Theoretical Study Of Kinetics Research Articles

How quickly does FLASH need to be delivered? A theoretical study of radiolytic oxygen depletion kinetics in tissues

Purpose. Radiation delivered over ultra-short timescales (‘FLASH’ radiotherapy) leads to a reduction in normal tissue toxicities for a range of tissues in the preclinical setting. Experiments have shown this reduction occurs for total delivery times less than a ‘critical’ time that varies by two orders of magnitude between brain (∼0.3 s) and skin (⪆10 s), and three orders of magnitude across different bowel experiments, from ∼0.01 to ⪆(1–10) s. Understanding the factors responsible for this broad variation may be important for translation of FLASH into the clinic and understanding the mechanisms behind FLASH. Methods. Assuming radiolytic oxygen depletion (ROD) to be the primary driver of FLASH effects, oxygen diffusion, consumption, and ROD were evaluated numerically for simulated tissues with pseudorandom vasculatures for a range of radiation delivery times, capillary densities, and oxygen consumption rates (OCR’s). The resulting time-dependent oxygen partial pressure distribution histograms were used to estimate cell survival in these tissues using the linear quadratic model, modified to incorporate oxygen-enhancement ratio effects. Results. Independent of the capillary density, there was a substantial increase in predicted cell survival when the total delivery time was less than the capillary oxygen tension (mmHg) divided by the OCR (expressed in units of mmHg/s), setting the critical delivery time for FLASH in simulated tissues. Using literature OCR values for different normal tissues, the predicted range of critical delivery times agreed well with experimental values for skin and brain and, modifying our model to allow for fluctuating perfusion, bowel. Conclusions. The broad three-orders-of-magnitude variation in critical irradiation delivery times observed in in vivo preclinical experiments can be accounted for by the ROD hypothesis and differences in the OCR amongst simulated normal tissues. Characterization of these may help guide future experiments and open the door to optimized tissue-specific clinical protocols.

Theoretical kinetic study of key reactions between ammonia and fuel molecules, part I: Hydrogen atom abstraction from alkanes by ṄH2 radical

Cross reactions between ṄH2 radical and fuel molecules are essential in determining the reactivity for ammonia/fuel blends. A systematic theoretical chemical kinetic investigation of hydrogen atom abstraction by ṄH2 radical from C1–C6 alkanes, n-heptane, and isooctane has been performed in this study. The geometry optimization, frequency, and zero-point energy calculations for all species related to the title reactions were calculated at the M06-2X/6-311++G(d,p) level of theory. One-dimensional hindered rotor treatment has also been performed at the same level of theory to obtain the potential energy surface for the low frequency torsional modes. Single-point energy was obtained at the QCISD(T)/cc-pVDZ, QCISD(T)/cc-pVTZ, MP2/cc-pVDZ, MP2/cc-pVTZ and MP2/cc-pVQZ levels of theory and then extrapolated to the complete basis set. Benchmark calculations for single point energy have also been carried out at the CCSD(T)-F12/cc-pVTZ, CCSD(T)-F12/cc-pVQZ, CCSD(T)-F12/aug-cc-pVTZ and CCSD(T)-F12/aug-cc-pVQZ levels of theory based on the geometries obtained at the CCSD(T)/cc-pVDZ level of theory for the reaction systems of methane + ṄH2 and ethane + ṄH2. The rate constants of 41 reactions in the temperature range 298.15–2000 K were calculated by using the Master Equation System Solver (MESS) with conventional transition state theory. Good agreement has been gained between our calculated results and the available theoretical and experimental data in the literature. Rate rules for different reaction sites have also been derived, and the average rate constants for primary, secondary, and tertiary sites were also reported on a per hydrogen atom basis. The impact of calculation data for the ammonia/alkane blends model has also been investigated. The calculated rate constants are valuable for the combustion kinetic model development for ammonia blending fuels.

2,1,3-Benzoselenadiazole-containing zinc(II) halide complexes: Chalcogen bonding in the solid state and catalytic activity in the Schiff condensation

XRD study of crystals of zinc(II) halide complexes featuring 2,1,3-benzoselenadiazole (L) ligands [ZnL2X2] (X = Cl, Br, I) indicated availability of intramolecular chalcogen bonds between the Se atom and the halide ligands, which affect a rotation barrier around the Zn–N bonds and thus control a geometry of these species. In the solid state, the Se atom additionally forms intermolecular chalcogen bonds with the N atom of the 2,1,3-benzoselenadiazole ligands, which results in formation of infinite 1D chains. Theoretical calculations and experimental kinetic study indicated that although the Se atom serves as a Lewis acid, coordination of the organic ligand to the zinc(II) center does not provide enhanced catalytic activity of the latter in the model reaction of the Schiff condensation.

