High-pressure Limit Research Articles

Can a single ammonia and water molecule enhance the formation of methanimine under tropospheric conditions?: kinetics of •CH2NH2 + O2 (+NH3/H2O).

The aminomethyl (•CH2NH2) radical is generated from the photo-oxidation of methylamine in the troposphere and is an important precursor for new particle formation. The effect of ammonia and water on the gas-phase formation of methanimine (CH2NH) from the •CH2NH2 + O2 reaction is not known. Therefore, in this study, the potential energy surfaces for •CH2NH2 + O2 (+NH3/H2O) were constructed using ab initio//DFT, i.e., coupled-cluster theory (CCSD(T))//hybrid-density functional theory, i.e., M06-2X with the 6-311++G (3df, 3pd) basis set. The Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) simulation with Eckart's asymmetric tunneling was used to calculate the rate coefficients and branching fractions relevant to the troposphere. The results show 40% formation of CH2NH at the low-pressure (<1 bar) and 100% formation of CH2NH2OO• at the high-pressure limit (HPL) condition. When an ammonia molecule is introduced into the reaction, there is a slight increase in the formation of CH2NH; however, when a water molecule is introduced into the reaction, the increase in the formation of CH2NH was from 40% to ∼80%. The calculated rate coefficient for •CH2NH2 + O2 (+NH3) [1.9 × 10-23cm3 molecule-1 s-1] and for CH2NH2 + O2 (+H2O) [3.3 × 10-17cm3 molecule-1 s-1] is at least twelve and six order magnitudes smaller than those for free •CH2NH2 + O2 (2 × 10-11cm3 molecule-1 s-1 at 298K) reactions, respectively. Our result is consistent with that of previous experimental and theoretical analysis and in good agreement with its isoelectronic analogous reaction. The work also provides a clear understanding of the formation of tropospheric carcinogenic compounds, i.e., hydrogen cyanide (HCN).

A Theoretical Kinetic Study on Concerted Elimination Reaction Class of Peroxyl-hydroperoxyl-alkyl Radicals (•OOQOOH) in Normal-alkyl Cyclohexanes.

The concerted elimination reaction class of peroxyl-hydroperoxyl alkyl radicals (•OOQOOH) plays a crucial role in the low-temperature combustion of normal-alkyl cyclohexanes. The generation of the relatively unreactive HO2 radicals in this reaction is one of the factors leading to the negative temperature coefficient (NTC) behavior, which hinders the low-temperature oxidation of normal-alkyl cyclohexanes. In this study, 44 reactions are selected and divided into 4 different subclasses according to the nature of the carbon atom where the H atom is eliminated and the reaction center position. Utilizing the CBS-QB3 method, we compute the energy barriers for the concerted elimination reactions of peroxyl-hydroperoxyl alkyl radicals. Following this, we assess both the high-pressure limit and pressure-dependent rate constants for all reactions by applying TST and RRKM/ME theory. These calculations allow for the development of rate rules, which come to fruition through an averaging process involving the rate constants of representative reactions within each subclass. Our work provides accurate rate constants and rate rules for this reaction class, which can aid in constructing more accurate combustion mechanisms for normal-alkyl cyclohexanes.

Theoretical Study on the Rate Constant of Spin‐forbidden Reaction SO2+O(3P) (+M)→SO3 (+M)

AbstractTo improve the understanding of the oxidation of SO2 to SO3, the non‐adiabatic transition state theory (NA‐TST) coupling with the master equation (ME) method was adopted to study the kinetic characteristics of the spin‐forbidden recombination reaction SO2+O(3P) (+M)→SO3 (+M). By determining the minimum energy crossing point (MECP) of the triplet‐singlet potential energy surfaces, the high‐pressure limit (HPL), k∞=3.81×108 T0.998 e−1214/T cm3 mol−1 s−1 and the low‐pressure limit (LPL), k0=4.78×1036 T−6.38 e−4610/T cm6 mol−2 s−1 were obtained, respectively. The LPL rate constants were compared with the reported experimental data and good consistency was found. Besides, the hopping probability near the MECP was also determined. In addition, this work shows the potential application of NA‐TST/ME method to investigate the kinetics of similar spin‐forbidden reactions.

