Statistical Rate Theory Research Articles

Computational Modeling of the Conformation-Dependent Atmospheric Reactivity of Criegee Intermediates.

The impacts of Criegee intermediates (CIs) on atmospheric chemistry depend significantly on the CI conformation. In this Perspective, I highlight examples of how electronic structure and statistical rate theory calculations, in conjunction with experiment, have revealed conformation-dependent details of both CI ground-state reactivity and electronic excitation. Calculations using single-reference electronic structure methods and conventional transition state theory have predicted that CIs with syn-alkyl or syn-vinyl substituents isomerize rapidly to vinyl hydroperoxides (VHPs) or dioxoles, both of which can decompose rapidly under atmospheric conditions. Ongoing computational research on hydroxyl radical (OH) roaming initiated by VHP dissociation requires the application of multireference electronic structure methods and variational transition state theory. CIs that lack both syn-alkyl and syn-vinyl substituents undergo either bimolecular reaction or π* ← π electronic excitation in the atmosphere. Accurate predictions of CI ultraviolet-visible spectra require multireference calculations with large active spaces and at least a second-order perturbative treatment of dynamic electron correlation. The extent to which electronic spectra can be diagnostic of the presence of specific CI conformers varies significantly with CI chemical identity.

Drying time of plaster within in-fill plasterboard walls and evaporation theory for water within gypsum: methodology, numerical simulation and application

ABSTRACT The desulphurisation-gypsum-filling-wall (DSG-FW) formed by pouring into plasterboard partitions with desulphurisation gypsum slurry can utilise low-quality desulphurisation gypsum and also improve the acoustic insulation issues arising from the use of hollow partitions. The construction schedule of this wall is unable to be controlled because the moisture transfer characteristics in DSG-FW are unclear. Therefore, it is important to determine the drying time and the moisture-transfer behaviour during the construction process. By using different thicknesses of gypsum samples, adding different amounts of perlite and adjusting the relative humidity, whether the gypsum board is covered, the drying and evaporating process of gypsum was compared and analysed. The evaporative characteristics of the desulphurisation gypsum core wall at different relative humidities were determined through experiments. In addition, an evaporation model was introduced for simplification and verification. The experimental results show that the drying of gypsum is correlated with relative humidity and gypsum thickness: the higher the relative humidity, the lower the evaporation rate; the thicker the gypsum, the longer the drying time; perlite and gypsum board have no effect on gypsum drying. The rate of evaporation of DSG-FW increases non-linearly with the decrease in relative humidity under which the DSG-FW is conditioned; the simplified Hertz-Knudsen-Langmuir (HKL-S) model can only be used to predict the evaporation behaviour within the higher relative humidity range (more than 55% RH), while the simplified statistical rate theory (SRT-S) model was verified as being able to predict the rate of evaporation across almost the whole relative humidity range. The SRT-S model can be used to predict and control the construction of DSG-FW in actual engineering work.

Trajectory-Based Time-Resolved Mechanism for Benzene Reductive Elimination from Cyclopentadienyl Mo/W Phenyl Hydride Complexes.

Calculated potential energy structures and landscapes are very often used to define the sequence of reaction steps in an organometallic reaction mechanism and interpret kinetic isotope effect (KIE) measurements. Underlying most of this structure-to-mechanism translation is the use of statistical rate theories without consideration of atomic/molecular motion. Here we report direct dynamics simulations for an organometallic benzene reductive elimination reaction, where nonstatistical intermediates and dynamic-controlled pathways were identified. Specifically, we report single spin state as well as mixed spin state quasiclassical direct dynamics trajectories in the gas phase and explicit solvent for benzene reductive elimination from Mo and W bridged cyclopentadienyl phenyl hydride complexes ([Me2Si(C5Me4)2]M(H)(Ph), M = Mo and W). Different from the energy landscape mechanistic sequence, the dynamics trajectories revealed that after the benzene C-H bond forming transition state (often called reductive coupling), σ-coordination and π-coordination intermediates are either skipped or circumvented and that there is a direct pathway to forming a spin flipped solvent caged intermediate, which occurs in just a few hundred femtoseconds. Classical molecular dynamics simulations were then used to estimate the lifetime of the caged intermediate, which is between 200 and 400 picoseconds. This indicates that when the η2-π-coordination intermediate is formed, it occurs only after the first formation of the solvent-caged intermediate. This dynamic mechanism intriguingly suggests the possibility that the solvent-caged intermediate rather than a coordination intermediate is responsible (or partially responsible) for the inverse KIE value experimentally measured for W. Additionally, this dynamic mechanism prompted us to calculate the kH/kD KIE value for the C-H bonding forming transition states of Mo and W. Surprisingly, Mo gave a normal value, while W gave an inverse value, albeit small, due to a much later transition state position.

