High-pressure Limit Research Articles

Theoretical and Kinetic Study of the Hydrogen Atom Abstraction Reactions of Esters with HȮ2 Radicals

This work details an ab initio and chemical kinetic study of the hydrogen atom abstraction reactions by the hydroperoxyl radical (HȮ2) on the following esters: methyl ethanoate, methyl propanoate, methyl butanoate, methyl pentanoate, methyl isobutyrate, ethyl ethanoate, propyl ethanoate, and isopropyl ethanoate. Geometry optimizations and frequency calculations of all of the species involved, as well as the hindrance potential descriptions for reactants and transition states, have been performed with the Møller-Plesset (MP2) method using the 6-311G(d,p) basis set. A validation of all of the connections between transition states and local minima was performed by intrinsic reaction coordinate calculations. Electronic energies for all of the species are reported at the CCSD(T)/cc-pVTZ level of theory in kcal mol(-1) with the zero-point energy corrections. The CCSD(T)/CBS (extrapolated from CCSD(T)/cc-pVXZ, in which X = D, T, Q) was used for the reactions of methyl ethanoate + HȮ2 radicals as a benchmark in the electronic energy calculations. High-pressure limit rate constants, in the temperature range 500-2000 K, have been calculated for all of the reaction channels using conventional transition state theory with asymmetric Eckart tunneling corrections. The 1-D hindered rotor approximation has been used for the low frequency torsional modes in both reactants and transition states. The calculated individual and total rate constants are reported for all of the reaction channels in each reaction system. A branching ratio analysis for each reaction site has also been investigated for all of the esters studied in this work.

First-principles kinetics of n-octyl radicals

Kinetics of the isomerisation and unimolecular degradation of n-octyl radicals have been studied with the reaction class transition state theory (RC-TST) method. To explore the kinetics of the 1,7-H migration reactions family, the accurate high-pressure limits of the rate constants for the reference reaction of this class (1-heptyl → 1-heptyl) have been calculated. Finally, both the achievements reported in this paper and previous developments are employed to obtain theoretical branching ratios of intramolecular H-transfers and unimolecular degradations of all possible n-octyl radicals; the results are in satisfactory agreement when compared to experiment. The application of the rates obtained to the simulation of a simple reactor is also reported.

Reaction of ketenyl radical with hydroxyl radical over C2H2O2 potential energy surface: A theoretical study

Abstract The potential energy surface of C 2 H 2 O 2 for HCCO + OH bimolecular reactions and the unimolecular decomposition of hydroxyketene (HOCH C O) are investigated employing high-level quantum chemical methods. Variable reaction coordinate transition state theory is used for the high-pressure limit rate constant calculation of the main reaction channels and RRKM-based multiwell master equation is used to calculate the pressure dependent rate constants and product branching ratios of these channels. The predicted rate constants are in good agreement with the limited experimental data available in the literature. The product distribution analysis shows that the association/decomposition of HCCO + OH to the formation of CO + 1 HCOH and CO + 3 HCOH channels are dominant in the whole temperature range of 500–2000 K below 1 atm, whereas at higher pressure and low temperature, the association reaction producing 3 CHCOOH becomes competitive. For the unimolecular decomposition of hydroxyketene, the formation of CO + 1 HCOH channel is found to be predominant over a wide range of temperatures and pressures. Rate constants of these reactions and thermodynamic parameters for species involved in these reactions are also provided.

Kinetic Modeling of Hydrogen Abstractions Involving Sulfur Radicals

One of the requisites for the development of detailed reaction networks is the availability of accurate kinetic data. Group additivity based models linking the Arrhenius parameters to structural characteristics of the transition state have proven to be a valuable tool to obtain those data. In this work, group additivity values are presented to allow a broad range of CH and SH hydrogen abstraction reactions by S radicals to be modeled. Rate coefficients in the temperature range from 300 to 1500 K are obtained by using the CBS-QB3 method in the high-pressure limit and are corrected for tunneling and anharmonicity of rotation about the transitional bond. A total of 149 reactions are studied. From these reactions, a total of 52 group additivity values and 35 resonance corrections are derived. The general applicability of the group additivity method is demonstrated for a test set containing 25 reactions. At 300 K, rate coefficients are on average reproduced within a factor of 2.8. The mean absolute deviations on the Arrhenius parameters are 2 kJ mol(-1) for the activation energy and 0.38 for log A in which A is the pre-exponential factor.

