CO H2 Reaction Research Articles

Dual oxidant and reactant interactions in NEPE pyrolysis: Experimental and kinetic modelling insights

The reaction of dual oxidants (NO2 and O2) and dual reactants (NH3 and CH2O) obtained from the pyrolysis of NEPE was investigated both experimentally and numerically. Experiments were performed in JSR at atmospheric pressure, at low and medium temperatures of 600–1000 K, with equivalence ratios of 0.67–1.33, and the species concentrations of reactants, intermediates, and products were measured. A detailed kinetic model was then developed, which can capture the experimental results. The kinetic analysis revealed that NO2 was the main oxidant at low temperatures, as the temperature increased, O2 became more competitive, reducing the flux of the NO2 consumption pathway. Additionally, NO shows a non-monotonic trend with increasing temperature. On the one hand, this is due to the competition between O2 and NO2, which leads to a decrease in the flux of NOx formation. On the one hand, at higher temperature, HCO tended to decompose to produce H, which converted NO2 to NO by the reaction NO2 + H = NO + OH, increasing the fluxes of NH2 and NO and leading to an increase in DeNOx pathway flux. Furthermore, the O2 concentration directly affected the activity of the reaction system. This is because O2 accelerates the reaction NO + HO2 = NO2 + OH through HO2 generated by the reaction HCO + O2 = CO + HO2, and this pathway not only replenishes NO2 but also provides a large amount of OH.

Investigation on laminar burning velocity measurements of premixed Ethane-Air mixture at higher pressure and temperature conditions

The externally heated diverging channel (EHDC) method was utilized to evaluate the laminar burning velocity (LBV) of premixed ethane-air flames at higher mixture temperature (350-620 K) and pressure (1-5 atm) conditions over different mixture strengths (ϕ = 0.7-1.3). The maximum LBV was noted at ϕ = 1.1 for all pressure and temperature conditions. The inverted parabolic behavior of the temperature exponent was obtained with the minima at ϕ = 1.1. The pressure exponent (β) exhibits a parabolic variation with its maxima at ϕ = 1.0. The current results are further analyzed and compared with the literature measurements, and the comprehensive kinetic model predictions of USC mech II, San Diego mech, and Aramco mech 1.3. The present LBV measurements match well with the kinetic model predictions of Aramco mech 1.3 at most of the mixture and pressure conditions. The present results propose the variation of pressure exponent (β) as a function of temperature ratio, and temperature exponent (α) as a function of pressure ratio for different mixture conditions (ϕ). A revised power-law correlation for α and β variations is also recommended as, Su=Su0Tu/Tu0∝0+∝11-Pu/Pu0)Pu/Pu0β0+β11-Tu/Tu0. The sensitivity analysis indicates the maximum decrement in the positive sensitivity for the chain branching reaction HCO + M ↔ H + CO + M (R30) across all the stated mixture conditions, owing to an increment in the third body effects along with pressure and temperature. The reaction pathway analysis reveals a maximum net reduction in the elemental flux (∼56 %) for the reaction between the species carbon monoxide (CO) and carbon dioxide (CO2) through different reactions, due to a rise in pressure and temperature.

Identification of the simplest sugar-like molecule glycolaldehyde towards the hot molecular core G358.93–0.03 MM1

ABSTRACT Glycolaldehyde (CH2OHCHO) is the simplest monosaccharide sugar in the interstellar medium, and it is directly involved in the origin of life via the ‘RNA world’ hypothesis. We present the first detection of CH2OHCHO towards the hot molecular core G358.93–0.03 MM1 using the Atacama Large Millimetre/Submillimetre Array (ALMA). The calculated column density of CH2OHCHO towards G358.93–0.03 MM1 is (1.52 ± 0.9) × 1016 cm−2 with an excitation temperature of 300 ± 68.5 K. The derived fractional abundance of CH2OHCHO with respect to H2 is (4.90 ± 2.92) × 10−9, which is consistent with that estimated by existing two-phase warm-up chemical models. We discuss the possible formation pathways of CH2OHCHO within the context of hot molecular cores and hot corinos and find that CH2OHCHO is likely formed via the reactions of radical HCO and radical CH2OH on the grain-surface of G358.93–0.03 MM1.