Bridge bearing damage identification based on statistical moment change rate

In this study, a comprehensive method is proposed to identify bridge bearing damage based on the statistical moment rate of change for simply supported beam bridges. The novelty of this study lies in two aspects: (1) establish the statistical moment change rate theory, which derive the relationship between statistical moment and modal shape; and (2) propose a new method for identifying vibration modes and bridge bearing damage using statistical moment and statistical moment change rate, respectively. The proposed method has a strong robustness with a high computational efficiency. Under the proposed method, the statistical moment theory is introduced and the statistical moment change rate index is constructed. Theoretical derivation shows that the index can accurately identify bearing damage. Subsequently, this method is investigated through numerical simulation and performed well in different situations, such as high-level measurement noise. Furthermore, it was validated via on-site testing on Qingshui bridge and a certain bridge in Chongqing. Additionally, the statistical moment change rate allows the damage location to be identified rapidly, thus providing a new method for bridge bearing damage identification.

Ab Initio and Statistical Rate Theory Exploration of the CH (X2Π) + OCS Gas-Phase Reaction.

The first theoretical results regarding the gas-phase reaction mechanism and kinetics of the CH (X2Π) + OCS reaction are presented here. This reaction has a proposed importance in the removal of OCS in regions of the interstellar medium (ISM) and has the potential to form the recently observed HCS/HSC isomers, with both constitutional isomers having recently been observed in the L483 molecular cloud in a 40:1 ratio. Statistical rate theory simulations were performed on stationary points along the reaction potential energy surface (PES) obtained from ab initio calculations at the RO-CCSD(T)/aug-cc-pV(Q+d)Z//M06-2X-D3/aug-cc-pV(Q+d)Z level of theory over the temperature and total density range of 150-3000 K and 1011-1024 cm-3, respectively, using a Master Equation analysis. Exploration of the reaction potential energy surface revealed that all three pathways identified to create CS + HCO products required surmounting barriers of 16.5 kJ mol-1 or larger when CH approached the oxygen side of OCS, rendering this product formation negligible below 1000 K, and certainly under low-temperature ISM conditions. In contrast, when CH approaches the sulfur side of OCS, only submerged barriers are found along the reaction potential energy surface to create HCCO + S or CO + HCS, both of which are formed via a strongly bound OCC(H)S intermediate (-358.9 kJ mol-1). Conversion from HCS to HSC is possible via a barrier of 77.8 kJ mol-1, which is still -34.1 kJ mol-1 below the CH + OCS entrance channel. No direct route from CH + OCS to H + CO + CS was found from our ab initio calculations. Rate theory simulations suggest that the reaction has a strong negative temperature dependence, in accordance with the barrierless addition of CH to the sulfur side of OCS. Product branching fractions were also determined from MESMER simulations over the same temperature and total density range. The product branching fraction of CO + HCS reduces from 79% at 150 K to 0.0% at 800 K, while that of HCS dissociation to H + CS + CO increases from 22% at 150 K to 100% at 800 K. The finding of CO + HCS as the major product at the low temperatures relevant to the ISM, instead of H + CS + CO, is in opposition to the current supposition used in the KIDA database and should be adapted in astrochemical models as another source of the HCS isomer.