Accident hypoxique en anesthésie pédiatrique dû à un réglage inadapté du respirateur

A 3-year-old child was anesthetized for ENT examination and surgery. After induction and tracheal intubation, the patient was ventilated (controlled mode). The respirator screen showed information compatible with a failure of intubation: no expired CO2, no expired flow, no alarm of high pressure limit, and no respiratory chest movement. A fall of SpO2 appeared rapidly which recovered after extubation and manual ventilation through a face mask and reintubation. The expiratory CO2 was present when the patient was ventilated manually and disappeared under controlled ventilation. The increase in the value of the maximal insufflation pressure allowed efficient ventilation with an expiratory CO2 curve and showed high ventilation pressure compatible with a bronchospasme. This case report shows that in case of bronchospasme, if the value of the maximal insufflation pressure is low, this may lead to an erroneous diagnosis of failure of intubation.

Non-Born–Oppenheimer molecular dynamics of the spin-forbidden reaction O(3P) + CO(X 1Σ+) → CO2($\tilde X{}^1\Sigma _g^ +$X̃Σg+1)

The lowest-energy singlet (1 (1)A') and two lowest-energy triplet (1 (3)A' and 1 (3)A") electronic states of CO2 are characterized using dynamically weighted multireference configuration interaction (dw-MRCI+Q) electronic structure theory calculations extrapolated to the complete basis set (CBS) limit. Global analytic representations of the dw-MRCI+Q∕CBS singlet and triplet surfaces and of their CASSCF∕aug-cc-pVQZ spin-orbit coupling surfaces are obtained via the interpolated moving least squares (IMLS) semiautomated surface fitting method. The spin-forbidden kinetics of the title reaction is calculated using the coupled IMLS surfaces and coherent switches with decay of mixing non-Born-Oppenheimer molecular dynamics. The calculated spin-forbidden association rate coefficient (corresponding to the high pressure limit of the rate coefficient) is 7-35 times larger at 1000-5000 K than the rate coefficient used in many detailed chemical models of combustion. A dynamical analysis of the multistate trajectories is presented. The trajectory calculations reveal direct (nonstatistical) and indirect (statistical) spin-forbidden reaction mechanisms and may be used to test the suitability of transition-state-theory-like statistical methods for spin-forbidden kinetics. Specifically, we consider the appropriateness of the "double passage" approximation, of assuming statistical distributions of seam crossings, and of applications of the unified statistical model for spin-forbidden reactions.

Quantum Chemical and Kinetics Study of the Thermal Gas Phase Decomposition of 2-Chloropropene

A detailed theoretical study of the kinetics of the thermal decomposition of 2-chloropropene over the 600-1400 K temperature range has been done. The reaction takes place through the elimination of HCl with the concomitant formation of propyne or allene products. Relevant molecular properties of the reactant and transition states were calculated for each reaction channel at 14 levels of theory. From information provided by the BMK, MPWB1K, BB1K, M05-2X, and M06-2X functionals, specific for chemical kinetics studies, high-pressure limit rate coefficients of (5.8 ± 1.0) × 10(14) exp[-(67.8 ± 0.4 kcal mol(-1))/RT] s(-1) and (1.1 ± 0.2) × 10(14) exp[-(66.8 ± 0.5 kcal mol(-1))/RT] s(-1) were obtained for the propyne and allene channels, respectively. The pressure effect over the reaction was analyzed through the calculation of the low-pressure limit rate coefficients and falloff curves. An analysis of the branching ratio between the two channels as a function of pressure and temperature, based on these results and on computed specific rate coefficients, show that the propyne forming channel is predominant.