Theoretical Kinetic Study on Hydrogen Abstraction Reactions from n-Pentane by NO2.

The interaction of fuel with NOx chemistry is important for the construction of the reaction mechanism and engine application. In this work, the reaction pathways of nC5H12 + NO2 were studied by high-level electronic structure calculations (DLPNO-CCSD(T)-F12/cc-pVTZ-F12//B2PLYPD3/cc-pVTZ). The rate constants were calculated by using the multistructural canonical transition-state theory with the Eckart tunneling method (TST/MS-T/ET). The studied condition is in a wide temperature range of 298-2400 K. The influence of MS-T anharmonicity and tunneling effect will be clarified for these site-specific H-abstraction pathways. The result reflects the large deviation introduced by the treatment of MS-T anharmonicity, especially at a high temperature. For the same type of reactions, the rate constants of H-abstraction both occurring at the secondary carbon are not almost identical. The branching ratios show that abstraction from the secondary site forming cis-HONO (R2c) contributes 36-78% to nC5H12 consumption in the temperature range of 298-2400 K. The current results show that the multistructural torsional anharmonicity has a crucial influence on the accurate estimation of branching ratios.

A theoretical kinetic study of 1-butyne, 2-butyne, and 3-methyl-1-butyne combustion

Propargylic radicals are an essential component in molecular weight growth kinetics and can react to form polycyclic aromatic hydrocarbons and soot. To establish rate rules for propargylic radicals, a total of 16 H-atom abstraction reactions of three alkynes, namely 1-butyne (1-C4H6), 2-butyne (2-C4H6), and 3-methyl-1-butyne (C5H8), were calculated using high-level quantum chemistry, in which four active radicals including Ḣ, ȮH, ĊH3, and HȮ2 were involved. Considering the completeness of the establishment of rate rules, the H-atom abstraction sites have included the primary and secondary site of 1-C4H6, primary site of 2-C4H6, and tertiary site of C5H8. The M06–2X/6–311++G(d,p) level of density function theory was used for the geometry optimization, vibrational frequency, and dihedral scan calculations. Single point energy calculations were carried out at the CCSD/cc-pVXZ (X = T, Q) level of coupled cluster theory. The rate coefficients for all of the abstraction reactions and the thermochemical quantities of 1-C4H6, 2-C4H6, and C5H8 and their corresponding radical products were calculated. The results indicate that H-atom abstraction by ȮH radicals has the fastest reaction rates for all three species. Moreover, abstraction from the tertiary site on C5H8 is the fastest, followed by abstraction of secondary hydrogen atoms on 1-C4H6 and primary hydrogen atoms on 2-C4H6, while abstraction of primary hydrogen atoms on 1-C4H6 are the slowest. NUIGMech1.3 (mechanism of version 1.3 proposed by National University of Ireland, Galway) has been updated based on our calculations, and ignition delay times (IDTs) were predicted using the updated mechanism. The predicted IDTs gave better agreement with the experiment data at pressures of 1, 30 and 50 bar. Sensitivity analyses were performed to understand observed model predicted differences at low to intermediate temperatures (700–1000 K) at 10 bar. The results show that the reactions 1-C4H6/2-C4H6 + HȮ2 and HȮ2 + HȮ2<=> H2O2 + O2 have a significant effect on promoting and inhibiting reactivity, respectively. Further Flux analyses show that the H-atom abstraction from 1-C4H6 and 2-C4H6 by ȮH radical is a dominant channel in changing branching ratios.