Temperature -and pressure-dependent branching ratios for 2,6-dimethylheptyl radicals (C9H19) + O2 reaction: An ab initio and RRKM/ME approach on a key component of bisabolane biofuel

Chemical kinetics mechanisms contain elementary reactions and associated Arrhenius rate parameters are necessary to understand the modeling of hydrocarbon autoignition chemistry. The complexity of such mechanisms is increased due to interest in operating next-generation combustion strategies in the low-temperature region (≤1000 K), which is governed by O2-addition to alkyl radicals (R), subsequent radical isomerization and decomposition steps thereafter. In this work, we report theoretically the reaction of molecular oxygen to the four isomers of 2,6-dimethylheptyl radicals (C9H19). The stationary points on potential energy surfaces (PES), rate constants, and branching ratios from C9H19 isomers initiated by O2 has been investigated by a combination of ab initio/density functional theory (CBS-QB3) and advanced statistical rate theory i.e., microcanonical variational transition state theory (μCVTST) and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) simulations. The temperature- and pressure-dependent rate constants and branching ratios over temperature range 400 K–1000 K (with an interval of 50 K) and with different pressures 0.001, 0.01, 0.1, 1, 10 and 100 bar were computed. The RRKM/ME calculations reveal for addition of O2 to the primary radical, 2,6-dimethylhept-1-yl + O2 reaction, the formation of 2-iso-butyl-4-methyl-tetrahydrofuran (a five membered cyclic ether) + OH via concerted C–C and O–O bond scission of primary-secondary QOOH and formation of ROO-1 is the kinetically favorable pathway below 600 K. For 2,6-dimethylhept-2-yl + O2, three competitive favorable channels lead to the formation of 5-iso-propyl-2,2-dimethyl-tetrahydrofuran + OH, a five membered cyclic ether formed coincident with OH in a chain-propagating step from decomposition of tertiary-secondary hydroperoxyalkyl (QOOH), formation of a six membered cyclic ether i.e., 2,2,6,6-tetramethylpyran and ROO-2, however at higher temperature (>900 K) and 2–6-dimethyl-2-heptene is contributed equally over other products. For 2,6-dimethylhept-3-yl + O2 reaction, leading to 5-iso-propyl-2,2-dimethyltetrahydrofuran + OH (same products as in ROO-2) from decomposition of secondary–tertiary hydroperoxyalkyl (QOOH). For 2,6-dimethylhept-4-yl + O2 reaction, it reveals the formation of chain-propagation channels leading to iso-butene + methyl-isobutanal + OH. The results of this study reveals the importance of temperature- and pressure-dependent branching ratios, which is necessary to understand the low temperature internal combustion mechanism. The unimolecular rate constants were also compared with the previously reported values for 2,6-dimethylheptyl radical + O2 and similar reaction system i.e., 2,5-dimethylhexyl radical + O2. The updated rate constants and branching ratios may also serve as general prototypes for low-temperature oxidation of branched alkanes of next-generation biofuels with similar structural motifs, such as bisabolane and farnesane.

Investigation of the reactions NCN + CH3, NCN + OH, and CH3 + OH behind shock waves

The NCN radical plays a key role for modeling prompt-NO formation in hydrocarbon flames. As both CH3 and OH radicals are present at high mole fraction level in the reaction zone, their radical-radical cross-reactions with NCN are of special interest, but no experimental rate constants are available. In contrast to the OH reaction, the reaction with CH3 is not considered in common detailed flame mechanisms as yet. It is shown in this study, however, that the NCN + CH3 reaction proceeds close to collision limit and therefore acts as one of the most important reactions to describe the fate of NCN in flames. NCN and OH concentration-time profiles were recorded behind shock waves. The thermal decomposition of cyanogen azide (NCN3) and tert‑Butyl hydroperoxide (TBHP) served as quantitative source of NCN and CH3/OH radicals. Representing a secondary reaction under the applied experimental conditions, the rate constant of the reaction CH3 + OH has been first determined from OH profiles with TBHP as precursor. With k(CH3 + OH) = 4.34 × 10–10 × (T / K)6.38 exp(+68.0 kJ mol–1 / RT) cm3 mol–1 s–1 (±40%, 925 – 1800 K, p ≈ 393 mbar) it was found to be in good agreement with available literature results. Rate constants of the reaction NCN + CH3 were then determined from NCN profiles by shock-heating gas mixtures with TBHP and NCN3. According to previous as well as our own statistical rate theory calculations, the only relevant product channel is the formation of CH2NH + CN. In the temperature range 857 – 1817 K and at a pressure of p ≈ 314 mbar, a high value for the rate constant k(NCN + CH3) = 5.40 × 1013 exp(+1.77 kJ mol–1 / RT) cm3 mol–1 s–1 (±30%) was found. For future implementation into flame models, we recommend the rate constant expression k(NCN + CH3) = 8.44 × 1019 (T / K) –1.76 exp(–15.2 kJ mol–1 / RT) cm3 mol–1 s–1 (±40%, 800 – 4000 K), which is based on a theoretically assisted extrapolation of the experimental data. Finally, OH profiles in the presence of CH3 and NCN have been analyzed. In agreement with previous theoretical estimates, the reaction is indeed comparably slow and therefore only a rough upper limit of k(NCN + OH) < 8 × 1012 cm3mol–1s–1 (918 – 1808 K, p ≈ 415 mbar) could be inferred from the experiments.