Simple cubic equation of state applied to hard-sphere, Lennard-Jones fluids, simple fluids and solids

Based on the expansion and extension of the virial equation of state (EOS) of hard-sphere fluids solved by the Percus–Yevick integration equation, a universal cubic (UC) EOS is developed. The UC EOS is applied to model hard-sphere and Lennard-Jones (LJ) fluids, simple Ar and N2 liquids at low temperatures, and supercritical Ar and N2 fluids at high temperatures, as well as ten solids, respectively. The three parameters are determined for the hard-sphere fluid by fitting molecular dynamics (MD) simulation data of the third to eighth virial coefficients in the literature; for other fluids by fitting isothermal compression data; and for solids by using the Einstein model. The results show that the UC EOS gives better results than the Carnahan–Starling EOS for compressibility of hard-sphere fluids. The Helmholtz free energy and internal energy for LJ fluids are predicted and compared with MD simulation data. The calculated pressures for simple Ar and N2 liquids are compared with experimental data. The agreement is fairly good. Eight three-parameter EOSs are applied to describe isothermals of ten typical solids. It is shown that the UC EOS gives the best precision with correct behavior at high-pressure limitation. The UC EOS considering thermal effects is used to analytically evaluate the isobaric thermal expansivity and isothermal compressibility coefficients. The results are in good agreement with experimental data.

Rate Rules, Branching Ratios, and Pressure Dependence of the HO2 + Olefin Addition Channels

In this work, we present high-pressure rate rules and branching ratios for the addition of HO2 to olefins through the concerted addition channel to form an alkyl peroxy radical (HO2 + olefin → RO2) and through the radical addition channel to form a β-hydroperoxy alkyl radical (HO2 + olefin → β-QOOH). These rate rules were developed by calculating rate constants for a series of addition reactions involving olefins with varying degrees of branching. The individual rate expressions were determined from electronic structure calculations performed at the CBS-QB3 level of theory combined with TST calculations. The calculated rate constants were found to be in good agreement with those reported in the literature. Next, we calculated apparent pressure- and temperature-dependent rate constants for HO2 addition to the terminal site of 1-butene using an energy-grained master equation (ME) approach and QRRK calculations with a modified strong collision (MSC) approximation. The two methods gave similar results for both reaction classes. We found that, for the radical addition reaction, the high-pressure limit for the stabilization channel is not reached until unusually high pressures (>1000 atm). Instead, this reaction leads to the direct formation of an oxirane + OH. In general, the results for the major channels are in reasonable agreement with prior theoretical and experimental data. Finally, to explicitly examine the effect of pressure, we compared concentration-time profiles for the reactions of HO2 plus butene in air that were obtained using both high-pressure and pressure-dependent mechanisms at 10 and 100 atm. These simulations showed that, contrary to general expectations, the manifestation of pressure falloff effects in kinetic modeling studies might be more prevalent at increasing pressures. This behavior is attributed to the reaction of β-QOOH with O2, the rate of which increases with increasing pressure of air. This bimolecular reaction competes with the unimolecular reactions of β-QOOH under conditions where falloff effects are important for that channel.

Effect of Roaming Transition States upon Product Branching in the Thermal Decomposition of CH3NO2

The kinetics for the thermal unimolecular decomposition of CH3NO2 and its structural isomer CH3ONO have been investigated by statistical theory calculations based on the potential energy surface calculated at the UCCSD(T)/CBS and CASPT3(8, 8)/6-311+G(3df,2p) levels. Our results show that for the decomposition of CH3NO2 at pressures less than 2 Torr, isomerization to CH3ONO via the recently located roaming transition state is dominant in the entire temperature range studied, 400-3000 K. However, at higher pressures, the formation of the commonly assumed products, CH3 + NO2, becomes competitive and at pressures higher than 200 Torr the production of CH3 + NO2 is exclusive. The predicted rate constants for 760 Torr and the high-pressure limit with Ar as diluent in the temperature range 500-3000 K, producing solely CH3 + NO2, can be expressed respectively by kd(760)(CH3NO2) = 2.94 × 10(55)T(-12.6) exp(-35500/T) s(-1) and kd(∞)(CH3NO2) = 5.88 × 10(24)T(-2.35) exp(-31400/T) s(-1). In the low pressure limit, the decomposition reaction takes place exclusively via the roaming TS producing internally excited CH3ONO, giving rise to both CH3O + NO and CH2O + HNO with the second-order rate constant kd(0)(CH3NO2) = 1.17 × 10(31)T(-10.94) exp(-32400/T) cm(3) molecule(-1) s(-1). For CH3ONO decomposition, a new roaming transition state connecting to the CH2O + HNO products has been located, lying 6.8 kcal/mol below the well-known four-member ring tight transition state and 0.7 kcal/mol below CH3O + NO. The rate constants predicted by similar calculations give rise to the following expressions for the thermal decomposition of CH3ONO in He: kd(760)(CH3ONO) = 8.75 × 10(41)T(-8.97) exp(-22600/T) s(-1) and kd(∞)(CH3ONO) = 1.58 × 10(23)T(-2.18) exp(-21100/T) s(-1) in the temperature range 300-3000 K. These results are in very good agreement with available experimental data obtained under practical pressure conditions. The much different branching ratios for the formation of CH3O + NO and CH2O + HNO in the decomposition of both CH3NO2 and CH3ONO are also given in this work.

