Calculation Of Reaction Rates Research Articles

Kinetics mechanism of pore pressure effect on CO2-water-rock interactions: An experimental study

Carbon capture, utilization and storage (CCUS) is an effective technology to reduce carbon dioxide (CO2) content in atmosphere, while due to the insufficient mastery of deterioration laws of reservoir rocks caused by CO2, the extensive application of carbon storage projects is hindered by the leakage risk. Therefore, it was aimed to study the kinetics mechanism of pore water pressure on CO2-water–rock interactions. First, the pressure effect on water–rock interactions was studied to obtain the relationship between ratio of solid–liquid contact area Ψ and pore pressure, and the controlling mechanism of Ψ on reaction rate ri was verified. On this basis, the pressure effect on CO2-water–rock interactions was further studied. Considering the effects on Ψ and solution acidity, the original CO2-applicable kinetics equation was modified to reduce the error of calculating reaction rate by 33.60% on average. Meanwhile, a forecast method was proposed for the reaction rate under different pressure conditions, and the average calculation error was 41.00% lower than that of original kinetics equation. After verifying the accuracy of these two theoretical methods to calculate reaction rate, the reliability to calculate porosity evolution process was further verified, and the calculation errors could be reduced by 29.52% and 31.81%, respectively.

Calculation of Reaction Rate and Uncertainty for d(p,γ)3He and 3He(d,p)4He Using Bayesian Statistical Approach in Astrophysics

Calculation of Reaction Rate and Uncertainty for d(p,γ)3He and 3He(d,p)4He Using Bayesian Statistical Approach in Astrophysics

Chemically derived quinary Cu2Co1–xNaxSnS4 photon absorber material and its photocatalytic application

The solar absorbing materials attract attention for the opto-electronic device applications such as solar cells and photocatalysis. Interest in photocatalytic materials to clean up the wastewater has started to increase. Generally, semiconductors sensitive to UV region are used. However, most of the light from the sun is in the visible region. Cu2CoSnS4 is a promising material that exhibits superior photo-response in the visible regions with an appropriate band gap. In this study, Cu2CoSnS4 films were prepared by the sol–gel method. Cu2Co1–xNaxSnS4 was synthesized for the first time by partly substituting sodium with cobalt. With this approach, noticeable improvements were observed in the physical properties of the material. With the sodium replacement, the Cu2Co1–xNaxSnS4 exhibited high photocatalytic performance toward the degradation of methylene blue (MB) under visible-light irradiation. According to the results, Cu2CoSnS4 and Cu2Co1–xNaxSnS4 samples degraded the MB solution by degradation efficacy values of 89.69% and 94.57%, respectively, in a short time like 40 min under visible light. This study shows that the substitution of sodium boosted the degradation efficiency by 5.4%. For the Cu2CoSnS4 and Cu2Co1–xNaxSnS4 samples, the computed reaction rate constants are 0.059 min−1 and 0.077 min−1, respectively.

Probing time-resolved plasma-driven solution electrochemistry in a falling liquid film plasma reactor: Identification of HO2- as a plasma-derived reducing agent.

Many applications involving plasma-liquid interactions depend on the reactive processes occurring at the plasma-liquid interface. We report on a falling liquid film plasma reactor allowing for in situ optical absorption measurements of the time-dependence of the ferricyanide/ferrocyanide redox reactivity, complemented with ex situ measurement of the decomposition of formate. We found excellent agreement between the measured decomposition percentages and the diffusion-limited decomposition of formate by interfacial plasma-enabled reactions, except at high pH in thin liquid films, indicating the involvement of previously unexplored plasma-induced liquid phase chemistry enabled by long-lived reactive species. We also determined that high pH facilitates a reduction-favoring environment in ferricyanide/ferrocyanide redox solutions. In situ conversion measurements of a 1:1 ferricyanide/ferrocyanide redox mixture exceed the measured ex situ conversion and show that conversion of a 1:1 ferricyanide/ferrocyanide mixture is strongly dependent on film thickness. We identified three dominant processes: reduction faster than ms time scales for film thicknesses >100 µm, •OH-driven oxidation on time scales of <10ms, and reduction on 15ms time scales for film thickness <100 µm. We attribute the slow reduction and larger formate decomposition at high pH to HO2- formed from plasma-produced H2O2 enabled by the high pH at the plasma-liquid interface as confirmed experimentally and by computed reaction rates of HO2- with ferricyanide. Overall, this work demonstrates the utility of liquid film reactors in enabling the discovery of new plasma-interfacial chemistry and the utility of atmospheric plasmas for electrodeless electrochemistry.