The role of dimers in complex forming reactions at low temperature: full dimension potential and dynamics of (H2CO)2 + OH reaction.

The (H2CO)2 + OH and H2CO-OH + H2CO reaction dynamics are studied theoretically for temperatures below 300 K. For this purpose, a full dimension potential energy surface is built, which reproduces well accurate ab initio calculations. The potential presents a submerged reaction barrier, as an example of the catalytic effect induced by the presence of the third molecule. However, quasi-classical and ring polymer molecular dynamics calculations show that the dominant channel is the dimer-exchange mechanism below 200 K, and that the reactive rate constant tends to stabilize at low temperatures, because the effective dipole of either dimer is reduced with respect to that of formaldehyde alone. The reaction complex formed at low temperatures does not live long enough to produce a complete energy relaxation, as assumed in statistical theories. These results show that the reactivity of the dimers cannot explain the large rate constants measured at temperatures below 100 K.

Mechanistic Insight and Intersystem Crossing Dynamics of the C(3P) + H2CO/D2CO Reaction

The reaction of atomic carbon, C(3P), with H2CO has been investigated using the direct dynamics trajectory surface hopping (DDTSH) method with Tully's fewest switches algorithm. The lowest lying ground triplet and single states are considered for the dynamics study at a reagent collision energy of 8.0 kcal/mol. From the trajectory calculations, we observed that CH2 + CO and H + HCCO are the two major product channels for the title reaction. The insertion mechanism of the C(3P) + H2CO reaction is rather complex and is followed by three distinct intermediates with no entrance channel barrier to the reaction on the B3LYP/6-31G(d,p) potential energy surfaces. The triplet insertion complexes are formed by three different approaches; "Sideways", "End-on" and "Head-on" attack of the triplet carbon atom toward H2CO molecule. Our dynamics calculations predict a new product channel (H + HCCO(X 2A'')) with a contribution of ∼46% of the overall products formation via ketocarbene intermediate through "Head-on" approach. Despite the weak spin-orbit coupling (SOC) interactions, intersystem crossing (ISC) via a ketocarbene intermediate has a small but significant contribution, about 2.3%, for the CH2 + CO channel. To understand the kinetic isotope effects on the reaction dynamics, we have extended our study for the C(3P) + D2CO reaction. It is seen that isotopic substitution of both the H atoms has a small reduction in the extent of ISC dynamics for the carbene formation. Our results, certainly, reveal the importance of the ketocarbene intermediate and the H + HCCO products channel as one of the major product formation channels in the title reaction, which was not reported earlier.

Experimental and Theoretical Investigation of Reactions of Formyl (HCO) Radicals in the Gas Phase: (I) Kinetics of HCO Radicals with Ethyl Formate and Ethyl Acetate in Tropospherically Relevant Conditions.

Formyl (HCO) radicals were generated in situ in the gas phase via the photolysis of glyoxal in N2 at 248 nm using the pulsed laser photolysis-cavity ring-down spectrometry technique, and the absorption cross-section of the radical was measured to be σHCO = (5.3 ± 0.9) × 10-19 cm2 molecule-1 at 298 K and 615.75 nm, which was the probing wavelength. The kinetics of the reactions of HCO radicals with ethyl formate (EF) and ethyl acetate (EA) were investigated experimentally in the temperature range of 260-360 (±2) K at a pressure of 60 Torr/N2. The absolute rate coefficient for the reaction between HCO and EF was measured to be (298 K) = (1.39 ± 0.30) × 10-14 cm3 molecule-1 s-1 at ambient temperature, whereas that for the reaction of HCO with EA was measured tobe (298 K) = (2.05 ± 0.43) × 10-14 cm3 molecule-1 s-1. The reaction of HCO with EA was faster than that with EF, which might be due to the greater stability of the formed radical intermediate due to hyperconjugative and inductive effects. The dependency of the measured kinetics on experimental pressures and laser fluences was examined within a certain range. To complement the experiments, kinetics of the title reactions in the temperature range of 200-400 K were deciphered theoretically via the canonical variational transition-state theory with small-curvature tunneling and interpolated single-point energy (CVT/SCT/ISPE) method using a dual-level approach at the CCSD(T)/cc-pVTZ//MP2/6-311++G(d,p) level of theory/basis set. A good degree of agreement was detected between the rate coefficients measured experimentally and those calculated theoretically both at room temperature and throughout the range of temperatures studied. The kinetic branching ratios and thermochemistry of the reactions were investigated to understand the thermodynamic feasibility and kinetic lability of each pathway throughout the studied temperatures. Atmospheric implications of these reactions of HCO radicals are also discussed.