Theoretical Kinetics studies of isoprene peroxy radical chemistry: The fate of Z-δ-(4-OH, 1-OO)-ISOPOO radical

The OH radical recycling mechanism in isoprene oxidation is one of the most exciting topics in atmospheric chemistry, and the corresponding studies expand our understanding of oxidation mechanisms of volatile organic compounds in the troposphere and provide reliable evidence to improve and develop conventional atmospheric models. In this work, we performed a detailed theoretical kinetics study on the Z-δ-(4-OH, 1-OO)-ISOPOO radical chemistry, which is proposed as the heart of OH recycling in isoprene oxidation. With the full consideration of its accumulation and consumption channels, we studied and discussed the fate of Z-δ-(4-OH, 1-OO)-ISOPOO radical by solving the energy-resolved master equation over a broad range of conditions, including not only room temperatures but also high temperatures of a forest fire or low temperatures and pressures of the upper troposphere. We found non-negligible pressure dependence of its fate at combustion temperatures (up to two orders of magnitude) and demonstrated the significance of both the multi-structural torsional anharmonicity and tunneling for accurately calculating kinetics of the studied system. More interestingly, the tunneling effect on the phenomenological rate constants of the H-shift reaction channel is also found to be pressure-dependent due to the competition with the O2 loss reaction. In addition, our time evolution calculations revealed a two-stage behavior of critical species in this reaction system and estimated the shortest half-lives for the Z-δ-(4-OH, 1-OO)-ISOPOO radical at various temperatures, pressures and altitudes. This detailed kinetics study of Z-δ-(4-OH, 1-OO)-ISOPOO radical chemistry offers a typical example to deeply understand the core mechanism of OH recycling pathways in isoprene oxidation, and provides valuable insights for promoting the development of relevant atmospheric models.

Технология использования чернового концентрата в качестве минералов-носителей

Технология использования чернового концентрата в качестве минералов-носителей

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.

Numerical Calculation and 3-D Imaging of the Arrhenius Temperature Integral

The Arrhenius temperature integral is typically used in non-isothermal kinetic analysis, which is widely applied in gas–solid reactions in separation processes. In previous studies, researchers provided various methods to solve the temperature integral, but the error usually became significant when the value of x (x = Ea/RT) was too large or too small. In this paper, we present a new series method and design a computer program to calculate the temperature integral. According to the precise calculation of the temperature integral, we first reveal the relationship among the integral, the temperature, and the activation energy, and we find an interesting phenomenon in which the 3-D image of the temperature integral is of self-similarity according to fractal theory. The work is useful for mechanism and theoretical studies of non-isothermal kinetics.

Experimental and Theoretical Studies of the Gas-Phase Kinetics of Epichlorohydrin with Tropospheric Oxidants

Experimental and Theoretical Studies of the Gas-Phase Kinetics of Epichlorohydrin with Tropospheric Oxidants

An experimental and theoretical kinetic modeling study of the thermal decomposition of methyl-2-methyl butanoate behind shock waves

In order to investigate the combustion chemistry of a branched ester namely Methyl-2-methylbutanoate (M2MB), thermal decomposition of M2MB diluted in Argon was carried out behind reflected shock waves in the temperature range 959 -1392 K using a single pulse shock tube (SPST). The post shock mixtures were analyzed quantitatively as well as qualitatively using gas chromatography (GC) and gas chromatography coupled with mass spectrometry (GC–MS). Methane (CH4), Ethane (C2H6), Ethylene (C2H4), Methanol (CH3OH), Propene (C3H6) and Methyl acrylate (C4H6O2) were the major products observed during thermal decomposition. The various oxygenated compounds were found as minor products. The first order rate constant observed for the decomposition of M2MB is kexpt(959–1392 K) = (2.13 ± 0.457) × 109 exp [(19200 ± 561)/T] s−1 The temperature dependent kinetics of the CC and CO homolytic bond cleavages as well as of the H-migration for the unimolecular decomposition reactions were calculated using RRKM/ME over the temperature range of 959–1392 K. A reaction scheme involving 42 species and 61 elementary reactions, was proposed to simulate the reactant and product concentration over the studied temperature range. A comparison study of decomposition kinetics of branched chain ester and long chain ester is discussed in this work. The concentration profiles for reactant and all the products observed from simulations and experimental results were observed to be in very good agreement.