Theoretical study on the kinetics and reaction mechanism involved in the reduction of quinone by 1‐benzyl‐1,4‐dihydronicotinamide

AbstractAlthough it is well known that coenzyme NAD(P)H is involved in anabolic and catabolic reactions in the living organism, there is still significant controversy over the reaction mechanism involved in this biochemical transformation. Thus, 1‐benzyl‐1,4‐dihydronicotinamide was used as a NAD(P)H model in the reduction reaction of 1,4‐benzoquinone (Q), 2,3,5,6‐tetrachloro‐1,4‐benzoquinone, and 2,3‐dicyano‐1,4‐benzoquinone in acetonitrile medium. The kinetic calculations support that formal hydride transfer is the main mechanism promoting Q reduction, while the two‐step process dominates 2,3‐dicyano‐1,4‐benzoquinone reduction. Interestingly, only the single‐electron transfer mechanism takes place when 2,3,5,6‐tetrachloro‐1,4‐benzoquinone is used, affording the corresponding semiquinone derivative as the main product. This mechanistic behavior is related to the presence or absence of electron‐withdrawing groups in the quinones used. Furthermore, the kinetic study results showed that calculated reaction rate constants are in close agreement with experimental results. The results support that formal hydride transfer on the reduction reaction of Q by 1‐benzyl‐1,4‐dihydronicotinamide in acetonitrile proceeds through a hydrogen coupled electron transfer mechanism. This theoretical analysis provides valuable knowledge that can be extrapolated to study the reduction of quinones performed by NADH and NADPH in physiological media.

Reaction Rates of OH Radicals with CH3OCF2CHFCF3 and CHF2CF2OCH2CF2CHF2: Measurements and Estimation Using Neural Network Method.

The reaction rates of OH radicals with CH3OCF2CHFCF3 (k1) and CHF2CF2OCH2CF2CHF2 (k2) were measured over a temperature range of 250-430 K. Kinetic measurements were performed using the flash and laser photolysis methods combined with a laser-induced fluorescence technique. The Arrhenius rate parameters were determined as k1 = (2.52 ± 0.25) × 10-12·exp[-(1390 ± 30)/T], k2 = (1.83 ± 0.20) × 10-12·exp[-(1420 ± 35)/T] cm3 molecule-1 s-1. The infrared absorption spectra of the two hydrofluoroethers were measured at approximately 298 K in 760 Torr of N2. The atmospheric lifetimes of CH3OCF2CHFCF3 and CHF2CF2OCH2CF2CHF2 have been estimated as 2.5 and 3.8 years, respectively, and their global warming potentials were determined as 245 and 405, respectively. Additionally, a method, using a three-layered feed-forward neural network, for estimating the rates of the reaction of the OH radicals with alkanes, ethers, and alcohols was investigated. The ratios of the calculated reaction rates to the observed ones agreed within a factor of 2. The ability of the neural network method to predict reaction rates was examined by using a leave-one-out test.