Pyrolysis of Methyl Formate and the Reaction of Methyl Formate with H Atoms: Shock Tube Experiments and Statistical Rate Theory.

Methyl formate (MF) is the smallest carboxylic ester and currently considered a promising alternative fuel. It can also serve as a model compound to study the combustion chemistry of the ester group, which is a typical structural feature in many biodiesel components. In the present work, the pyrolysis of MF was investigated behind reflected shock waves at temperatures between 1430 and 2070 K at a nominal pressure of 1.1 bar. Both time-resolved hydrogen atom resonance absorption spectroscopy (H-ARAS) and time-resolved time-of-flight mass spectrometry (TOF-MS) were used for species detection. Additionally, the reaction of MF and perdeuterated MF-d4 with H atoms was investigated at temperatures between 1000 and 1300 K at nominal pressures of 0.4 and 1.1 bar with H-ARAS. In the latter experiments, ethyl iodide served as precursor for H atoms. Rate coefficients of seven parallel unimolecular decomposition channels of MF and five parallel reaction channels of the MF + H reaction were calculated from statistical rate theory on the basis of molecular and transition state data from quantum chemical calculations. These calculated rate coefficients were implemented into an MF pyrolysis/oxidation mechanism from the literature, and the experimental concentration-time profiles of H (from ARAS) as well as MF, CH3OH, HCHO, and CO (from TOF-MS) were modeled. It turned out that the literature mechanism, which was originally validated against flow-reactor experiments, ignition delay times, and laminar burning velocities, was generally able to fit also the concentration-time profiles from the shock tube experiments reasonably well. The agreement could still be improved by substituting the original rate coefficients, which were estimated from structure-reactivity relationships, by the values calculated from statistical rate theory in the present work. Details of the channel branching are discussed, and the updated mechanism is given, also in machine-readable form.

Effect of formic acid on O2 + OH˙CHOH → HCOOH + HO2 reaction under tropospheric condition: kinetics of cis and trans isomers.

Formic acid (HCOOH) is one of the highly abundant acids in the troposphere. It is important in the formation of atmospheric aerosols and impacts the acidity of rainwater. In the present scenario, the model chemistry of HCOOH(FA) sources and sinks is poorly understood. In this work, we apply quantum chemical methods coupled with advanced statistical rate theories to understand the production of FA and its catalytic behavior under tropospheric conditions. The potential energy surfaces (PES) for O2 + OH˙CHOH and O2 + OH˙CHOH(+FA) reactions were constructed using the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level and rate constants for the production of FA were predicted using Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) simulation with Eckart tunnelling and canonical variational transition state theory (CVT) with small curvature tunnelling (SCT) between the temperature range of 200-320 K and pressure range of 0.001 to 10 bar. The reaction follows the formation of cis and trans intermediates followed by spontaneous decomposition via concerted HO2 elimination to form stable post intermediates, which then leads to the straight formation of cis and trans formic acid. The current study also helps understand the role of cis and trans contribution to the total rate constants. The results show that O2 + OH˙CHOH is dominated by both cis and trans isomers; however, the trans isomer plays a more important role in the catalytic reaction. This result is due to the formation of a strong hydrogen-bonded complex in the trans isomer, which is dominated by the enthalpy factor rather than the entropy factor. The results predict that the catalytic effect of FA on the O2 + OH˙CHOH reaction is important when the concentration of FA is not included in the calculations; however, it has no effect under tropospheric conditions, when the FA concentration is included in the calculation. As a result, the total effective reaction rate constants are smaller. This work provides experimental/theoretical confirmation of Franco et al. who predicted that methanediol is the precursor for the FA formation, resolving an open problem in the kinetics of the gas phase reaction. O2 + OH˙CHOH and O2 + OH˙CHOH(+FA) probably explain other diol systems, which can help explain the formation of other atmospheric acids that affect aerosol formation and cloud evolution.