Properties of liquid iron along the melting line up to Earth-core pressures

We report a molecular dynamics study of the transport coefficients and the infinite frequency shear modulus of liquid iron at high temperatures and high pressures. We observe a simultaneous rise of both the shear viscosity and the diffusion coefficient along the melting line and estimate whether liquid iron can vitrify under Earth-core conditions. We show that in the conditions of the model studied in our work iron demonstrates a moderate increase of viscosity along the melting line. It is also demonstrated that at the limit of high temperatures and high pressures the liquid iron behaves similarly to the soft sphere system with exponent n ≈ 4.6.

The comparison of two speed-dependent Voigt line profile models

Numerical calculations of the ratio of line width and maximum value of two speed-dependent Voigt line profiles to the respective values of their speed-independent analogs are performed in the high-pressure limit. The intercomparison demonstrates considerable disagreement between different speed-dependent models.

Exact Theory of Dense Amorphous Hard Spheres in High Dimension. II. The High Density Regime and the Gardner Transition

We consider the theory of the glass phase and jamming of hard spheres in the large space dimension limit. Building upon the exact expression for the free-energy functional obtained previously, we find that the random first order transition (RFOT) scenario is realized here with two thermodynamic transitions: the usual Kauzmann point associated with entropy crisis and a further transition at higher pressures in which a glassy structure of microstates is developed within each amorphous state. This kind of glass-glass transition into a phase dominating the higher densities was described years ago by Elisabeth Gardner, and may well be a generic feature of RFOT. Microstates that are small excitations of an amorphous matrix-separated by low entropic or energetic barriers-thus emerge naturally, and modify the high pressure (or low temperature) limit of the thermodynamic functions.

Theoretical and Kinetic Study of the Reactions of Ketones with HȮ2 Radicals. Part I: Abstraction Reaction Channels

This work presents an ab initio and chemical kinetic study of the reaction mechanisms of hydrogen atom abstraction by the HO2 radical on five ketones: dimethyl, ethyl methyl, n-propyl methyl, iso-propyl methyl, and iso-butyl methyl ketones. The Møller-Plesset method using the 6-311G(d,p) basis set has been used in the geometry optimization and the frequency calculation for all the species involved in the reactions, as well as the hindrance potential description for reactants and transition states. Intrinsic reaction coordinate calculations were carried out to validate all the connections between transition states and local minima. Energies are reported at the CCSD(T)/cc-pVTZ//MP2/6-311G(d,p) level of theory. The CCSD(T)/cc-pVXZ method (X = D, T, Q) was used for the reaction mechanism of dimethyl ketone + HO2 radical in order to benchmark the computationally less expensive method of CCSD(T)/cc-pVTZ//MP2/6-311G(d,p). High-pressure limit rate constants have been calculated for all the reaction channels by conventional transition state theory with asymmetric Eckart tunneling corrections and 1-D hindered rotor approximations in the temperature range 500-2000 K.

