11 research outputs found

    Ensayo aleatorizado del cierre de orejuela izquierda vs varfarina para la prevención de accidentes cerebrovasculares tromboembólicos en pacientes con fibrilación auricular no relacionada con valvulopatía. Estudio PREVAIL

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    The successful application of poly­(<i>N</i>-vinylcaprolactam)-based microgels requires a profound understanding of their synthesis. For this purpose, a validated process model for the microgels synthesis by precipitation copolymerization with the cross-linker <i>N</i>,<i>N</i>′-methylenebis­(acrylamide) is formulated. Unknown reaction rate constants, reaction enthalpies, and partition coefficients are obtained by quantum mechanical calculations. The remaining parameter values are estimated from reaction calorimetry and Raman spectroscopy measurements of experiments with different monomer/cross-linker compositions. Because of high cross-propagation reaction rate constants, simulations predict a fast incorporation of the cross-linker. This agrees with reaction calorimetry measurements. Furthermore, the gel phase is predicted as the major reaction locus. The model is utilized for a prediction of the internal particle structure regarding its cross-link distribution. The highly cross-linked core reported in the literature corresponds to the predictions of the model

    Discussion of the Separation of Chemical and Relaxational Kinetics of Chemically Activated Intermediates in Master Equation Simulations

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    Chemical activation of intermediates, such as hydrogen abstraction products, is emerging as a basis for a fully new reaction type: hot β-scission. While for thermally equilibrated intermediates chemical kinetics are typically orders of magnitude slower than relaxational kinetics, chemically activated intermediates raise the issue of inseparable chemical and relaxational kinetics. Here, this separation problem is discussed in the framework of master equation simulations, proposing three cases often encountered in chemistry: insignificant chemical activation, predominant chemical activation, and the transition between these two limits. These three cases are illustrated via three example systems: methoxy (CH<sub>3</sub>Ȯ), diazenyl (ṄNH), and methyl formate radicals (CH<sub>3</sub>OĊO). For diazenyl, it is found that hot β-scission fully replaces the sequence of hydrogen abstraction and β-scission of thermally equilibrated diazenyl. Building on the example systems, a rule of thumb is proposed that can be used to intuitively judge the significance of hot β-scission: if the reverse hydrogen abstraction barrier height is comparable to or larger than the β-scission barrier height, hot β-scission should be considered in more detail

    Ab Initio Calculations of Thermochemical Properties of Methanol Clusters

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    Highly accurate ab initio calculations of binding enthalpies and entropies of gas phase clusters of methanol have been performed, yielding uncertainties smaller than 1 kJ/mol per hydrogen bond in the Gibbs free energy of reaction. This requires quantum chemical RIMP2 and CCSD­(T) post-Hartree–Fock methods with basis sets up to aug-cc-pV5Z for energy calculations. An analysis of topological symmetry and hindered rotor effects proves necessary for reliable entropies. This approach goes beyond the rigid rotor plus harmonic oscillator method implemented in standard quantum mechanics software tools. The results demonstrate that (1) thermochemical methanol cluster properties can nowadays be obtained by ab initio methods with an accuracy comparable to or even better than that of the experimental data available, especially for larger species that cannot be studied directly by experiments and (2) cooperativity effects and state-dependent cluster distributions cause a strongly varying average enthalpy and entropy per bond as a function of temperature and density for methanol

    Reactions of Chemically Activated Formic Acid Formed via HĊO + ȮH

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    The chemistry of formyl radicals plays an important role in the kinetic modeling of oxygenated hydrocarbons. Here, the fate of rovibrationally excited formic acid produced via HĊO + ȮH is evaluated in a RRKM/Master Equation study. For that purpose, the HĊO + ȮH potential energy surface is studied theoretically using high-level quantum mechanics. Direct reaction from HĊO + ȮH to the bimolecular products is found to dominate for most relevant conditions due to formic acid well-skipping. The kinetics of these well-skipping reactions can only be evaluated when including the unimolecular intermediate, formic acid. Further, hydrogen abstraction from rovibrationally excited formic acid is found to be important at low-temperature conditions and for high radical concentrations

    Reactions of Chemically Activated Formic Acid Formed via HĊO + ȮH

    No full text
    The chemistry of formyl radicals plays an important role in the kinetic modeling of oxygenated hydrocarbons. Here, the fate of rovibrationally excited formic acid produced via HĊO + ȮH is evaluated in a RRKM/Master Equation study. For that purpose, the HĊO + ȮH potential energy surface is studied theoretically using high-level quantum mechanics. Direct reaction from HĊO + ȮH to the bimolecular products is found to dominate for most relevant conditions due to formic acid well-skipping. The kinetics of these well-skipping reactions can only be evaluated when including the unimolecular intermediate, formic acid. Further, hydrogen abstraction from rovibrationally excited formic acid is found to be important at low-temperature conditions and for high radical concentrations

    Pressure-Dependent Rate Constant Predictions Utilizing the Inverse Laplace Transform: A Victim of Deficient Input Data

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    <i>k</i>(<i>E</i>) can be calculated either from the Rice–Ramsperger–Kassel–Marcus theory or by inverting macroscopic rate constants <i>k</i>(<i>T</i>). Here, we elaborate the inverse Laplace transform approach for <i>k</i>(<i>E</i>) reconstruction by examining the impact of <i>k</i>(<i>T</i>) data fitting accuracy. For this approach, any inaccuracy in the reconstructed <i>k</i>(<i>E</i>) results from inaccurate/incomplete <i>k</i>(<i>T</i>) description. Therefore, we demonstrate how an improved mathematical description of <i>k</i>(<i>T</i>) data leads to accurate <i>k</i>(<i>E</i>) data. Refitting inaccurate/incomplete <i>k</i>(<i>T</i>), hence, allows for recapturing <i>k</i>(<i>T</i>) information that yields more accurate <i>k</i>(<i>E</i>) reconstructions. The present work suggests that accurate representation of experimental and theoretical <i>k</i>(<i>T</i>) data in a broad temperature range could be used to obtain <i>k</i>(<i>T</i>,<i>p</i>). Thus, purely temperature-dependent kinetic models could be converted into fully temperature- and pressure-dependent kinetic models