A comparative study of experiments and theories on steady-state evaporation of water

A precise description of energy and mass transport across the liquid-vapor interface of water is central in disciplines spanning from climatology to seawater desalination. We present a critical assessment of six recent experimental data sets that report temperature jumps, vapor pressures, and evaporation rates during steady-state evaporation of water. The experimental data were used to test available theories. Three state-of-the-art theories that take the resistance of the liquid-vapor interface into account were compared; statistical rate theory, non-equilibrium thermodynamics, and kinetic theory of gases. Statistical rate theory appears to under-predict the difference between the saturation pressure and the actual pressure of the vapor phase. Interface transfer coefficients for water compatible with non-equilibrium thermodynamics theory were determined. These coefficients predict the right order of magnitude of the evaporation fluxes from the different data sets. However, inconsistencies between the different data sets and indications of systematic measurement errors were identified during the determination and evaluation of these coefficients. The condensation coefficient in kinetic theory of gases computed from the experimental data span two orders of magnitude. All three theories were found to depend much on a precise determination of the conditions at the interface, in particular on the difference between the vapor phase pressure and the saturation pressure. Already a shift of 1–5 Pa changes the predicted evaporation rates significantly. For certain experiments, a change of 2 Pa modifies the evaporation rate predicted by statistical rate theory by one order of magnitude. Overall, we show that determination of vapor pressures to a higher accuracy (<0.3 Pa) is needed to enhance the understanding of evaporation mechanisms and which theory to use.

Re-evaluation of rate constants for the reaction N2H4 (+ M) ⇄ NH2 + NH2 (+ M)

Rate constants for the dissociation/recombination reaction N2H4 (+ M) ⇄ NH2 + NH2 (+ M) are determined by a combination of quantum-chemical calculations and statistical unimolecular rate theory. Between 1100 and 2500 K, limiting low-pressure rate constants for hydrazine dissociation in the bath gas Ar of k0 = [Ar] 6.1 × 1020 (T/1000 K)−7.3 exp (-34490 K/T) cm3 mol−1 s−1, limiting high-pressure rate constants of k∞ = 7.6 × 1016 (T/1000 K)−1.0 exp(-33600 K/T) s − 1, and center broadening factors of the falloff curves (between 1100 and 1600 K in Ar) of Fcent = 0.71 exp(- T/1460 K) + 0.29 exp(- T/21 K) + exp(-13400 K/T) were calculated. Using equilibrium constants KC = 1.7 × 103 (T/1000 K)−1.5 exp(-33,460 K/T) mol cm−3, between 300 and 600 K limiting low-pressure rate constants for the reverse recombination of NH2 radicals in the bath gas N2 of krec,0 = [N2] 4.4 × 1020(T/400 K)−6.9 exp (-1630 K/T) cm6 mol−2 s−1, limiting high-pressure rate constants of krec,∞ = 4.4 × 1013 (T/1000 K)0.44 exp(-140 K/T) cm3 mol-1 s−1, and center broadening factors (between 300 and 600 K in N2) of Fcent = exp(- T/1130 K) + exp (-10340 K/T) were obtained. A comparison with experimental results for hydrazine dissociation from the literature suggests incomplete falloff extrapolations toward k∞ and experimental problems in the determination of k0 at temperatures above about 1600 K. Implications of the present re-evaluated rate constants for the modeling of high temperature ammonia oxidation kinetics are discussed, showing an only small influence of their precise values on the overall properties of the process.