A novel method for predicting the thermal stabilization temperature of organic Rankine cycle system working fluids based on transition state theory

The use of organic working fluids in organic Rankine cycle (ORC) systems may lead to thermal decomposition, which negatively impacts system performance and safety. In this work, a simplified theoretical method based on the transition state theory is proposed to predict the ideal maximum thermal stabilization temperature of organic working fluids. This method involves considering the initial decomposition reaction of the working fluids and calculating the total reaction rate of the initial decomposition. By setting an acceptable decomposition rate, the ideal maximum thermal stabilization temperature of the working fluid can be determined. Using this method, the total reaction rate constants of several typical hydrocarbon, hydrofluorocarbon and siloxane working fluids were obtained in the temperature range of 300∼700 K. The predicted maximum operational temperatures were in good agreement with most experimental results. The branch ratio analyses show that C–C bond cleavage contributed mainly to the decomposition of hydrocarbons, the removal of HF and C–C bond cleavage contributed mainly to that of HFCs, and the Si–C bond cleavage contributed mainly to that of siloxanes. Only considering these main reactions in the reaction rate calculation can greatly reduce calculation cost without significantly impacting the prediction accuracy.

Improved numerical simulation model for nuclear reaction rate calculations in high-speed plasma collisions

Beam–target reactions in fusion plasmas play an important role in both magnetic confinement fusion and inertial confinement fusion in the condition of low-density plasmas with high-velocity interactions. The traditional method for calculating beam–target reaction rate neglects the transport process of incident particles in inhomogeneous plasmas, leading to errors providing that the temperature and density in the transport path of incident particles vary obviously. An improved method considering the transport process is proposed in this paper to eliminate the deficiencies. Then the method is employed in high-speed plasma collision studies. When the initial plasma density and temperature are set to $0.5\,{\rm g}\,{\rm cm}^{-3}$ and 100 eV, it is found that the beam–target reaction rate calculated by the traditional method is almost identical to that by our method if the collision velocity is less than 600 km s $^{-1}$ . However, the traditional method is not suitable for study as the collision velocity gets higher, inducing obvious differences, which can reach 70 % at 1000 km s $^{-1}$ . The improved method will make large corrections to evaluate the importance of the non-negligible beam–target reaction for inertial confinement fusion schemes with large implosion velocity such as double-cone ignition and impact ignition, in which the high-speed plasmas collide with each other to realize plasma ignition.

Recent results and future perspectives with solid targets at LUNA

The stellar evolution and chemical make-up of the Universe are determined by nuclear reactions occurring in a wide variety of stellar sites. Precise determinations of the cross sections of these reactions are crucial for the calculation of reaction rates and for the development of stellar evolution models. The Laboratory for Underground Nuclear Astrophysics (LUNA) collaboration has been at the forefront of the direct measurement of nuclear reactions at the low energies of astrophysical interest for the last 35 years. The many significant results achieved at LUNA have been made possible due to the low background conditions uniquely available thanks to its location deep underground at the Laboratori Nazionali del Gran Sasso. Another key aspect of these successes is due to the experience of the LUNA collaboration in the production and characterization of a variety of solid targets used in reaction measurements. In this review, the main production techniques of solid targets are described, as well as the common methods adopted for target degradation monitoring. We also present the results of recent measurements using these targets and the future plans of the LUNA collaboration for measurements using solid targets at the LUNA400 kV and the new Ion Beam Facility (IBF) 3.5 MV are also presented.

Calculation of astrophysical reaction rate and uncertainty for T(d,n)4He using Bayesian statistical approach

Abstract One of the best methods to investigate and calculate a desired quantity using available limited data is the Bayesian statistical method, which has been recently entered the field of nuclear astrophysics and can be used to evaluate the astrophysical S-factors, the cross sections and, as a result, the nuclear reaction rates of Big Bang Nucleosynthesis. This study tries to calculate the astrophysical S-factor and the rate of reaction T(d,n)4He as an important astrophysical reaction with the help of this method in energies lower that electron repulsive barrier, and for this purpose, it uses the R-Software, which leads to improved results in comparison with the non-Bayesian methods for the mentioned reaction rate.

