter evaluation in global kinetic model of NO oxidation on a diesel oxidation catalyst extended by the reactions describing PtOx formation and reduction. Though the rate coefficient for PtOx formation with NO2 is higher than with O2, the overall PtOx formation rate and the steady-state PtOx level are higher with O2 at typical component concentrations (8% O2 or 250 ppm NO2). Similar activation energies were observed for both PtOx formation reactions. Validity of the model was verified also at lower O2 and NO2 concentrations (4% O2 or 125 ppm NO2). The developed model is thus capable of sufficiently precise prediction of catalyst deactivation at various O2 and NO2 concentrations relevant to diesel exhaust gas.

Photoinduced polycyclic aromatic hydrocarbon dehydrogenation

The physical and chemical conditions in photodissociation regions (PDRs) are largely determined by the influence of far ultraviolet radiation. Far-UV photons can efficiently dissociate molecular hydrogen, a process that must be balanced at the HI/H2 interface of the PDR. Given that reactions involving hydrogen atoms in the gas phase are highly inefficient under interstellar conditions, H2 formation models mostly rely on catalytic reactions on the surface of dust grains. Additionally, molecular hydrogen formation in polycyclic aromatic hydrocarbons (PAHs) through the Eley–Rideal mechanism has been considered as well, although it has been found to have low efficiency in PDR fronts. In a previous work, we have described the possibility of efficient H2 release from medium to large sized PAHs upon photodissociation, with the exact branching between H-/H2-loss reactions being molecule dependent. Here, we investigate the astrophysical relevance of this process, by using a model for the photofragmentation of PAHs under interstellar conditions. We focus on three PAHs cations (coronene, ovalene, and circumcoronene), which represent three possibilities in the branching of atomic and molecular hydrogen losses. We find that, for ovalene (H2-loss dominated) the rate coefficient for H2 formation reaches values of the same order as H2 formation in dust grains. This result suggests that this hitherto disregarded mechanism can account, at least partly, for the high level of molecular hydrogen formation in dense PDRs.

Quantitative Measurements of Chemiluminescence in a Laminar Methane–Air Premixed Flame and Comparison to Numerical Methods

Quantitative measurements of chemiluminescence emissions as a result of CH*, OH*, C2*, and CO2* were conducted in CH4/air premixed flames at different equivalence ratios (θ = 0.7–1.33), and numerical results based on one-dimensional flame simulations were compared to these data. A wavelength-dependent and optical-path-corrected emission calibration method was applied to quantify the emission of each excited species. The numerically simulated emission intensities of OH* and CH* are within the same order of magnitude as the experimental values for the same flame conditions. When self-absorption and cooling water thermosteresis are taken into account, the simulated OH* values are 1.0–1.4 times greater than the experimental results, while the CH* values are approximately 2.3–4.1 times greater. These findings suggest that more work is required to refine the temperature dependence factors for the CH* formation rate coefficients at high temperatures.

Experimental and theoretical studies of the reactions of ground-state sulfur atoms with hydrogen and deuterium.

The gas-phase kinetics of S(3P) atoms with H2 and D2 have been studied via the laser flash photolysis-resonance fluorescence technique. S atoms were generated by pulsed photolysis of CS2 at 193 nm and monitored by time-resolved fluorescence at 181 nm. The rate coefficients for H2 (k1) and D2 (k2), respectively, are summarized as k1(600-1110 K) = 3.0 × 10-9 exp-1.317×105-2.703×107K/T8.314 T/K cm3 molecule-1 s-1 and k2(770-1110 K) = 2.2 × 10-14 (T/298 K)3.55 exp(-5420 K/T) cm3 molecule-1 s-1. Error limits are discussed in the text. The rate coefficients for formation of SH(SD) + H(D) on a newly developed triplet potential energy surface were characterized via ring polymer molecular dynamics and canonical variational transition-state theory. There is excellent agreement above about 1000 K between theory and experiment. At lower temperatures, the experimental rate coefficient is substantially larger than the results computed for the adiabatic reaction, suggesting a significant role for intersystem crossing to the singlet potential energy surface at lower temperatures.