Kinetics for the reaction of Criegee intermediate CH2OO with n-butyraldehyde and its atmospheric implications

The Criegee intermediates (CIs), formed through ozone oxidation of unsaturated hydrocarbons, can undergo reactions with various carbonyl compounds in the atmosphere. In this study, we have investigated the 1,3-cycloaddition reaction of n-butyraldehyde with the simplest Criegee intermediate, CH2OO, using the OH laser-induced fluorescence (LIF) method. The experiments were conducted over a temperature and pressure range of 283–318 K and 5–100 Torr, respectively. Our results reveal that the bimolecular rate constant of the reaction is dependent on both temperature and pressure. Specifically, the measured rate constants at 100 Torr are (3.51 ± 0.63), (2.97 ± 0.53), (2.70 ± 0.49), and (2.52 ± 0.45) × 10−12 cm3 molecule−1 s−1 at temperatures of 283, 298, 308, and 318 K, respectively, with a total error of 18%. By fitting the pressure-dependent rate constants according to the Lindemann mechanism, we obtained a high-pressure limit rate constant of (3.05 ± 0.55) × 10−12 cm3 molecule−1 s−1 at 298 K. The Arrhenius plot of the temperature-dependent rate constants yields an activation energy of (−1.71 ± 0.30) kcal mol−1 and a pre-exponential factor of (1.66 ± 0.24) × 10−13 cm−3 s−1 (total error). Furthermore, our findings indicate that the title reaction exhibits a more pronounced temperature-dependent effect compared to that of CH2OO with water dimer and SO2, as well as that of OH with n-butyraldehyde. This observation suggests that the significance of title reaction in the atmosphere increases at lower temperature.

First-principles study on the upper pressure limit of diamond under multiaxial stress

In this study, we investigated the upper pressure limit of diamond. A method for predicting the upper pressure limit of diamonds under multiaxial stress is developed by introducing stress angles α based on first-principles calculations. The normal stresses of diamond in three mutually orthogonal orientations [111], 112¯, and 11¯0 are set as σ111tanα=σ112¯=σ11¯0. The three most important characteristics, namely, the theoretical strength, maximum dynamic stability stress, and phase transition stress, are evaluated and analysed to further determine the upper pressure limit of diamond. The calculated results indicate that the upper pressure limit of diamond can reach up to 1.265 TPa in the [111] orientation under multiaxial compressions, which is approximately 1.25 times higher than that of our calculated phase transition stress (σ[111] = 1008.3 GPa) and 1.18–1.28 times higher than that of the current upper pressure limit 990 (Phys. Rev. Lett. 108 (2012) 045704)–1075 GPa (Proc. Natl Acad. Sci. U. S. A. 103 (2006) 1204) at a hydrostatic pressure, which is a new record. This study provides a possible approach to obtain a high pressure limit by multiaxial compression, which will advance the progress of in-situ high-pressure anvil technology.

Two-dimensional non-linear hydrodynamics and nanofluidics

A water monolayer squeezed between two solid planes experiences strong out-of-plane confinement effects while expanding freely within the plane. As a consequence, the transport of such two-dimensional water combines hydrodynamic and nanofluidic features, intimately linked with each other. In this paper, we propose and explicitly solve a non-linear hydrodynamic equation describing two-dimensional water flow with viscosity parameters deduced from molecular dynamic simulations. We demonstrate that the very ability of two-dimensional water to flow in short channels is governed by the second (dilatational) viscosity coefficient, leading to flow compression and velocity saturation in the high-pressure limit. The viscosity parameter values depend strongly on whether graphene or hexoganal boron nitride layers are used to confine 2D water that offers an interesting opportunity to obtain various nanofluids out of the same water molecules just by using alternate materials to fabricate the 2D channels.