Neutron source strength verification via reaction rate measurement and Monte Carlo simulation

Determining the neutron source strength is essential to ensure optimal use of the source and to implement adequate radiation protection measures. In this study, the source strength of two (α,n) neutron sources – a newly acquired 185 GBq 241AmBe and a legacy source 74 GBq 239PuBe – were verified. Source strength verification was performed by measuring the induced activities in irradiated 115In foils and comparing the results to MCNP calculated reaction rates. The results for 241AmBe agree well with a maximum and average deviations of 8.1% and 4.1%, respectively. While for 239PuBe the maximum deviation is 11.7% and the average deviation is 6.3%. Results of this study provide a reliable reference for the application of the sources, in particular, for the 241AmBe which is used as the external neutron source of the PRR1-Subcritical Assembly for Training, Education, and Research (PRR1-SATER), and for the design of a neutron irradiator that will repurpose the 239PuBe.

The Influence of an Extended π Electron System on the Electrochemical Properties and Oxidizing Activity of a Series of Iron(III) Porphyrazines with Bulky Pyrrolyl Substituents.

The present study investigates four iron(II/III) porphyrazines with extending pyrrolyl peripheral substituents to understand the impact of introduced phenyl rings on the macrocycle's electrochemical and spectroelectrochemical properties as well as their activity in oxidation reactions. The electrochemical studies showed six well-defined redox processes and quasi-reversible one-electron transfers-two originating from the iron cation and four related to the ring. Adding phenyl rings to the periphery increased the electrochemical gap by 0.1 V. The UV-Vis spectra changes were observed at the applied potential of -1.3 V with the presence of additional red-shifted bands. The oxidizing studies showed increased efficiency in the oxidation reaction of the reference substrate in the cases of Pz1 and Pz2 in both studied oxygen atom donors. The calculated reaction rates in t-BuOOH were 12.0 and 15.0 mmol/min, respectively, for Pz1 and Pz2, compared to 6.4 for Pz3 and 1.8 mmol/min for Pz4. The study identified potential applications for these porphyrazines in mimicking cytochrome P450 prosthetic groups for oxidation and hydroxylation reactions in the future.

Polyrate 2023: A computer program for the calculation of chemical reaction rates for polyatomics. New version announcement