Pressure Dependence of Rate Coefficients for Formation and Dissociation of Pentachlorodisilane and Related Chemical Activation Reactions

Four entrance channels to the pentachlorodisilane (Si2HCl5) potential energy well, namely, SiCl2 + SiHCl3, SiCl4 + SiHCl, Cl3SiSiCl + HCl, and SiCl3 + SiHCl2, were analyzed in detail through transition state theory, Rice–Ramsperger–Kassel–Marcus (RRKM) theory, and solution of the multichannel master equation. The stationary points in the potential energy surface were optimized, and their vibrational frequencies and rotational constants calculated at the (U)B3LYP/6–31+G(d,p) level of theory; the (U)CCSD(T)/aug-cc-pVTZ level was then used for accurate estimation of activation energies. The pressure and temperature dependence of the rate coefficients of the channels related to Si2HCl5 stabilization/dissociation was determined along a wide range of conditions, for the first time. All channels showed strong pressure dependence in the four cases, at least at low-to-moderate pressure conditions. Each entrance channel leads to the formation of different products under different conditions, and the mechanism was analyzed in detail. The results indicated that at atmospheric pressure the reactions are in the falloff region, and therefore do not support the adoption of high-pressure limit rate coefficients in reaction models designed for simulation of systems at atmospheric or subatmospheric pressure conditions.

H i-to-H2 Transition Layers in the Star-forming Region W43

Abstract The process of atomic-to-molecular (H i-to-H2) gas conversion is fundamental for molecular-cloud formation and star formation. 21 cm observations of the star-forming region W43 revealed extremely high H i column densities, of 120–180 , a factor of 10–20 larger than predicted by H i-to-H2 transition theories. We analyze the observed H i with a theoretical model of the H i-to-H2 transition, and show that the discrepancy between theory and observation cannot be explained by the intense radiation in W43, nor be explained by variations of the assumed volume density or H2 formation rate coefficient. We show that the large observed H i columns are naturally explained by several (9–22) H i-to-H2 transition layers, superimposed along the sightlines of W43. We discuss other possible interpretations such as a non-steady-state scenario and inefficient dust absorption. The case of W43 suggests that H i thresholds reported in extragalactic observations are probably not associated with a single H i-to-H2 transition, but are rather a result of several transition layers (clouds) along the sightlines, beam-diluted with diffuse intercloud gas.

Theoretical study of resonances and recombination in low-energy collisions of three identical bosons

Resonances and three-body recombination in low-energy collisions of three identical bosons are studied theoretically using hyperspherical coordinates. Two different methods are used and compared calculating positions and widths of three-body resonances: the complex absorbing potential and the eigenchannel R-matrix approaches. Good agreement between results of the two approaches is found. Cross section and rate coefficient for dimer formation in three-body recombination are obtained.

State-to-state study of the D + H2(v = 0,j) collisions and its astrophysical implications

We report fully quantum time-independent calculations for the collisional excitation and dissociation of H2 by D, two astrophysically relevant processes. Our calculations are based on the H3 global potential energy surface (PES) by Mielke et al. We do observe an increase of the reactive HD formation rate coefficients with increasing rotational state of the reactant at low temperatures. At higher temperatures, D + H2(v = 0, j = 0–5) → HD + H rate coefficients tend to be similar whereas, for j > 5, the reactivity significantly increases. We also found that HD molecules are preferentially formed in excited rotational states irrespectively from the initial rotational state of H2. At low temperatures, the reactive process is negligible compared to the inelastic ones. The availability of rotationally resolved collisional data for chemical processes involving HD is fundamental for a reliable modelling of internal molecular state's kinetics. As a consequence, a proper and more realistic description of the HD formation and cooling mechanisms due to this molecular species is expected by the use of these new reaction rate coefficients, especially at low temperatures.