Evolution characteristics of under-expanded accompanying jet under extreme heat condition

This work is focused to study heat and mass transfer characteristics as well as evolution mechism of the wake flow field of the aerodynamic vehicle (ADV) with base-bleed unit (BBU) under extreme heat condition. Here, we have used a new type of annular nozzle for BBU to improve the structure of the wake flow field and provide more mass and energy for the wake flow field. Based on H2-CO reaction mechanism for the post-combustion from the base-bleed gas and supersonic inflow, numerical models of unsteady accompanying jet coupled with the burning of the propellant are established for annular and circular nozzles. Comparisons of chamber pressure and jet shape of computational results are done with experimental studies, and a reasonable match has been obtained in these comparisons. Further, an under-expanded supersonic annular jet with containing central jet develops into a subsonic annular flow, presenting some distinguishing features compared with the decay process of an under-expanded circular jet. The base pressure coefficient for the annular nozzle increases by 21.8% and the peak of average base temperature is improved by 15.4%, which means this new type of annular nozzle is more beneficial to the increase of heat energy and pressure of the ADV base.

A theoretical study on the kinetics for ethanol reaction with formyl radical

C2H5OH is one of important renewable fuels. The mechanism for the C2H5OH + HCO reaction has been investigated by a potential energy surface calculation at the B3LYP/aug-cc-pVTZ (optimization) and CCSD(T)/cc-pVTZ (single-point) levels. Our results show that the HCO free radical can abstract the H atoms in the OH group giving CH3CH2O + CH2O or in the CH2 group giving CH3CHOH + CH2O. The rate constant results by TST calculations considering tunneling corrections show that the second pathway is dominate in all the calculation temperature range of 300-2000K.

Laminar flame speed of ethanol/ammonia blends–An experimental and kinetic study

Blending bio-ethanol with ammonia is an interesting approach to reach carbon-neutrally combustion systems. As a fundamental parameter, laminar flame speed for different blends of ethanol/ammonia is explored using the spherical expanding flame technique under constant pressure conditions. A comparison to the few recent literature experimental results is proposed. An empirical correlation is developed to estimate the laminar flame speed of any ammonia/ethanol mixture as a function of the equivalence ratio at 1 bar and 423 K. Some simulation results are also provided to compare kinetics mechanisms accuracy to experimental data and identify the most relevant sensitive reactions. Two mechanisms are compared, one from the literature and another from the fusion of two mechanisms originally developed for pure ethanol and pure ammonia respectively. One major conclusion is that none of these mechanisms allows sufficient agreement with experimental data and more in-depth studies are still needed to provide high accurate kinetics mechanisms for ethanol/ammonia mixture. Sensitivity analysis highlights an important difference between the sensitive reactions with pure ammonia and ethanol blends. The key role of carbon reactions HCO(+M)<=>H+CO(+M) and CO+OH<=>CO2+H on the laminar flame speed is shown for blends containing more than 50% of ethanol.

Mechanism and sites requirement for CO hydrogenation to CH3OH over Cu/CeO2 catalysts

This study investigated the Cu-CeO2 interactions for CO hydrogenation to methanol by applying rate measurements, chemical titration/desorption, together with in-situ/operando spectroscopy techniques under realistic reaction conditions. Ce components were enriched in the surface region of Cu/CeO2 catalysts while Ce3+ and Ce4+ atoms co-existed during CO hydrogenation. The ratio of Ce3+ to Cu0 in the surface region of catalysts increased linearly with the Ce content. The turnover rates of CH3OH formation increased as a single-valued function of the ratio, irrespective to the individual contact extent between Cu and CeO2 clusters or Ce content in the Cu/CeO2 catalysts, indicating that the Cu0-Ce3+ sites pair acts as active sites for CO hydrogenation to methanol over Ce-CeO2 samples. This study also unraveled that formyl and formate species, as reactive surface intermediates, were co-adsorbed on the Cu0-Ce3+ sites, and their hydrogenation routes occurred concurrently for CH3OH formation during CO-H2 reactions.