Polyrate is a suite of computer programs for the calculation of chemical reaction rates of polyatomic species (including atoms and diatoms as special cases) by variational transition state theory (VTST); conventional transition state theory is also supported. Polyrate can calculate the rate constants for both bimolecular reactions and unimolecular reactions, and it can be applied to reactions in the gas phase, liquid solution phase, or solid state and to reactions at gas–solid interfaces. Polyrate can perform VTST calculations on gas-phase reactions with both tight and loose transition states. For tight transition states it uses the reaction-path (RP) variational transition state theory developed by Garrett and Truhlar, and for loose transition states it uses variable-reaction-coordinate (VRC) variational transition state theory developed by Georgievskii and Klippenstein. The RP methods used for tight transition states are conventional transition state theory, canonical variational transition state theory (CVT), and microcanonical variational transition state theory (μVT) with multidimensional semiclassical approximations for tunneling and nonclassical reflection. For VRC calculations, rate constants may be calculated for canonical or microcanonical ensembles or energy- and total-angular-momentum resolved microcanonical ensembles. Pressure-dependent rate constants for elementary reactions can be computed using system-specific quantum RRK theory (SS-QRRK) with the information obtained from high-pressure-limit VTST calculation as input by using the SS-QRRK utility code. Alternatively, Polyrate 2023 may be interfaced with TUMME 2023 for a master-equation treatment of pressure dependence or to obtain phenomenological rate constants for complex mechanisms. Potential energy surfaces may be analytic functions evaluated by subroutines, or they may be implicit surfaces defined by electronic structure input files or interface subroutines containing energies, gradients, and force constants (Hessians). For the latter, Polyrate can be used in conjunction with various interfaces to electronic structure programs for direct dynamics, and it has routines designed to make such interfacing straightforward. Polyrate supports six options for direct dynamics, namely (i) straight single-level direct dynamics, (ii) zero-order interpolated variational transition state theory (IVTST-0), (iii) first-order interpolated variational transition state theory (IVTST-1), (iv) interpolated variational transition state theory by mapping (IVTST-M), (v) variational transition state theory with interpolated single-point energies (VTST-ISPE), and (vi) variational transition state theory with interpolated optimized corrections (VTST-IOC). Polyrate can handle multistructural and torsional-potential anharmonicity in conjunction with the MSTor program. Polyrate 2023 contains 112 test runs, and 46 of these are for direct dynamics calculations; 85 of the test runs are single-level runs, and 27 are dual-level calculations. New version program summaryProgram Title: Polyrate 2023CPC Library link to program files:https://doi.org/10.17632/vpf3y9cbhf.1Developer's repository link:https://doi.org/10.5281/zenodo.8213313Licensing Provisions: Apache-2.0 for the program and CC-BY-4.0 for the manualProgramming language: Fortran 90Does the new version supersede the previous version?: YesJournal references of previous versions: Comput. Phys. Commun. 47 (1987) 91 [1]; Comput. Phys. Commun. 71 (1992) 235 [2]; Comput. Phys. Commun. 88 (1995) 341 [3].Nature of problem: Polyrate 2023 can calculate unimolecular and bimolecular chemical reaction rates of polyatomic species (and also of atoms and diatoms as special cases) by using variational transition state theory with multidimensional tunneling [4-8] or by conventional transition state theory [9]. The program uses either reaction-path variational transition state theory for tight transition states or variable-reaction-coordinate variational transition state theory for loose transition states. Rate constants may be calculated for canonical or microcanonical ensembles with multidimensional semiclassical approximations for tunneling and nonclassical reflection and for reactions in the gas phase or solid state or at gas–solid interfaces. In addition, rates may be calculated for vibrationally adiabatic and vibrationally diabatic state-selected reactions in which one or more of the vibrational modes is restricted to the ground state or to the first excited state, while the translational, rotational, and remaining vibrational modes are treated thermally [10,11]. The program may also be used to find stationary geometries of reactants, products, and transition states and to calculate reaction paths, Arrhenius parameters, and equilibrium constants.Solution method: Energies, gradients, and Hessians are obtained from analytic potential energy functions or from electronic structure input data, where the input data may be supplied in files or by direct interface to electronic structure codes. A number of methods are available for calculating the reaction path. Nontorsional anharmonicity may be included under the assumption of independent generalized normal modes. Torsional potential anharmonicity and multiple-structure anharmonicity can be included along the reaction path using the formulas of the multi-structural torsional (MS-T) method with one input structure [12-16]. Small-curvature [2,17], large-curvature [18,19], and optimized multidimensional [20,21] semiclassical tunneling methods are available as options to calculate tunneling contributions to transmission coefficients. Pressure-dependent rate constants for elementary reactions can be computed using system-specific quantum Rice–Ramsperger–Kassel theory (SS-QRRK) [22]. Phenomenological rate constants for complex reactions can be obtained in conjunction with the TUMME 2023 program [23-25].Interfaces to other programs: Polyrate 2023 can run as a standalone program, and many interface options are also available [24], including: CRATE, GAMESSPLUSRATE, GAUSSRATE, JAGUARATE, MC-TINKERRATE, MORATE, MULTILEVELRATE, NWCHEMRATE.Reason for new version: The new version greatly increases