CO Oxidation Reaction by Platinum Clusters on the Surface of Multiwalled Carbon Nanotubes: Experimental and Theoretical Study of Kinetics in a Wide Range of O2/CO Ratios

This work presents a systematic study of the kinetic aspects of CO oxidation reaction catalyzed by platinum nanoparticles (NPs) supported on the surface of multiwalled carbon nanotubes (MWCNTs). The investigation presented is closely related to the actual practical task of air purification in enclosed spaces. Therefore, the catalytic reaction was carried out in the presence of an excess of oxygen (5 vol.%) and over a wide range of CO concentrations from 50 ppm to 1600 ppm. For the catalyst characterization, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were applied. Kinetic modelling based on the Langmuir–Hinshelwood and Mars-van Krevelen mechanisms was taken as a basis, using the results obtained on Pt foil. Simulation of CO oxidation reaction on platinum NPs at temperatures above 90 °C was carried out using a kinetic model describing the reaction mechanism on bulk platinum. The description of the kinetics of CO oxidation reaction on Pt NPs over the entire temperature range, including the low temperatures down to −40 °C, required the introduction of the steps characterizing an additional concerted mechanism related to CO-assisted O2 dissociation. Using the presented model, some predictions of the kinetic behaviour of the system were made.

Theoretical study of hydrogen adsorption kinetics: Mg17Al12 vs pure Mg

The adsorption and dissociation of hydrogen molecules on Mg17Al12(100), Mg17Al12(110), Mg17Al12(111) and Mg(0001) surfaces are investigated by the first-principles calculations. We find that the H adsorption on Mg17Al12 systems are all stronger than that of Mg(0001). Among them, the lowest adsorption energies of H atoms are obtained in the Mg17Al12(110) system, which is −0.278 and −0.247 eV/H when the H coverage is 1/11 and 2/11 ML, respectively. Furthermore, the Mg17Al12(110) surface presents an energy path of hydrogen dissociation with a minimum barrier of 0.59 eV, which is smaller than that of Mg(0001) surface (0.84 eV). The electronic structure analysis illustrates that the bonding between H atoms and Mg17Al12(110) surface is enhanced, although H atoms obtain similar charges on the Mg17Al12(110) and Mg(0001) surfaces. Moreover, the lowered dissociation barrier on the three studied Mg17Al12 surfaces, are likely ascribed to the stronger hybridization of H 1s and Al 3s orbitals, leading to the accelerated hydrogen adsorption kinetics on Mg17Al12 surface with respect to the pure Mg surface.

Theoretical Kinetic Studies on Thermal Decomposition of Glycerol Trinitrate and Trimethylolethane Trinitrate in the Gas and Liquid Phases.

Glycerol trinitrate (NG) and trimethylolethane trinitrate (TMETN), as typical nitrate esters, are important energetic plasticizers in solid propellants. With the aid of high-precision quantum chemical calculations, the Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation theory and the transition state theory have been employed to investigate the decomposition kinetics of NG and TMETN in the gas phase (over the temperature range of 300-1000 K and pressure range of 0.01-100 atm) and liquid phase (using water as the solvent). The continuum solvation model based on solute electron density (SMD) was used to describe the solvent effect. The thermal decomposition mechanism is closely relevant to the combustion properties of energetic materials. The results show that the RO-NO2 dissociation channel overwhelmingly favors other reaction pathways, including HONO elimination for the decomposition of NG and TMETN in both the gas phase and liquid phase. At 500 K and 1 atm, the rate coefficient of gas phase decomposition of TMETN is 5 times higher than that of NG. Nevertheless, the liquid phase decomposition of TMETN is a factor of 5835 slower than that of NG at 500 K. The solvation effect caused by vapor pressure and solubility can be used to justify such contradictions. Our calculations provide detailed mechanistic evidence for the initial kinetics of nitrate ester decomposition in both the gas phase and liquid phase, which is particularly valuable for understanding the multiphase decomposition behavior and building detailed kinetic models for nitrate ester.

Experimental and theoretical study of the low-temperature kinetics of the reaction of CN with CH2O and implications for interstellar environments.