Successive H-atom Addition to Solid OCS on Compact Amorphous Solid Water

Abstract Carbonyl sulfide (OCS) is an abundant sulfur (S)-bearing species in the interstellar medium. It is present not only in the gas phase, but also on interstellar grains as a solid; therefore, OCS very likely undergoes physicochemical processes on icy surfaces at very low temperatures. The present study experimentally and computationally investigates the reaction of solid OCS with hydrogen (H) atoms on amorphous solid water at low temperatures. The results show that the addition of H to OCS proceeds via quantum tunneling, and further addition of H leads to the formation of carbon monoxide (CO), hydrogen sulfide (H2S), formaldehyde (H2CO), methanol (CH3OH), and thioformic acid (HC(O)SH). These experimental results are explained by our quantum chemical calculations, which demonstrate that the initial addition of H to the S atom of OCS is the most predominant, leading to the formation of OCS-H radicals. Once the formed OCS-H radical is stabilized on ice, further addition of H to the S atom yields CO and H2S, while that to the C atom yields HC(O)SH. We have also confirmed, in a separate experiment, the HC(O)SH formation by the HCO reactions with the SH radicals. The present results would have an important implication for the recent detection of HC(O)SH toward G+0.693–0.027.

Dynamics of Reaction CH3CHI + O2 Investigated via Infrared Emission of Products CO, CO2, and OH.

The reaction CH3CHI + O2 has been commonly employed in laboratories to produce a methyl-substituted Criegee intermediate CH3CHOO, but the detailed dynamics of this reaction remain unexplored. We carried out this reaction by irradiating a flowing mixture of CH3CHI2 (∼70 mTorr) and O2 (∼4 and 8 Torr) at 308 or 248 nm and observed infrared emission of the products with a step-scan Fourier-transform spectrometer. Upon irradiation at 248 nm with O2 ∼4 Torr, a Boltzmann distribution of CO (v ≤ 4, J ≤ 25) with average vibrational energy (12 ± 2) kJ mol-1 and of OH (v = 1, J ≤ 5.5) were observed and assigned to be produced from the decomposition of CH3C(O)OH* to form CO + CH3OH and OH + CH3CO, respectively. The observed broadband emission of CO2 was simulated with two vibrational distributions of average energies (42 ± 3) and (114 ± 6) kJ mol-1 and assigned to be produced from the decomposition of CH3C(O)OH* and (methyl dioxirane)*, respectively. The results upon irradiation of the sample at 308 nm are similar, likely indicating a small fraction of energy partition into these products and rapid thermalization of CH3CHI*. Compared with reaction CH2I + O2, the title reaction yielded products with much less internal excitation, consistent with the expectation that these observed products receive much less fraction of available energy upon fragmentation when an additional methyl moiety was present in the parent. The large-v component of CO observed in experiments of CH2I + O2 at 248 nm, produced from secondary reaction HCO + O2, was absent in this work because the corresponding secondary reaction CH3CO + O2 in decomposition of CH3CHOO* produces α-lactone + OH or H2CO + CO + OH.

Valence Photoionization and Autoionization of the Formyl Radical.