the functionality, stability, and accuracy; the code has been cleaned up, and some bugs have been fixed.Summary of revisions:1.The variable-reaction-coordinate method with either single-faceted or multi-faceted dividing surfaces [26–30] has been implemented for barrierless association reactions and dissociation reactions without an intrinsic barrier, i.e., reactions without a barrier when considered in the downhill direction. This implementation is parallelized using MPI. In calculating the total number of available states for this method, the integration over the projection on the principal axis of the total angular momentum is calculated analytically. The user now has a choice of using the original variational transition state theory based on a reaction path (VTST-RP) or using variational transition state theory based on a variable reaction coordinate (VTST-VRC).2.Options to carry out VTST-RP calculations with generalized normal modes in curvilinear coordinates, in particular in nonredundant [31–33] or redundant [34] internal coordinates, were added. All standard force constant types (stretches, nondegenerate bends, torsions, and degenerate bends) are available. Curvilinear coordinates are recommended for all calculations based on reaction paths.3.The number-of-states calculation that is required for microcanonical variational theory (μVT) [35] is now programmed much more efficiently by the extended Beyer-Swinehart method [36,37]. Now there is no significant difference between CVT and μVT as far as computational cost.4.The improved canonical variational theory (ICVT) option is removed. Whenever one needs a more accurate treatment of the threshold, one can simply use μVT without significantly increasing the computational cost, so there is no longer any motivation for using ICVT.5.The first-order interpolated variational transition state theory (IVTST-1) method [38], interpolated variational transition state theory by mapping (IVTST-M) [39,40], including a spline-under-tension option, and several variational transition state theory with interpolated corrections (VTST-IC) [40] algorithms were added. IVTST-M allows arbitrary numbers of points along the reaction path and arbitrary numbers of points at which Hessians are computed, and it leads to faster convergence of direct dynamics calculations. VTST-IC methods include variational transition state theory with interpolated single-point energies (VTST-ISPE).6.Interfaces for direct dynamics were made more convenient. The PTUMME keyword to print an interface file readable by the TUMME 2023 program [23-25] has been added.7.An option for performing calculations with classical rather than quantal partition functions was added.8.The code has been modified to calculate wells in the reactant and/or product channels. One can optimize the reactants, the products, and the saddle point in separate runs. Quasi-Newton optimization with BFGS updating [41–44] was added. The eigenvector following (EF) algorithm [45–47] was added for geometry optimization of transition states and other stationary states.9.The canonical unified statistical model (CUS) [6,48–50] was added.10.The re-orientation of the dividing surface (RODS) [51,52] and variational reaction path (VRP) [53] algorithms were implemented. These methods allow VTST calculations with larger step sizes.11.An option was added to perform nonequilibrium solvation calculations [54].12.Version 4 of the large-curvature ground state tunneling method (LCG4) [55] was added as the default large-curvature tunneling (LCT) method, and it is now also used in the optimized multidimensional tunneling (OMT) [56] and microcanonical optimized multidimensional (μOMT) [20] tunneling methods. The interpolated algorithm for efficient large-curvature tunneling calculations (ILCT) [6,35] was also added.13.New WKB vibrational anharmonicity [58,59] capability was added.14.An option was added to scale the vibrational frequencies [60,61].15.A keyword was added to allow users to raise low frequencies to a certain number (e.g., 50 or 100 cm−1) to approximately account for anharmonicity [62,63].16.Several separable options for including anharmonicity in hindered internal rotations were added [64].17.The single-structure MS-T method [12,14] was added with coupled torsional potentials [16]. This capability is added both for a saddle point and for including torsional anharmonicity along a reaction path. Multi-structural [13] and multi-path [65–67] variational transition state theory calculations can be performed by combining the data from multiple Polyrate 2023 and MSTor outputs.18.The SS-QRRK code for pressure dependent rate constants [22,68,69] has been included as a utility code. This method can include variational effects, multidimensional tunneling, and various vibrational and torsional anharmonicity options into the computed pressure dependent rate constants and the SS-QRRK microcanonical rate constants. The FE integral for the normalized Boltzmann-weighted number of unimolecular states above the threshold energy can be evaluated via numerical integration instead of using the approximate analytical formula [70].19.The standard-state free energy of activation profile is calculated along a reaction path using a standard state of 1 bar rather than an unconventional pressure.20.The generalized free energy of activation can now be calculated using logarithms of the vibrational partition functions to avoid underflow. Similarly, a shifting constant is used to solve the overflow and underflow problem in the case that the imaginary action integral (the semiclassical tunneling integral) is too large.21.The code was rearranged to allow parallel processing using MPI.22.All common blocks were replaced by Fortran 90 modules. Memory for arrays is dynamically allocated. Therefore, compiling the code with a predefined system size (e.g., number of atoms) is no longer required.23.Various bug fixes have been made, and some errors have been corrected. The restart option was improved. The code was made more portable, many options were made more convenient, defaults or keywords were improved, the output and manual were improved, and many new test runs were added.