Theoretical study of the complex reaction of O(3P) with trans-2-butene

The complex triplet potential energy surface for the reaction of the triplet oxygen atom O(3P) with cis-2-butene is investigated at the CBS-QB3 level of theory. The different possible isomerization and dissociation pathways, including both O-additions and H-abstractions, are thoroughly studied. Our calculations show that as found for the trans-2-butene reaction, in the high-pressure limit, the major product is CH3CHC(O)H + CH3 (P1), whereas in the low-pressure limit the most thermodynamically stable product forms CH3CO + CH3CH2 (P4). The experimental negative activation energy reported for the addition step is very well reproduced at the CBS-QB3 level of theory. Various thermodynamic and kinetic values of interest for these reactions are predicted for the first time. A discussion on the negative activation energy for the addition step of the trans- and cis-2-butene reactions with O(3P) focussing on the addition reactant complexes is presented.

Ab Initio Thermal Rate Calculations of HO + HO = O(3P) + H2O Reaction and Isotopologues

The forward and reverse reactions, HO + HO ⇌ O((3)P) + H2O, which play roles in both combustion and laboratory studies, were theoretically characterized with a master equation approach to compute thermal reaction rate constants at both the low and high pressure limits. Our ab initio k(T) results for the title reaction and two isotopic variants agree very well with experiments (within 15%) over a wide temperature range. The calculated reaction rate shows a distinctly non-Arrhenius behavior and a strong curvature consistent with the experiment. This characteristic behavior is due to effects of positive barrier height and quantum mechanical tunneling. Tunneling is very important and contributes more than 70% of total reaction rate at room temperature. A prereactive complex is also important in the overall reaction scheme.

Rate constants of the reaction Fe + O2 + M ↔ FeO2 + M in the low- and high-pressure limits

The thermochemical and kinetic characteristics of the reaction Fe + O2 + M ↔ FeO2 + M are obtained based on an analysis within the framework of an RRKM model with account of the excited electronic states of the three isomeric forms of the FeO2 molecule. A new method for determining the dissociation energy and the entry energy barrier for the recombination reaction is proposed. The enthalpy of formation and Gibbs energy function of the FeO2 molecule are determined: Δ f H°(FeO2, 0) = 77 ± 10 kJ mol−1; Φ°(FeO2, T), J mol−1 K−1 = 421.4552 + 59.9779lnx − 0.0073327x −2 + 0.9598x −1 (x = 10−4 T; 250 < T < 3000 K). The best description of the available experimental data on the rate constants of reactions (I) and (−I) in the low-pressure limit in the framework of the proposed model is achieved with k 1,0(250 < T < 3000 K) = 2.8 · 1018(T/1000)−2.6exp (−3002/T) cm6 mol−2 s−1 and k −1,0(1000 < T < 3000 K) = 3.5 · 1019(T/1000)−4.0 × exp(−44371/T) cm3 mol−1 s−1. The corresponding high-pressure rate constants are k 1,∞(250 < T < 3000 K) = 1.2 · 1014exp(−2646/T) cm3 mol−1 s−1 and k -1,∞ (1000 < T < 3000 K) = 1.0 · 1015(T/1000)−1.2exp (−43657/T) s−1. It is shown that the available experimental data on the rate constants of reactions (I) and (−I) can be satisfactorily described only with consideration given to the electronically excited states of the FeO2 molecule.

Breakdown threshold in the microwave field at low and high pressures in electronegative gas mixtures

The dependence of the critical value of the microwave field amplitude in electronegative mixtures, which corresponds to the local equality of the ionization frequency and the dissociation attachment frequency, is investigated theoretically in a wide range of pressures with allowance for electron temperature pulsations. The resultant values are compared with the critical value for the static field. The range of pressures in which electron temperature pulsations become significant is determined, and the excess of the critical amplitude over the value in the static field is calculated in the high-pressure limit. Simple formulas are recommended for estimating the critical value of the microwave field amplitude.