We have used 308 nm photolysis of acetaldehyde to measure a photoionization spectrum of the formyl (HCO) radical between 8 and 11.5 eV using an 11 meV FWHM photoionization energy resolution. We have confirmed that the formyl radical is the carrier of the spectrum by generating an identical spectrum of the HCO product in the Cl + H2CO reaction. The spectrum of HCO and its deuterated isotopologue (DCO) have several resolved autoionizing resonances above the Franck-Condon envelope, which we assign to autoionization after initial excitation into neutral 3sσ and 3p Rydberg states converging to the first triplet excited state of HCO+(ã 3A'). The quantum defects for these states are δ3sσ = 1.06 ± 0.02 and δ3p = 0.821 ± 0.019. We report absolute photoionization cross-section measurements of σHCOPI(9.907 eV) = 4.5 ± 0.9 Mb, σHCOPI(10.007 eV) = 4.8 ± 1.0 Mb, σHCOPI(10.107 eV) = 6.0 ± 1.2 Mb, σHCOPI(10.107 eV) = 5.7 ± 1.2 Mb, and σHCOPI(10.304 eV) = 10.6 ± 2.2 Mb relative to the photoionization cross section of the methyl radical. The absolute cross-section measurements are a factor of ∼1.5 larger than those determined in past studies, although the presence of strong autoionizing features supports a dependence on photoionization energy resolution. We propose that the semiempirical model of Xu and Pratt for estimation of free radical photoionization cross sections is more accurate when applied with a reference species containing the same atoms as the free radical rather than isoelectronic species with different atoms.

Accurate Rate Constants for the Forward and Reverse H + CO ↔ HCO Reactions at the High-Pressure Limit.

The forward and reverse H + CO ↔ HCO reactions are important for combustion chemistry and have been studied from a wide variety of theoretical and experimental techniques. However, most of the chemical kinetic investigations concerning these processes are focused on low pressures or fall-off regions. Hence, a high-level electronic structure treatment was employed here in order to provide accurate rate constant values for these reactions at the high-pressure limit along temperatures from 50 to 4000 K. In relative terms, the variational effects on rate constants are shown to be almost as important at high temperatures as quantum tunneling corrections at the lowest temperatures investigated. The activation energies fitted by using modified and traditional Arrhenius’ equations for the forward rate constants from 298 to 2000 K are, respectively, equal to 2.64 and 3.89 kcal mol–1, while similar fittings provided, respectively, 1.96 and 3.22 kcal mol–1, considering now forward rate constants from a temperature range of 298–373 K. This last activation energy determination (3.22 kcal mol–1) is in better agreement with the commonly referenced experimental value of 2.0 ± 0.4 kcal mol–1, also obtained from traditional fittings in the range 298–373 K, than the value attained from a broader temperature range fitting (3.89 kcal mol–1). However, some additional care must be considered along these comparisons once the experimental reaction rate measurements have been done for the trimolecular H + CO + M → HCO + M reaction instead. Anyway, the usage of appropriate temperature ranges is fundamental for reliable activation energy comparisons, although the remaining deviation between theory and experiment is still large and is possibly caused by the different pressure regimes assessed in each case. Finally, we roughly estimated that the high-pressure limit for the HCO decomposition into H and CO can be achieved along temperatures ranging from ∼246 and ∼255 K downward, respectively, at pressures of 1.1 and 9.6 atm, although further experiments should be carried out in order to improve these estimates. On the other hand, pressures larger than 9.8 × 104 atm are required for the aforementioned dissociation reaction to attain the high-pressure limit at 700 K. Therefore, the rate constants determined here are probably applicable in combustion studies at lower temperatures.

Influence of the double bond position in combustion chemistry of methyl butene isomers: A shock tube and laser absorption study

AbstractShock tube experiments have been carried out on 2‐methyl‐1‐butene (2M1B), 2‐methyl‐2‐butene (2M2B), and 3‐methyl‐1‐butene (3M1B)—the three isomers of methyl butene compound. Carbon monoxide (CO) time‐histories and ignition delay times are obtained behind reflected shockwaves over the temperature range of 1350‐1630 K and pressures of 8.3‐10.5 atm with stoichiometric mixtures of 0.075% fuel in O2/Ar. Comparative ignition study reveals that 3M1B ignites significantly faster than the other two isomers, while 2M1B dissociates earlier but ignites later than 2M2B. Possible mechanisms for this behavior are discussed with ignition delay time sensitivity and reaction path analysis. In addition, time‐resolved CO measurements are compared with three different reaction mechanisms from the literature. Sensitivity analyses have been carried out to identify important reactions that need attention to accurately predict the chemistry of these isomers. Further investigation into the rates of unimolecular fuel decomposition reactions and C3H3 + O2 = CH2CO + HCO reaction are suggested based on the current investigation.