Theoretical study on hydrogen abstraction reactions from pentane isomers by NO2

The rate constants for the H-abstraction reactions of C5H12 + NO2 are calculated by high-accuracy electronic structure calculations and transition state theory (TST). The investigation includes three pentane isomers, neopentane, n-pentane, and iso-pentane, each of which leads to the formation of three different HONO isomers, namely cis-HONO, trans-HONO, and HNO2. The lowest energy conformers of all relevant species are determined through geometry optimization, frequency analysis, and one-dimensional hindered rotor scanning at the M06-2X/6-311++G(d,p) level of theory, and are further re-optimized using the B2PLYP-D3/cc-pVTZ method. The single point energies of all the reactants, products, and transition states are obtained at the DLPNO-CCSD(T)/CBS level of theory. Traditional transition state theory was employed to derive the rate constants, which were then fitted to obtain the kinetic parameters in the Arrhenius expression. Simulation results show improved performance in ignition delay times and oxidation species prediction, when the calculated reaction rate constants are implemented in the literature n-pentane kinetic models. The results offer enhanced comprehension of the chemistry of C5H12 + NO2, and will aid in the understanding of the mutual reactions of large hydrocarbon fuels and NOx.

Virtual undergraduate chemical engineering labs based on density functional theory calculations

Abstract Offering virtual laboratories to chemical engineering students is likely to extend beyond the COVID-19 era. Thus, there is a need to develop more simulation-based tasks in a wide blend of subjects, spanning thermodynamics, chemical reactions, and unit operations. Molecular and material modeling based on density functional theory (DFT) calculations provides limitless opportunities to train students on how to obtain thermokinetic values that are typically measured experimentally. Through a series of illustrative cases, this contribution provides detailed procedures pertinent to calculating reaction rate constants, standard enthalpies of reactions, redox potentials, equilibrium reaction constants, and surface-assisted mechanisms. We hope that DFT-operated virtual labs will assist students in attaining learning outcomes and promote their self-learning in the above-mentioned subjects. From a broader prospective, the presented case studies are expected to encourage students to appreciate the practical applications of DFT, not only in the academic but also in the industrial domains.

Caracal: A Versatile Ring Polymer Molecular Dynamics Simulation Package.

A new open-source program package named Caracal covering simulations of molecular systems with ring polymer molecular dynamics (RPMD) is presented. It combines a powerful RPMD implementation including chemical reaction rate calculations and biased periodic and nonperiodic samplings with a collection of easy to set up potential energy surface (PES) methodologies, thus delivering an all-inclusive approach. Most implemented PESs are based on the QMDFF and EVB-QMDFF methods. Where the quantum mechanically derived force field (QMDFF) can be set up for an arbitrary molecular system in a black-box fashion, the empirical valence bond (EVB)-QMDFF connects two QMDFFs and is able to represent the PES of a chemical reaction. With our previously published flavors of this composite method, PESs for almost arbitrary gas phase thermal ground state reactions can be set up. Given an optimized reaction path, the mechanism of the reaction can be classified and RPMD rate constants can be obtained via umbrella sampling and recrossing calculations on an EVB-QMDFF PES. Further, QMDFFs can be polymerized for the description of liquid systems. In this paper, the internal structure as well as the handling philosophy of Caracal are outlined. Further, examples of the different possible kinds of calculations are given.

How far can we stretch the timescale with RETIS?

Molecular dynamics (MD) and Monte Carlo (MC) have long coexisted as two main independent branches of molecular simulation. In the late eighties, however, algorithms based on the combination of both were created such as hybrid Monte Carlo which uses large MD steps as MC moves. An entirely different kind of combination emerged a decade later via the transition path sampling (TPS) method in which MD trajectories are not just part of the MC move, but also form the state space being sampled. Algorithms like replica exchange transition interface sampling (RETIS) exploit this idea to compute reaction rates via a series of TPS simulations. RETIS yields results identical to hypothetical long MD runs, but with exponentially reduced computation time. This perspective describes the RETIS method and discusses recent and future advancements that will enable the study of even longer molecular timescales with reasonable computational resources.