    Pressure-Dependent Rate Constant Predictions Utilizing the Inverse Laplace Transform: A Victim of Deficient Input Data

    No full text
    <i>k</i>(<i>E</i>) can be calculated either from the Rice–Ramsperger–Kassel–Marcus theory or by inverting macroscopic rate constants <i>k</i>(<i>T</i>). Here, we elaborate the inverse Laplace transform approach for <i>k</i>(<i>E</i>) reconstruction by examining the impact of <i>k</i>(<i>T</i>) data fitting accuracy. For this approach, any inaccuracy in the reconstructed <i>k</i>(<i>E</i>) results from inaccurate/incomplete <i>k</i>(<i>T</i>) description. Therefore, we demonstrate how an improved mathematical description of <i>k</i>(<i>T</i>) data leads to accurate <i>k</i>(<i>E</i>) data. Refitting inaccurate/incomplete <i>k</i>(<i>T</i>), hence, allows for recapturing <i>k</i>(<i>T</i>) information that yields more accurate <i>k</i>(<i>E</i>) reconstructions. The present work suggests that accurate representation of experimental and theoretical <i>k</i>(<i>T</i>) data in a broad temperature range could be used to obtain <i>k</i>(<i>T</i>,<i>p</i>). Thus, purely temperature-dependent kinetic models could be converted into fully temperature- and pressure-dependent kinetic models

    Pressure-Dependent Rate Constant Predictions Utilizing the Inverse Laplace Transform: A Victim of Deficient Input Data

    No full text
    <i>k</i>(<i>E</i>) can be calculated either from the Rice–Ramsperger–Kassel–Marcus theory or by inverting macroscopic rate constants <i>k</i>(<i>T</i>). Here, we elaborate the inverse Laplace transform approach for <i>k</i>(<i>E</i>) reconstruction by examining the impact of <i>k</i>(<i>T</i>) data fitting accuracy. For this approach, any inaccuracy in the reconstructed <i>k</i>(<i>E</i>) results from inaccurate/incomplete <i>k</i>(<i>T</i>) description. Therefore, we demonstrate how an improved mathematical description of <i>k</i>(<i>T</i>) data leads to accurate <i>k</i>(<i>E</i>) data. Refitting inaccurate/incomplete <i>k</i>(<i>T</i>), hence, allows for recapturing <i>k</i>(<i>T</i>) information that yields more accurate <i>k</i>(<i>E</i>) reconstructions. The present work suggests that accurate representation of experimental and theoretical <i>k</i>(<i>T</i>) data in a broad temperature range could be used to obtain <i>k</i>(<i>T</i>,<i>p</i>). Thus, purely temperature-dependent kinetic models could be converted into fully temperature- and pressure-dependent kinetic models

    Hydrogen Abstraction from <i>n</i>‑Butyl Formate by H<sup>•</sup> and HO<sub>2</sub><sup>•</sup>

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    The combustion chemistry of esters has been elucidated in the past through the study of smaller formates and acetates. Hydrogen abstraction from the fuel as an initiation step is mostly modeled based on estimations for similar abstractions from nonoxygenated hydrocarbons. This study reports computed ab initio rates for abstractions by H<sup>•</sup> and HO<sub>2</sub><sup>•</sup> radicals from the recently proposed biofuel candidate <i>n</i>-butyl formate. The energies are evaluated with a double hybrid density functional that performs especially well for barrier heights (B2KPLYP/aug-cc-pvtz). Hindered rotation of HO<sub>2</sub><sup>•</sup> with respect to <i>n</i>-butyl formate is treated using accurate eigenvalue summation and shows large influence on the rates. Transition states at the γ and δ positions are still influenced by the formate group. The abstraction from the γ carbon by HO<sub>2</sub><sup>•</sup> is slowest, although proceeding over the lowest barriers, due to the important influence of transition state entropies. A comparison with smaller esters and <i>n</i>-butanol shows that estimated rates deviate within 1 order of magnitude from the ab initio computations for similar groups in <i>n</i>-butyl formate

    Automated Chemical Kinetic Modeling via Hybrid Reactive Molecular Dynamics and Quantum Chemistry Simulations

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    An automated scheme for obtaining chemical kinetic models from scratch using reactive molecular dynamics and quantum chemistry simulations is presented. This methodology combines the phase space sampling of reactive molecular dynamics with the thermochemistry and kinetics prediction capabilities of quantum mechanics. This scheme provides the NASA polynomial and modified Arrhenius equation parameters for all species and reactions that are observed during the simulation and supplies them in the ChemKin format. The ab initio level of theory for predictions is easily exchangeable, and the presently used G3MP2 level of theory is found to reliably reproduce hydrogen and methane oxidation thermochemistry and kinetics data. Chemical kinetic models obtained with this approach are ready to use for, e.g., ignition delay time simulations, as shown for hydrogen combustion. The presented extension of the ChemTraYzer approach can be used as a basis for methodological advancement of chemical kinetic modeling schemes and as a black-box approach to generate chemical kinetic models
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