Effect of salt on the adsorption of ionic surfactants at the air-water interface

• NaCl has significant effect on the equilibrium and dynamic surface tension of ionic surfactant solutions. • The SRT model predicts the adsorption dynamics more accurately than the DKC model. • Salt bridging leads to the compact packing of ionic surfactant at the air-water interface. The combined effect of ionic surfactants and inorganic salts on the interfacial properties is of great importance to various industrial processes. In this work, the effect of ionic surfactants (SDS and CTAB), in the presence of inorganic salt (NaCl), on the surface tension is systematically investigated using experiments, theoretical models, and all-atom molecular dynamics simulations. The equilibrium and dynamic surface tension of surfactant-salt-water systems in a salinity range of 0.01–0.1 M are measured using the pendant drop method. The theoretical models viz., statistical rate theory and diffusion kinetic controlled, suggest that adsorption drives the migration of surfactant molecules to the interface. Finally, the molecular dynamics simulations illustrate increased surfactant packing at the air-liquid interface in the presence of salt due to counterions bridging between the surfactant molecules, which results in a lower value of surface tension for both the surfactants.

Probing the transient evolution and heat transfer of nascent liquid film during initial condensation

A hybrid theoretical and simulation method is employed to investigate the transient characteristics of heat and mass transfer of nascent liquid film during initial condensation of argon vapor on a subcooled substrate. The method combines a non-equilibrium thermodynamics analysis with molecular dynamics simulations to provide the kinetics and evolution of the atoms. Throughout the evolution process, three different stages can be recognized by the variation of interfacial mass flux. Initially, the solid-vapor interfacial effect is dominated and the argon atoms are adsorbed as clusters and aggregate on the surface. In the transition stage, the mass flux increases and the adsorption behavior almost vanishes, a nascent liquid-like film is formed. When the times exceed a critical value of tc, the condensation process is initiated and the solid-vapor interface is replaced by the vapor-liquid interface. As a result, the mass flux keeps constant. During the adsorption process, the driven force of is theoretical determined through the cluster evolutions. With the decrease of cooling temperature, the mass flux is enlarged and the time for the transition becomes earlier. The mass flux determined from MD simulations coincides well with the theoretical analysis from statistical rate theory. Meanwhile, the spatial evolutions of the atoms are also traced. The atoms in the near wall region present a crystal-like structure. Compared with that in bulk vapor phase, the atoms in the liquid film have a stronger mobility. For the heat and mass transportation, the interfacial temperature jump and the thermal resistance is increased with the increase of cooling temperature. Two independent methods are adopted to obtain the heat and mass fluxes. The slop of the relationship between the heat and mass fluxes shows close agreement with the latent heat of argon, which reveals the occurrence of condensation.

Molecular and theoretical identification of adsorption phase transition behaviors via thermo-kinetics analysis

In order to identify the adsorption phase transition behaviors, a series of thermo-kinetics investigation is performed through theoretical analysis integrated with molecular dynamics simulations. Vary with the vapor pressures and cooling temperatures, three regions are distinguished during the phase transition process, they are stable adsorption region, metastable adsorption and condensation regions. For the stable state, the adsorption equilibrium is reached and the thickness of adsorption layer remains constant. In the metastable state, the thickness of the adsorption layer grows slightly. In the final state, condensation is initiated, a liquid film is formed and it thickens unlimitedly. From the theoretical adsorption model and statistical rate theory, the thermos-kinetics behaviors are presented and the temperature jump across the interface is discussed. Also, the boundaries for dividing the transition process are determined. Results show that the critical pressure ratio for the transition from stable state to the metastable adsorption is decreased with the increase of solid cooling temperature. Once the temperate is beyond 129 K, the critical pressure ratio keeps constant. For the transition from metastable adsorption to condensation, the critical condition is determined from the kinetic behaviors of adsorption clusters. With the increase of pressure ratio, the clusters are formed and aggregated. Once the pressure ratio approaches a certain value, the first-order phase transition occurs, then a homogenous liquid film is formed. The adsorption phase transition process presented by molecular dynamics simulations coincides well with that predicted from theoretical models.