Column selection considerations in compact capillary liquid chromatography

Recent years have seen significant advances in compact, portable capillary LC instrumentation. This study explores the performances of several commercially available columns within the pressure and flow limits of both the columns and one of these compact LC instruments. The commercially available compact capillary LC system with UV-absorbance detector used in this study is typically operated using columns in the 0.15–0.3 mm internal diameter (i.d.) range. Efficiency measurements (i.e., theoretical plates, N) for six columns with i.d.s in this range and of varying lengths and pressure limits, packed with stationary phases of different particle diameters and morphologies, were made using a mixture of standard alkylphenones. Kinetic plot comparisons between columns that vary by one (or more) of these parameters are described, along with calculated kinetic performance and Knox-Saleem limits. These theoretical performance descriptions provide insight into optimal operating conditions when using capillary LC systems. Based on kinetic plot evaluation of available capillary columns in the 0.2–0.3 mm i.d. range with a conservative upper pressure limit of 330 bar packed with superficially porous particles, a 25 cm column could generate ∼47,000 plates in 7.85 min when operated at 2.4 µL/min. For comparison, more robust 0.3 mm i.d. columns (packed with fully porous particles) that can be operated at higher pressures than can be provided by the pumping system (conservative pump upper pressure limit of 570 bar), a ∼20 cm column could generate nearly 40,000 plates in 5.9 min if operated at 6 µL/min. Across all capillary LC columns measured, higher pressure limits and shorter columns can provide the best throughput when considering both speed and efficiency.

The gas-phase pyrolysis of cyclopropylamine. A computational study on the kinetics and reaction mechanism

Small amines represent an accessible model to understand the reactivity of the nitrogenous components of biomass during combustion and pyrolysis processes. In this work, the kinetics and mechanism of the gas-phase pyrolysis of cyclopropylamine, c-C3H5NH2, have been studied by ab initio and density functional calculations. From the derived information high-pressure limit rate coefficients were estimated using the transition state theory over the 400 – 1400 K temperature range. The results support a stepwise process, in which the initial ring-opening stage determines the rate through competitive direct, biradical and carbene mechanisms to give 1-aminopropene, CH3CH=CHNH2, and 1-propanimine, CH3CH2CH=NH, as reactive intermediates. The Arrhenius equation obtained at the G3(MP2)//B3LYP/6–311++G(3df,3pd) level of theory, log10 (koverall,∞/s-1) = (15.47 ± 0.11) – (57.68 ± 0.32) kcal mol-1 (2.303 RT)-1, agrees very well with the experimental expression determined by Parry and Robinson. According to these findings, the carbenic pathway is most important up to ∼500 K, above which the biradical mechanism predominates. Subsequent reaction steps include the addition of the CH3CH=CHNH2 and CH3CH2CH=NH species to another c-C3H5NH2 molecule to produce c-C3H5NHCH(NH2)CH2CH3, which afterwards decomposes into c-C3H5N=CHCH2CH3 and NH3. The results also reveal relatively high electronic barriers for the interconversion between the geometric isomers of CH3CH=CHNH2 and CH3CH2CH=NH, suggesting that the contribution of these processes is negligible. Further calculations of the title reaction in the presence of selected aliphatic amines RNH2 (R = Me, Et,n-Pr, i-Pr, allyl) confirm that the unimolecular fission of the ring is the rate determining step.

Theoretical Study of Intramolecular H-Migration Reactions of Peroxyl Radicals of JP-10 (Exo-Tetrahydrodicyclopentadiene)

ABSTRACT Intramolecular H-migration reactions of peroxyl radicals of JP-10 (C10H15OO•) play an important role in developing the oxidation mechanism of JP-10 at low temperature. However, accurate rate coefficients for these reactions are lacking in the literature. Therefore, 33 H-migration reactions of C10H15OO•, which belong to six series of reactions, were investigated. Quantum chemical computations at the CBS-QB3 level reveal that C10H15OO• do not obey the rule that the energy barrier of 1,4-H migration reaction is higher than that of 1,5-H migration reaction because of the special steric cyclic structure of C10H15OO• and the ring strain in the transition state. The transition state theory and Rice-Ramsberger-Kassel-Marcus/Master-Equation theory were used to obtain high-pressure limit rate coefficients and pressure-dependent rate coefficients, respectively. The results show that H-migration reactions of C10H15OO• have obvious pressure-dependent effect. The most competitive reaction was obtained among 33 reactions. Moreover, branching ratios of each series of reactions were used as weight ratios to lump this series of reactions. Six series of reactions were lumped as six lumped reactions by the isomer lumping method. This will provide accurate rate coefficients for constructing a mechanism with the low number of species and suitability for computational fluid dynamics simulation.