Investigation on the dual-fuel active-thermal atmosphere combustion strategy based on optical diagnostics and numerical simulations

The dual-fuel active-thermal atmosphere combustion strategy has the potential to make the homogeneous combustion compression ignition more controllable, achieving low fuel consumption and engine emissions. However, the detailed combustion process and chemical interaction between the high- and low- reactivity fuels have not been fully understood. This work investigated the premixed iso-octane combustion assisted by the two-stage reaction of n-heptane on an optical engine. High-speed imaging technique was applied to visualize the natural flame luminosity and a data-processing method based on Cantera was proposed to evaluate the detailed chemical kinetics process. Results indicate that the high-temperature heat release stage changes from partially premixed combustion with distinct blue flames into the mixing-controlled combustion, showing evident signs of soot radiation when the two-stage ignition transits into three-stage ignition. Heat release from the premixed n-heptane is initially dominated by reaction HCO + O2 = CO + HO2 and turns into hydrogen-oxygen reactions with increasing temperature. In the primary reacting regions for the cases with latter injection timings, reactions C3H5 + H(+M) = C3H6(+M) and C3H3 + H(+M) = C3H4(+M) play the major role, which promotes the formation of soot precursors. Formaldehyde can represent the mixture reactivity. Reaction pathway of premixed n-heptane generated reactive species that promote the consumption of iso-octane. Reactions of iso-octane also enhance the formation of reactive species that further accelerate the combustion process. Retardation of the iso-octane injection timing reduces the local mixture reactivity and weakens the chemical interaction between iso-octane and n-heptane. This lowers the heat release rate, and, objectively, results in the transition of two-stage ignition to three-stage ignition.

Effect of advanced intake valve closing on the thermo-chemical characteristics of the homogeneous combustion in a DME fueled HCCI engine

This numerical study was performed to analyze the thermo-chemical characteristics of homogeneous combustion on the effect of advanced intake valve closing timing in a DME fueled HCCI engine. To achieve this, the engine simulation was performed using the detailed chemical kinetic mechanism, including 453 reactions for dimethyl ether (CH3OCH3), and the modified NOX mechanism was applied to analyze the NOX reaction pathway of the DME combustion. The experimental study was performed to validate the engine analysis using a single cylinder CI engine with the injection timing of BTDC 40 degree and IVC timing of BTDC 128 degree. The injection quantity and pressure were set to 8 mg and 50 MPa at 1500 RPM, respectively. The results of engine performance such as cylinder pressure, cylinder temperature, heat release rate, and IMEP were decreased and the ISFC and ignition delay were increased when the IVC timing was advanced since the overall rates of a chemical reaction and the reaction rate of OH radicals are reduced. Especially, the reduction of CH2OCH2O2H radicals was a very important factor it affects the ignition delay in the low temperature reaction. Consequently, it influences the LTC and HTC reactions and makes a difference in the fuel oxidation rate and combustion reaction. Furthermore, the OH radicals also had the greatest effect to the CO oxidation and NOX production. The CO and NOX emissions were primarily occurred by the forward reactions of HCO + O2 → CO + HO2 and HO2 + NO → NO2 + OH during the combustion period, respectively. The chemical reactivity and CO oxidation reaction were reduced as the fuel mass fraction increased in the low combustion temperature. In addition, the reaction path of NOX emission related to the thermal NOX reaction did not occur due to the low combustion temperature. Thus, the thermo-chemical characteristics of the DME homogeneous combustion of this work suggest that the chemical reaction of fuel oxidation and exhaust emissions are limited as the effect of chemical oxidation rate, reaction rate, and the OH radical depending on the IVC timing changes.

Investigation of the chemical kinetics process of diesel combustion in a compression ignition engine using the large eddy simulation approach

This work investigated the chemical kinetics processes of diesel combustion under compression ignition combustion conditions using the large eddy simulation approach. An overall reasonable agreement with the experimental combustion results was obtained. To further investigate the combustion details, a data-processing method was adopted to calculate and identify the representative reactions for the species consumption and heat release in each computational cell. It was found that during the initial ignition period, the combustion domain was primarily composed of three combustion regions, including the upstream exothermic region (Region 1), upstream endothermic region (Region 2), and downstream exothermic region (Region 3). The exothermic heat release in Region 1 and endothermic heat release in Region 2 were both dominated by the reaction C7H15O2-2 = C7H15-2 + O2, while Region 3 was dominated by the reaction HCO + O2 = CO + HO2, which laid the foundation for the further intense heat release process. The exhaust gas recirculation (EGR) has a significant effect on the heat release feature during the diffusion combustion process. For the high EGR case, the endothermic heat release was dominated by the reactions OH + H2 = H + H2O and C2H3(+M) = C2H2 + H(+M), rather than by the reaction C7H15O2-2 = C7H15-2 + O2 as at the low EGR rate.