Mechanical Extrusion Offers Advantages Over Ball Activation for Downhole Tools

Technology Update One of the most innovative recent advances in downhole drilling tool activation has been the introduction of mechanical extrusion technology, incorporating the use of rigid metal darts to improve the cycling and control of a variety of processes and devices. Increasingly, operators are recognizing mechanical extrusion by means of dart activation as a time-, cost-, and labor-saving alternative to traditional ball activation methods. For example, conventional cycling of drilling bypass valves has used a polymer extrusion process that relies on balls as the activation device. However, Churchill Drilling Tools has developed an effective and accurate mechanical extrusion method that uses a dart-based valve. The system can perform activations up to five times faster than polymer methods. The company’s MX (mechanical extrusion) system allows the use of rigid Smart Darts for multicycle control and exploits their robustness and resilience at high pressures and temperatures (up to 660°F) to deliver greater operating speed, reliability, and performance. The system’s unique feature is its two configurable shearing modes, which solves reliability and power delivery problems. It requires a radius as small as 0.0075 in. for power transmission, thereby making implementation extremely compact. With the ability to control and power three or more tools independently, users eliminate the conventional inner diameter (ID) and obstruction conflicts between different tools in a string. This has implications for mono-cycling ball and shear systems, which commonly obstruct the bore after use. These can be turned into multicycling, nonobstructive, and multi-tool systems. The system can also enhance multicycling ball-activated tools that are based on polymer extrusion by improving performance and delivering greater multitool compatibility and interoperability. The system allows the mechanical extrusion to be delivered in a range of settings to suit the specific cycle application. With a variety of latching, sealing, and flow path geometries, as well as the adjustable shear mode ratings, this element of the system provides a tangible change in string design and program implementation. For example, in bypass valve implementation, users can cover multiple flow path contingencies with a single valve in hole and choose from the range of darts to establish the optimal setting for the situation encountered. The system advances the capability of simple hydromechanical control and its power and reliability benefits into areas that previously needed more complex electronic systems. Being mechanical, it is not subject to the same high pressure and high temperature limits that affect electronics and electrical components. Smart Darts enable a multicycle circulating sub (circ sub) tool to deliver a reliable and versatile bypass on demand. Whatever the drilling application, whether planned or a contingency, the system enables a rapid and reliable switching to the optimal flow path configuration.

Porosity of Pillared Clays Studied by Hyperpolarized 129Xe NMR Spectroscopy and Xe Adsorption Isotherms

The influence of the layer charge on the microstructure was studied for a series of three hybrid pillared interlayered clays based on the organic dication Me(2)DABCO(2+) and charge reduced synthetic fluorohectorites. To get a detailed picture of the local arrangements within the interlayer space, multinuclear solid-state NMR spectroscopy was performed in conjunction with high-resolution (129)Xe MAS NMR, temperature-dependent wide-line 1D and 2D (129)Xe NMR, and Ar/Ar(l) and Xe/Xe(l) physisorption measurements. The resulting layer charge (x) for the three samples are 0.48, 0.44, and 0.39 per formula unit (pfu). The samples exhibit BET equivalent surfaces between 150 and 220 m(2)/g and pore volumes which increase from 0.06 to 0.11 cm(3)/g while the layer charge reduces. 1D and 2D (1)H, (13)C, (19)F, and (29)Si MAS data reveal that the postsynthetic charge reduction induces regions with higher defect concentrations within the silicate layers. Although the pillars tend to avoid these defect-rich regions, a homogeneous and regular spacing of the Me(2)DABCO(2+) pillars is established. Both the Ar/Ar(l) physisorption and (129)Xe NMR measurements reveal comparable pore dimensions. The trend of the temperature-dependent wide-line (129)Xe spectra as well as the exchange in the EXSY spectra is typical for a narrow 2D pore system. (129)Xe high-resolution experiments allow for a detailed description of the microstructure. For x = 0.48 a bimodal distribution with pore diameters between 5.9 and 6.4 Å is observed. Reducing the layer charge leads to a more homogeneous pore structure with a mean diameter of 6.6 Å (x = 0.39). The adsorption enthalpies ΔH(ads) determined from the temperature-dependent (129)Xe chemical shift data fit well to the ones derived from the Xe/Xe(l) physisorption measurements in the high-pressure limit while the magnitude of ΔH(ads) in the low-pressure limit is significantly larger. Thus, the (129)Xe data are influenced by adsorbate-adsorbent as well as adsorbate-adsorbate interactions.