Rate coefficients for the reaction of CN with CH2O were measured for the first time below room temperature in the range 32-103 K using a pulsed Laval nozzle apparatus together with the Pulsed Laser Photolysis-Laser-Induced Fluorescence technique. The rate coefficients exhibited a strong negative temperature dependence, reaching (4.62 ± 0.84) × 10-11 cm3 molecule-1 s-1 at 32 K, and no pressure dependence was observed at 70 K. The potential energy surface (PES) of the CN + CH2O reaction was calculated at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory, with the lowest energy channel to reaction characterized by the formation of a weakly-bound van der Waals complex, bound by 13.3 kJ mol-1, prior to two transition states with energies of -0.62 and 3.97 kJ mol-1, leading to the products HCN + HCO or HNC + HCO, respectively. For the formation of formyl cyanide, HCOCN, a large activation barrier of 32.9 kJ mol-1 was calculated. Reaction rate theory calculations were performed with the MESMER (Master Equation Solver for Multi Energy well Reactions) package on this PES to calculate rate coefficients. While this ab initio description provided good agreement with the low-temperature rate coefficients, it was not capable of describing the high-temperature experimental rate coefficients from the literature. However, increasing the energies and imaginary frequencies of both transition states allowed MESMER simulations of the rate coefficients to be in good agreement with data spanning 32-769 K. The mechanism for the reaction is the formation of a weakly-bound complex followed by quantum mechanical tunnelling through the small barrier to form HCN + HCO products. MESMER calculations showed that channel generating HNC is not important. MESMER simulated the rate coefficients from 4-1000 K which were used to recommend best-fit modified Arrhenius expressions for use in astrochemical modelling. The UMIST Rate12 (UDfa) model yielded no significant changes in the abundances of HCN, HNC, and HCO for a variety of environments upon inclusion of rate coefficients reported here. The main implication from this study is that the title reaction is not a primary formation route to the interstellar molecule formyl cyanide, HCOCN, as currently implemented in the KIDA astrochemical model.

Hydrodeoxygenation of acetic acid over Ni-promoted Cu-based catalysts: a theoretical mechanism and kinetic study

Reaction mechanism and descriptor of HDO process of acetic acid on (NiCu) bimetallic catalysts.

Kinetics of Kapalbhati: A mathematical interpretation of force, pressure, centre of gravity and centre of mass

This article introduces the theoretical study of kinetics of kapalbhati, how mathematical parameters correlated to each other. Kapalbhati is high frequency yoga breathing (exhalation breathing rate up to 2 Hz) literally means shining forehead (kapala = cranium or forehead, bhati = shining or even knowledge in Sanskrit). It is known widely for its positive effects on abdominal muscles and overall health. Kapalbhati plays an important role to enhance the abdominal muscle strength and its muscle tone as well.

Theoretical kinetics studies on the temperature and pressure dependence of the reaction of ammonia with the Criegee intermediate CH2OO

Theoretical kinetics studies on the temperature and pressure dependence of the reaction of ammonia with the Criegee intermediate CH2OO

A theoretical kinetic study of the reactions of NH2 radicals with methanol and ethanol and their implications in kinetic modeling

AbstractAmino (NH2) radicals play a central role in the pyrolysis and oxidation of ammonia. Several reports in the literature highlight the importance of the reactions of NH2 radicals with fuel in NH3‐dual‐fuel combustion. Therefore, we investigated the reactions of NH2 radicals with methanol (CH3OH) and ethanol (C2H5OH) theoretically. We explored the various reaction pathways by exploiting CCSD(T)/cc‐pV(T, Q)Z//M06‐2X/aug‐cc‐pVTZ level of theory. The reaction proceeds via complex formation at the entrance and exit channels in an overall exothermic process. We used canonical transition state theory to obtain the high‐pressure limiting rate coefficients for various channels over the temperature range of 300–2000 K. We discerned the role of various channels in the potential energy surface (PES) of NH2 + CH3OH/C2H5OH reactions. For both reactions, the hydrogen abstraction pathway at the OH‐site of alcohols plays a minor role in the entire T‐range investigated. By including the title reactions into an extensive kinetic model, we demonstrated that the reaction of NH2 radicals with alcohols plays a paramount role in accurately predicting the low‐temperature oxidation kinetics of NH3‐alcohols dual fuel systems (e.g., shortening the ignition delay time). On the contrary, these reactions have negligible importance for high‐temperature oxidation kinetics of NH3‐alcohol blends (e.g., not affecting the laminar flame speed). In addition, we calculated the rate coefficients for NH2 + CH4 = CH3 + NH3 reaction that are in excellent agreement with the experimental data.

To the theory of remagnetization kinetics of ferromagnetic nanoparticles. Non linear effects

The paper deals with theoretical study of remagnetization kinetics of a single-domain ferromagnetic particles in an alternating magnetic field, arbitrary directed with respect to the particle axis of easy magnetization. Period of the field is not supposed large as compared with the characteristic time of the particle Neel remagnetization. The nonlinear magnetization of the particle is considered. At the assumption of the particle strong magnetic anisotropy, a new equation for its average magnetic moment is derived.