Reaction Profiles and Kinetics for Radical-Radical Hydrogen Abstraction via Multireference Coupled Cluster Theory.

Radical-radical abstractions in hydrocarbon oxidation chemistry are disproportionation reactions that are generally exothermic with little or no barrier yet are underappreciated and poorly studied. Such challenging multireference electronic structure problems are tackled here using the recently developed state-specific multireference coupled cluster methods Mk-MRCCSD and Mk-MRCCSD(T), as well as the companion perturbation theory Mk-MRPT2 and the established MRCISD, MRCISD+Q, and CASPT2 approaches. Reaction paths are investigated for five prototypes involving radical-radical hydrogen abstraction: H + BeH → H2+ Be, H + NH2 → H2 + NH, CH3 + C2H5 → CH4 + C2H4, H + C2H5 → H2 + C2H4, and H + HCO → H2 + CO. Full configuration interaction (FCI) benchmark computations for the H + BeH, H + NH2, and H + HCO reactions prove that Mk-MRCCSD(T) provides superior accuracy for the interaction energies in the entrance channel, with mean absolute errors less than 0.3 kcal mol-1 and percentage deviations less than 10% over the fragment separations of relevance to kinetics. To facilitate combustion studies, energetics for the CH3 + C2H5, H + C2H5, and H + HCO reactions were computed at each level of theory with correlation-consistent basis sets (cc-pVXZ, X = T, Q, 5) and extrapolated to the complete basis set (CBS) limit. These CBS energies were coupled with CASPT2 projected vibrational frequencies along a minimum energy path to obtain rate constants for these three reactions. The rigorous Mk-MRCCSD(T)/CBS results demonstrate unequivocally that these three reactions proceed with no barrier in the entrance channel, contrary to some earlier predictions. Mk-MRCCSD(T) also reveals that the economical CASPT2 method performs well for large interfragment separations but may deteriorate substantially at shorter distances.

Formation of COMs through CO hydrogenation on interstellar grains

Context. Glycoaldehyde, ethylene glycol, and methyl formate are complex organic molecules that have been observed in dark molecular clouds. Because there is no efficient gas-phase route to produce these species, it is expected that a low-temperature surface route existst that does not require energetic processing. CO hydrogenation experiments at low temperatures showed that this is indeed the case. Glyoxal can form through recombination of two HCO radicals and is then further hydrogenated. Aims. Here we aim to constrain the methyl formate, glycolaldehyde, and ethylene glycol formation on the surface of interstellar dust grains through this cold and dark formation route. We also probe the dependence of the grain mantle composition on the initial gas-phase composition and the dust temperature. Methods. A full CO hydrogenation reaction network was built based on quantum chemical calculations for the rate constants and branching ratios. This network was used in combination with a microscopic kinetic Monte Carlo simulation to simulate ice chemistry, taking into account all positional information. After benchmarking the model against CO-hydrogenation experiments, simulations under molecular cloud conditions were performed. Results. Glycoaldehyde, ethylene glycol, and methyl formate are formed in all interstellar conditions we studied, even at temperatures as low as 8 K. This is because the HCO + HCO reaction can occur when HCO radicals are formed close to each other and do not require to diffuse. Relatively low abundances of methyl formate are formed. The final COM abundances depend more on the H-to-CO ratio and less on temperature. Only above 16 K, where CO build-up is less efficient, does temperature start to play a role. Molecular hydrogen is predominantly formed through abstraction reactions on the surface. The most important reaction leading to methanol is H2CO + CH3O → HCO + CH3OH. Our simulations are in agreement with observed COM ratios for mantles that have been formed at low temperatures.