Theoretical Investigation of Rate Rules for H-Intermigration Reactions for Cyclic Alkylperoxy Radicals

As a starting channel, the H-intermigration reaction of alkylperoxy radicals (ROO radicals) that yields hydroperoxyl alkyl radicals (QOOH radicals) determines the low-temperature chemistry of alkanes. In this work, this type of reaction was investigated for typical cyclic alkanes, which are important fuel components and soot precursors, using theoretical ab initio methods. First, all the molecular geometries and vibrational frequencies were computed using the density functional theory method and the single point energies were refined using the post-Hartree fork method (M062X/6-311G(d,p)//DLPNO-CCSD(T)/CBS). Then, high-pressure limit rate constants were evaluated with tight transition state theory, with which tunneling effects were considered using the Eckart model and low-frequency torsion modes were modeled as hindered rotors. Pressure-dependent rate constants were also calculated for typical reaction channels. Rate expressions in the Arrhenius form for 91 reactions are proposed. All reactions were categorized into seven reaction types and the rate rule for each reaction type was estimated with uncertainty factors of three to six. These rules can be potentially used in the development of low-temperature kinetic mechanisms for cycloalkanes. A comparison between different reaction types was also performed and the favorable channels are discussed.

Kinetics of the Simplest Criegee Intermediate CH2OO Reaction with tert-Butylamine

The kinetics of the simplest Criegee intermediate (CH2OO) reaction with tert-butylamine ((CH3)3CNH2) was studied under pseudo-first-order conditions with the OH laser-induced fluorescence (LIF) method at the temperature range of 283-318 K and the pressure range of 5-75 Torr. Our pressure-dependent measurement showed that at 5 Torr─the lowest pressure measured in the current experiment─this reaction was under the high-pressure limit condition. At 298 K, the reaction rate coefficient was measured to be (4.95 ± 0.64) × 10-12 cm3 molecule-1 s-1. The title reaction was observed to be negative temperature-dependent; the activation energy of (-2.82 ± 0.37) kcal mol-1 and the pre-exponential factor of (4.21 ± 0.55) × 10-14 cm3 molecule-1 s-1 were derived from the Arrhenius equation. The rate coefficient of the title reaction is slightly larger than (4.3 ± 0.5) × 10-12 cm3 molecule-1 s-1 of the CH2OO reaction with methylamine; the electron inductive effect and the steric hindrance effect might play a role in contributing to such difference.

Contact pressure dependent mechanisms of ultralow wear PTFE composites

One of the most dramatic reductions in the wear of PTFE has been achieved by compositing PTFE with as little as 1–5 wt% of alumina particles; this has been reported to produce wear rates K ∼10−7 mm3/Nm. The mechanisms associated with this reduction in wear are multifaceted, including 1) preventing crack propagation and delamination of the PTFE wear surface, 2) promoting tribochemistry and more recently 3) tribologically-induced breaking of the filler into nanoscale fragments to stabilize and reinforce tribofilms. However, in an effort to keep experiments comparable, many of the studies throughout the literature have focused on a narrow contact pressure range. In these experiments, we explored the effects of contact pressure on the tribological behavior of different PTFE and alumina composites, one of which is reported to achieve ultra-low wear (∼10−7 mm3/Nm) and another that is reported to only have mild reductions in wear (∼1 × 10−5 mm3/Nm) compared to unfilled PTFE (∼4 × 10−4 mm3/Nm). We found that with decreased contact pressures, the PTFE-alumina composite that was previously reported as high wear could achieve ultralow wear rates. The PTFE-alumina composite previously reported to achieve ultralow wear achieved ultralow wear at a range of low to high contact pressures, with a higher pressure limit corresponding to increases in wear. The friction behavior of PTFE-alumina composites was found to be highly dependent on contact pressure, with increasing pressures resulting in decreasing friction coefficients (∼0.5–0.17 over a 0.62–8.5 MPa range). This effect became more pronounced when the contact pressure was incrementally varied during testing resulting in up to a 70% decrease or increase in friction coefficient due to increasing or decreasing the pressure, respectively. IR spectra of the polymer wear surface showed that tribofilms rich in carboxylates and metal oxides form at the full range of contact pressures tested, even at the extremes. This formation of tribofilms at the sliding interface not only contributes to the ultralow wear of these materials, but plays a role in the friction behavior observed. From this, we gained new insight into the role, functionality and limitations of the alumina fillers.