Prediction model of LNG weathering using net mass and heat transfer

In an liquefied natural gas (LNG) storage tank, the composition of LNG changes with time owing to the boil-off gas (BOG) generation, and it is a crucial characteristic in designing a BOG management system. Until recently, the vapor phase temperature and the heat exchange at the interface have been utilized to predict the non-equilibrium thermodynamic behavior of LNG and BOG. However, the measured vapor phase temperature cannot represent the entire vapor phase owing to thermal stratification, and it is difficult to quantify the heat transfer at the interface rigorously in a large-scale tank. Thus, this paper proposes a prediction model of LNG weathering, which excludes vapor phase modeling. To facilitate this approach, the net mass transfer was calculated using the statistical rate theory, which converts a temperature term into pressure, and the net heat transfer at the interface was also applied. To verify the proposed model, we performed a long-term (50 days) experiment in a large-scale tank (50 m3) containing nitrogen and hydrocarbons, under isobaric operation. The simulation results showed good agreement with the measured data. In addition, the applicable range for the net expression equations was studied by measuring the temperature difference between the two phases.

An Experimental and Theoretical Investigation of the Gas-Phase C(3P) + N2O Reaction. Low Temperature Rate Constants and Astrochemical Implications.

The reaction between atomic carbon in its ground electronic state, C(3P), and nitrous oxide, N2O, has been studied below room temperature due to its potential importance for astrochemistry, with both species considered to be present at high abundance levels in a range of interstellar environments. On the experimental side, we measured rate constants for this reaction over the 50-296 K range using a continuous supersonic flow reactor. C(3P) atoms were generated by the pulsed photolysis of carbon tetrabromide at 266 nm and were detected by pulsed laser-induced fluorescence at 115.8 nm. Additional measurements allowing the major product channels to be elucidated were also performed. On the theoretical side, statistical rate theory was used to calculate low temperature rate constants. These calculations employed the results of new electronic structure calculations of the 3A″ potential energy surface of CNNO and provided a basis to extrapolate the measured rate constants to lower temperatures and pressures. The rate constant was found to increase monotonically as the temperature falls (kC(3P)+N2O (296 K) = (3.4 ± 0.3) × 10-11 cm3 s-1), reaching a value of kC(3P)+N2O (50 K) = (7.9 ± 0.8) × 10-11 cm3 s-1 at 50 K. As current astrochemical models do not include the C + N2O reaction, we tested the influence of this process on interstellar N2O and other related species using a gas-grain model of dense interstellar clouds. These simulations predict that N2O abundances decrease significantly at intermediate times (103 - 105 years) when gas-phase C(3P) abundances are high.

Effect of Water and Formic Acid on ·OH + CH4 Reaction: An Ab Initio/DFT Study

In this work, we used ab initio/DFT method coupled with statistical rate theory to answer the question of whether or not formic acid (HCOOH) and water molecules can catalyze the most important atmospheric and combustion prototype reaction, i.e., ·OH (OH radical) + CH4. The potential energy surface for ·OH + CH4 and ·OH + CH4 (+X) (X = HCOOH, H2O) reactions were calculated using the combination of hybrid-density functional theory and coupled-cluster theory with Pople basis set [(CCSD(T)/ 6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd)]. The results of this study show that the catalytic effect of HCOOH (FA) and water molecules on the ·OH + CH4 reaction has a major impact when the concentration of FA and H2O is not included. In this situation the rate constants for the CH4 + HO···HCOOH (3 × 10−9 cm3 molecule−1 s−1) reaction is ~105 times and for CH4 + H2O···HO reaction (3 × 10−14 cm3 molecule−1 s−1 at 300 K) is ~20 times higher than ·OH + CH4 (~6 × 10−15 cm3 molecule−1 s−1). However, the total effective rate constants, which include the concentration of both species in the kinetic calculation has no effect under atmospheric condition. As a result, the total effective reaction rate constants are smaller. The rate constants when taking the account of the FA and water for CH4 + HO···HCOOH (4.1 × 10−22 cm3 molecule−1 s−1) is at least seven orders magnitude and for the CH4 + H2O···HO (7.6 × 10−17 cm3 molecule−1 s−1) is two orders magnitude smaller than ·OH + CH4 reaction. These results are also consistent with previous experimental and theoretical studies on similar reaction systems. This study helps to understand how FA and water molecules change the reaction kinetic under atmospheric conditions for ·OH + CH4 reaction.