Characterization of Fast Neutron Transmission Through an Iron Shield

In this paper we give an analysis of the neutron transmission through an iron sphere using Monte Carlo and transport theory methods based on ENDF/B-VII.1 general purpose library. The motivation for this investigation comes from a well-known deficiency in the iron inelastic data from the older library evaluation (ENDF/B-V), giving a concern for a fast neutron flux underestimation within the reactor pressure vessels. In order to benchmark the next-generation ENDF/B-VI iron data, the U.S. Nuclear Regulatory Commission and the former Czechoslovakian National Research Institute have jointly preformed several experiments in 1990s, addressing neutron leakage spectra obtained for a 252Cf fission source in a centre of an iron sphere. It was shown that the ENDF/B-VI iron cross section, containing several improvements over previous evaluations, will not entirely resolve the neutron spectrum discrepancies observed at high neutron energies. Since safety analyses of reactor pressure vessel embrittlement are often based on neutron transport calculations using specific multigroup cross section libraries, simulation of this benchmark was performed using a hybrid shielding methodology of ADVANTG3.0.3 and MCNP6.1.1b codes. Comparison of calculated and referenced dosimeter activation rates are presented for several "standard" nuclear reactions, often used in reactor pressure vessel dosimetry. For that purpose, the new IRDFF-II special library from the IAEA Nuclear Data Services was used as a reference source of dosimetry cross sections. The MCNP6.1.1b code was used for calculation of reaction rates, which were also compared with previous IRDFF-1.05 special library and general purpose ENDF/B-VII.1 library.

A quantum chemical and kinetics study on the unimolecular reactions of fuel radicals formed during the oxidation of n‐propylamine

AbstractThe major intermediate product under co‐combustion conditions of hydrocarbon fuels with ammonia are amines. Herein, we have theoretically investigated the unimolecular reactions of fuel radicals formed during the oxidation of n‐propylamine. Quantum chemical calculations were performed to obtain the optimized structures and vibrational frequencies of reactants, products, and transition states at the B3LYP/aug‐cc‐pVTZ theory level. CCSD(T) single point energy calculations with the basis sets of aug‐cc‐pVDZ and aug‐cc‐pVTZ were further performed against the optimized structures, and the energies were extrapolated into infinite basis limit. Pressure‐dependent rate constants were calculated using Rice–Ramsperger–Kassel–Marcus/master equation analysis. The major reaction pathways for the fuel radicals were found to be β‐scission reactions at CC or CN bonds, and minor pathways were internal hydrogen shift and β‐scission reactions at CH or NH bonds. The calculated reaction rate constant trends were attributed to the decrease in CC and CN bond dissociation energies due to substitution effects from the amino group.

Complementarity and action mechanisms of Fe2+‐activated persulfate and H2O2 system

The Fe2+‐activated persulfate and H2O2 (Fe2+/persulfate/H2O2) system achieved 94% removal of four mixed sulfonamides with 300‐s treatment and possessed excellent complementarity and stability over a wide pH (3–11) and temperature (5–65°C) range. The quenching and electron spin resonance spectroscopy results confirmed the coexistence of sulfate radicals and hydroxyl radicals in the coupled system, which were responsible for the elimination of sulfonamides under ambient conditions. The reaction rate constants of sulfate radicals and hydroxyl radicals at possible reactive sites distinguished the difference in removal ratios of four sulfonamides, according to experimental determination and density functional theory calculations. The removal ratio of sulfathiazole was higher than others because its calculated reaction rate constants of sulfate radicals and hydroxyl radicals were higher than those of sulfamerazine, sulfamethoxazole, and sulfamethazine. The finding provided a reference for investigating the removal mechanism of mixed organic pollutants in the presence of multiple free radicals.