High-temperature mid-IR absorption and reaction kinetics of 2-methyl-1,3-dioxolane: An experimental and theoretical study

This work reports the mid-IR spectroscopy and reaction kinetics of 2-methyl-1,3-dioxolane (2M13DO). We carried out spectroscopic measurements to deduce temperature-dependent absorption cross-sections of 2M13DO over a broad wavelength range of 8.4–10.5 μm (950–1190 cm−1). For these measurements, we employed a rapidly tuning MIRcat-QT™ laser that can be operated either at a fixed wavelength or scanned mode over wide wavelength regions. By operating the laser at a fixed wavelength, we monitored the decay of 2M13DO behind reflected shock waves over T5 = 1050–1400 K and P5 = 0.7 and 2.6 bar. Our measured concentration time-histories of 2M13DO allowed us to directly extract the overall rate coefficients for the unimolecular decomposition of 2M13DO using the first-order rate law. We did not observe any pressure dependence in the measured rate coefficients, indicating that the reaction is close to the high-pressure limit. By employing the W1U composite method, we explored the important pyrolysis reaction pathways of 2M13DO in the reactive potential energy surface. Three important reaction channels, namely, 2M13DO → CH2CHOCH2CH2OH (IM1), 2M13DO → 2CH3CHO (P3), 2M13DO → CH3 + 1,3-dioxolan-2-yl (P4) were identified. Below 700 K, IM1 forming channel is dominant, whereas CH3CHO formation is dominant under our experimental conditions. Above 1500 K, the radical forming channel (CH3+P4) takes over other channels. At higher temperatures, the contribution of the radical forming channel continually increases, accounting for ∼ 99% at 2000 K. We used the stochastic RRKM-ME model to predict the pressure and temperature dependence of the rate coefficients, k(T, P), and time-resolved species profiles. Our theory showed excellent agreement with the measured rate coefficients. These are the first direct determination of the rate coefficients of the unimolecular decomposition of 2M13DO.

Understanding the kinetics and atmospheric degradation mechanism of chlorotrifluoroethylene (CF2CFCl) initiated by OH radicals.

The atmospheric degradation of chlorotrifluoroethylene (CTFE) by OH˙ was investigated using density functional theory (DFT). The potential energy surfaces were also defined in terms of single-point energies derived from the linked cluster CCSD(T) theory. With an energy barrier of -2.62 to -0.99 kcal mol-1 using the M06-2x method, the negative temperature dependence was determined. The OH˙ attack on Cα and Cβ atoms (labeled pathways R1 and R2, respectively) shows that reaction R2 is 4.22 and 4.42 kcal mol-1, respectively, more exothermic and exergonic than reaction R1. The main pathway should be the addition of OH˙ to the β-carbon, resulting in ˙CClF-CF2OH species. At 298 K, the calculated rate constant was 9.87 × 10-13 cm3 molecule-1 s-1. The TST and RRKM calculations of rate constants and branching ratios were performed at P = 1 bar and in the fall-off pressure regime over the temperature range of 250-400 K. The formation of HF and ˙CClF-CFO species via the 1,2-HF loss process is the most predominant pathway both kinetically and thermodynamically. With increasing temperature and decreasing pressure, the regioselectivity of unimolecular processes of energized adducts [CTFE-OH]˙ gradually decreases. Pressures greater than 10-4 bar are often adequate for assuring saturation of the estimated unimolecular rates when compared to the RRKM rates (in high-pressure limit). Subsequent reactions involve the addition of O2 to the [CTFE-OH]˙ adducts at the α-position of the OH group. The [CTFE-OH-O2]˙ peroxy radical primarily reacts with NO and then directly decomposes into NO2 and oxy radicals. "Carbonic chloride fluoride", "carbonyl fluoride", and "2,2-difluoro-2-hydroxyacetyl fluoride" are predicted to be stable products in an oxidative atmosphere.

Theoretical study of hydrogen abstraction by HO2 radicals from primary straight chain amines CnH2n+1-NH2 (n = 1-4).