Improved computational modeling of the kinetics of the acetylperoxy + HO2 reaction.

The acetylperoxy + HO2 reaction has multiple impacts on the troposphere, with a triplet pathway leading to peracetic acid + O2 (reaction (1a)) competing with singlet pathways leading to acetic acid + O3 (reaction (1b)) and acetoxy + OH + O2 (reaction (1c)). A recent experimental study has reported branching fractions for these three pathways (α1a, α1b, and α1c) from 229 K to 294 K. We constructed a theoretical model for predicting α1a, α1b, and α1c using quantum chemical and Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) simulations. Our main quantum chemical method was Weizmann-1 Brueckner Doubles (W1BD) theory; we combined W1BD and equation-of-motion spin-flip coupled cluster (SF) theory to treat open-shell singlet structures. Using RRKM/ME simulations that included all conformers of acetylperoxy-HO2 pre-reactive complexes led to a 298 K triplet rate constant, k1a = 5.11 × 10-12 cm3 per molecule per s, and values of α1a in excellent agreement with experiment. Increasing the energies of all singlet structures by 0.9 kcal mol-1 led to a combined singlet rate constant, k1b+1c = 1.20 × 10-11 cm3 per molecule per s, in good agreement with experiment. However, our predicted variations in α1b and α1c with temperature are not nearly as large as those measured, perhaps due to the inadequacy of SF theory in treating the transition structures controlling acetic acid + O3 formation vs. acetoxy + OH + O2 formation.

Gas Phase Protolysis of Trisarylzincate Anions.

We have applied a combination of tandem-mass spectrometry, quantum-chemical calculations, and statistical rate theory computations to examine the gas phase reactions between the trisarylzincate anions ArXZnPh2- (ArX = p-X-C6H4, X = NMe2, OMe, Me, H, F, and Cl) and 2,2,2-trifluoroethanol at T = 310 ± 20 K. The observed reactions bring about the protonation of one of the aryl anions, which is then released as the corresponding arene, while the formed alkoxide binds to the zinc center. The protonation is faster for the more electron-rich aryl groups and shows a linear Hammett plot if the rate constant for X = NMe2 is discarded from the analysis. Although the reactions are highly exothermic, they proceed only with relatively low efficiencies (0.1% ≤ φ ≤ 1.3%). According to the quantum-chemical calculations, this behavior can be ascribed to the reactions proceeding through a double-well potential with a tight transition structure located at the central barrier. Based on these potential energy surfaces, the statistical rate theory computations can reproduce the measured rate constants within factors of 2 to 8. A comparison of the protolysis of the trisarylzincates with that of the corresponding free aryl anions demonstrates how the coordination to the metal center not only stabilizes the carbanions energetically but also moderates their reactivity. Thus, our gas phase study contributes to a better understanding of the fundamentals of organometallic reactivity.

Quasi-equilibrium evaporation characteristics of oxygen in the liquid–vapor interfacial region

Evaporation is a ubiquitous non-equilibrium phenomenon which plays a vital role in both micro and macro phase transition systems. Leading evaporation theories, such as the kinetic gas theory and the statistical rate theory, rely on the interfacial temperature data to predict the mass flux. Compared to water and other room temperature fluids, the interfacial temperature measurement for cryogenic fluids were seldom reported, which means the evaporation calculations at cryogenic temperatures have to use regular correlations or coefficients developed from non-cryogenic fluids. In the present study, a visualized and controlled evaporation system was designed and fabricated. Liquid oxygen was used as the testing fluid since it is the common oxidant of the cryogenic propellants. The temperature distributions in the interfacial regions of steady-state evaporating liquid oxygen were obtained at a set of mass flux and pressure. Temperature discontinuities up to 0.79 K were observed across the liquid–vapor interface of evaporating oxygen. It was found that the liquid was overheated. The measurement implied that the examined evaporation theories lose their accuracy in predicting mass flux rate and temperature jump at cryogenic temperatures by deviations on several orders of magnitude. A modified expression for the evaporation coefficient was provided which shows higher accuracy than early expressions especially in atmospheric pressure regions.