11 research outputs found

### Ensayo aleatorizado del cierre de orejuela izquierda vs varfarina para la prevenciÃ³n de accidentes cerebrovasculares tromboembÃ³licos en pacientes con fibrilaciÃ³n auricular no relacionada con valvulopatÃa. Estudio PREVAIL

The successful application of polyÂ(<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>â€²-methylenebisÂ(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

Chemical activation
of intermediates, such as hydrogen abstraction
products, is emerging as a basis for a fully new reaction type: hot
Î²-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>OÌ‡), diazenyl (NÌ‡NH),
and methyl formate radicals (CH<sub>3</sub>OCÌ‡O). For diazenyl,
it is found that hot Î²-scission fully replaces the sequence
of hydrogen abstraction and Î²-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 Î²-scission:
if the reverse hydrogen abstraction barrier height is comparable to
or larger than the Î²-scission barrier height, hot Î²-scission
should be considered in more detail

### Ab Initio Calculations of Thermochemical Properties of Methanol Clusters

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
CCSDÂ(T) post-Hartreeâ€“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 HCÌ‡O + OÌ‡H

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 HCÌ‡O + OÌ‡H is evaluated
in a RRKM/Master Equation study. For that purpose, the HCÌ‡O
+ OÌ‡H potential energy surface is studied theoretically using
high-level quantum mechanics. Direct reaction from HCÌ‡O + OÌ‡H
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 HCÌ‡O + OÌ‡H

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 HCÌ‡O + OÌ‡H is evaluated
in a RRKM/Master Equation study. For that purpose, the HCÌ‡O
+ OÌ‡H potential energy surface is studied theoretically using
high-level quantum mechanics. Direct reaction from HCÌ‡O + OÌ‡H
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

<i>k</i>(<i>E</i>) can be calculated either
from the Riceâ€“Ramspergerâ€“Kasselâ€“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

<i>k</i>(<i>E</i>) can be calculated either
from the Riceâ€“Ramspergerâ€“Kasselâ€“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

<i>k</i>(<i>E</i>) can be calculated either
from the Riceâ€“Ramspergerâ€“Kasselâ€“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>â€‘Butyl Formate by H<sup>â€¢</sup> and HO<sub>2</sub><sup>â€¢</sup>

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>â€¢</sup> and HO<sub>2</sub><sup>â€¢</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>â€¢</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 Î³ and Î´ positions
are still influenced by the formate group. The abstraction from the
Î³ carbon by HO<sub>2</sub><sup>â€¢</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

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