Hydrogen abstraction reactions by HO2 radicals from four primary amines including methylamine (MA), ethylamine (EA), n-propylamine (PA), and n-butylamine (BA), are investigated and the effect of the functional group on rate constants at different reaction sites is examined. A hybrid functional BH&HLYP coupled with cc-pVTZ as the basis set is utilized to determine geometry optimizations, frequencies, and connections between transition states and corresponding local minima. By comparing the reaction energies obtained by several density functional theory methods to those obtained using the gold-standard CCSD(T)/CBS(T-Q) method, the M08-HX/maug-cc-pVTZ combination is identified as the best suitable method with a mean unsigned deviation of 0.81 kcal mol-1. This method is then applied to construct the potential energy surface for all the reaction systems. High-pressure limit rate constants at 500-2500 K are calculated through variation transition state theory and conventional transition state theory, including a one-dimensional hindered rotor treatment and asymmetrical Eckart tunneling correction. The branching ratio analysis suggests that the hydrogen abstraction at the C site adjacent to the NH2 functional group (α reaction site) dominates the reactions. Linear Bell-Evans-Polanyi and Bell-Evans correlations are observed for the hydrogen abstractions at the C reaction sites. Furthermore, a scheme to estimate the rate constants for the CnH2n+1-NH2 + HO2 reaction system is presented.

Temperature and pressure dependent rate constants of the reactions of OH• with cyclopentene from variational TST and SS-QRRK methods.

In this work, the pressure- and temperature-dependent reaction rate constants for the hydrogen abstraction and addition of hydroxyl radicals to the unsaturated cyclopentene were studied. Geometries and vibrational frequencies of reactants, products, and transition states were calculated using density functional theory, with single-point energy corrections determined at the domain-based local pair natural orbital-coupled-cluster single double triple/cc-pVTZ-F12 level. The high-pressure limit rate constants were calculated using the canonical variational transition state theory with the small-curvature tunneling approximation. The vibrational partition functions were corrected by the effects of torsional and ring-puckering anharmonicities of the transition states and cyclopentene, respectively. Variational effects are shown to be relevant for all the hydrogen abstraction reactions. The increasing of the rate constants by tunneling is significant at temperatures below 500K. The pressure dependence on the rate constants of the addition of OH• to cyclopentene was calculated using the system-specific quantum Rice-Ramsperger-Kassel model. The high-pressure limit rate constants decrease with increasing temperature in the range 250-1000K. The falloff behavior was studied at several temperatures with pressures varying between 10-3 and 103 bar. At temperatures below 500K, the effect of the pressure on the addition rate constant is very modest. However, at temperatures around and above 1000K, taking pressure into account is mandatory for an accurate rate constant calculation. Branching ratio analyses reveal that the addition reaction dominates at temperatures below 500K, decreasing rapidly at higher temperatures. Arrhenius parameters are provided for all reactions and pressure dependent Arrhenius parameters are given for the addition of OH• to cyclopentene.

An Experimental Study of 2-Propanol Pyrolysis Chemistry.

In the present study, 2-propanol pyrolysis experiments were conducted in a rapid compression facility for a range of temperatures from 965 to 1193 K, pressures from 4.4 to 10.0 atm conditions, and times ranging from 2 to 47 ms after end-of-compression. Mixtures were composed of 2-propanol, nitrogen, and argon with the 2-propanol concentration held constant at 1.5% by mole fraction. The production of seven stable intermediate species (methane, acetylene, ethene, ethane, acetaldehyde, propene, and acetone) were measured using fast-gas sampling and gas chromatography. The high concentrations of propene observed experimentally indicated thermal decomposition of 2-propanol via dehydration was significant at all conditions studied. The observation of the simultaneous presence of methane and acetone indicated H atom abstraction from 2-propanol by H and CH3 radicals was also significant at all conditions. The relative concentrations of methane and acetone indicated an increase in the 2-propanol + CH3 channel at higher temperature. The experimental data showed negligible sensitivity to over a factor-of-two increase in pressure, indicating pressure-dependent reactions, like the thermal decomposition of 2-propanol via dehydration, were in the high-pressure limit. The experimental results were compared with model predictions made using a recently developed kinetic mechanism for C3-C4 alcohols, and the results showed generally good agreement. The most significant discrepancies were for 2-propanol consumption at the highest temperature condition (T = 1193 K), where 2-propanol consumption was predicted as much higher by the model (by more than an order of magnitude) compared with the experimental results, and at the lowest temperature (T = 965 K), ethane production was predicted as much lower (by more than an order of magnitude) compared with the experimental results.

Pressure Dependence of Rate Coefficients of Unimolecular and Chemical Activation Reactions Connected to the Potential Energy Wells of Si2H2Cl4, Si2Cl6, and Si2Cl4 via Rice-Ramsperger-Kassel-Marcus Calculations.

Rate coefficients for elementary reactions connected to the potential energy wells of Si2H2Cl4, Si2Cl6, and Si2Cl4, which are important Si2 species in chemical vapor deposition (CVD) processes that use chlorosilanes as silicon source gases, were determined through the Rice-Ramsperger-Kassel-Marcus theory under various conditions of temperature and pressure. The optimized structures and vibrational frequencies of the reactants, products, and transition state were obtained using (U)B3LYP/6-31+G(d,p), and the single-point energies of the optimized structures were recalculated using the coupled cluster method with single and double excitations plus triple perturbation (U)CCSD(T) with complete basis set extrapolation. Many of the unimolecular decomposition channels and chemical activation reactions investigated in this work were found to be in the fall-off regime under subatmospheric to moderately high-pressure conditions so that it is expected that accurate modeling of the gas phase in the chlorosilane CVD reactor requires careful determination of the rate coefficients as functions of temperature and pressure for the conditions of interest, instead of using high-pressure limit rate coefficients. The rate coefficients determined here were expressed through Chebyshev coefficients and also modified Arrhenius parameters to be used in simulations of systems under a wide range of temperature and pressure conditions.

High-temperature mid-IR absorption spectra and reaction kinetics of 1,3-dioxolane

Cyclic saturated ethers are proposed as alternative fuels for future sustainability. In particular, dioxolanes have been identified as promising biofuel candidates. However, pyrolysis and oxidation of dioxolanes are not well understood. In this work, we measured the temperature-dependent absorption cross-section of 1,3-dioxolane (13DO) in the mid-IR region over 8.4 – 10.5 μm (950 –1190 cm−1). We applied the spectroscopic knowledge to investigate the temperature and pressure dependence of the rate coefficients of the unimolecular decomposition of 13DO over the temperature range of 1000–1400 K and pressures near 0.7 and 2.4 bar. We employed a rapidly tuning MIRcat-QT™ laser in conjugation with a shock tube to carry out high-temperature spectroscopic and pyrolysis measurements. Measured IR spectra of 13DO exhibited a strong temperature dependence. For the kinetic study, we employed the vibrational feature of 13DO near 1120.5 cm−1 to monitor the decay of 13DO quantitatively. The measured rate coefficients did not show any discernible pressure dependence. This indicates that the rate coefficients of the unimolecular decomposition of 13DO are close to the high-pressure limit under the present experimental conditions. In addition, we rationalized the temperature and pressure dependence of the reaction by employing electronic structure/RRKM-ME calculations. Our theoretical model accurately captured the experimental trends of the rate coefficients of the unimolecular decomposition of 13DO.

Cost-Effective Implementation of Multiconformer Transition State Theory for Alkoxy Radical Unimolecular Reactions.

Alkoxy radicals are important intermediates in the gas-phase oxidation of volatile organic compounds (VOCs) determining the nature of the first-generation products. An accurate description of their chemistry under atmospheric conditions is essential for understanding the atmospheric oxidation of VOCs. Unfortunately, experimental measurements of the rate coefficients of unimolecular alkoxy radical reactions are scarce, especially for larger systems. As has previously been done for peroxy radical hydrogen shift reactions, we present a cost-effective approach to the practical implementation of multiconformer transition state theory (MC-TST) for alkoxy radical unimolecular (H-shift and decomposition) reactions. Specifically, we test the optimal approach for the conformational sampling as well as the best value for a cutoff of high-energy conformers. In order to obtain accurate rate coefficients at a reduced computational cost, an energy cutoff is employed to reduce the required number of high-level calculations. The rate coefficients obtained with the developed theoretical approach are compared to available experimental rate coefficients for both 1,5 H-shifts and decomposition reactions. For all but one of the reactions tested, the calculated MC-TST rate coefficients agree with experimental results to within a factor of 7. The discrepancy for the final reaction is about a factor of 15, but part of the discrepancy is caused by pressure effects, which are not included in MC-TST. Thus, for the fastest alkoxy reactions, deviation from the high-pressure limit even at 1 bar should be considered.