Thermal Rate Constants For Reactions Research Articles

Semiclassical reaction rate constant of non-adiabatic system by means of initial value representation

We applied the semiclassical initial value representation (SC-IVR) method with the classical electron analog to deal with electronically non-adiabatic reactive barriers and calculated the thermal reaction rate constant. The symmetric form of the flux–flux correlation function is used, and the matrix element of the Boltzmannized flux operator is derived using imaginary-time path integral techniques. This combined use of non-adiabatic SC-IVR and non-adiabatic path-integral sampling (NA-IVR&PI) is beyond the treatment of mean-field, and we analyze its performance in comparison with some existing mean-field methods for different scenarios of potential energy surface. We found a systematically better performance. One exception is the deep tunneling regime, where the mean-field ring-polymer instanton is superior. In the golden rule limit, where all these mean-field methods break down, the significant advantage of NA-IVR&PI is shown by using the electronic states for description of the dividing surface.

Structure-Based Reaction Descriptors for Predicting Rate Constants by Machine Learning: Application to Hydrogen Abstraction from Alkanes by CH3/H/O Radicals.

Accurate determination of the thermal rate constants for combustion reactions is a highly challenging task, both experimentally and theoretically. Machine learning has been proven to be a powerful tool to predict reaction rate constants in recent years. In this work, three supervised machine learning algorithms, including XGB, FNN, and XGB-FNN, are used to develop quantitative structure-property relationship models for the estimation of the rate constants of hydrogen abstraction reactions from alkanes by the free radicals CH3, H, and O. The molecular similarity based on Morgan molecular fingerprints combined with the topological indices are proposed to represent chemical reactions in the machine learning models. Using the newly constructed descriptors, the hybrid XGB-FNN algorithm yields average deviations of 65.4%, 12.1%, and 64.5% on the prediction sets of alkanes + CH3, H, and O, respectively, whose performance is comparable and even superior to the corresponding one using the activation energy as a descriptor. The use of activation energy as a descriptor has previously been shown to significantly improve prediction accuracy ( Fuel 2022, 322, 124150) but typically requires cumbersome ab initio calculations. In addition, the XGB-FNN models could reasonably predict reaction rate constants of hydrogen abstractions from different sites of alkanes and their isomers, indicating a good generalization ability. It is expected that the reaction descriptors proposed in this work can be applied to build machine learning models for other reactions.

Influence of Molecular Parameters on Rate Constants of Thermal Dissociation/Recombination Reactions: The Reaction System CF4 ⇄ CF3 + F.

The possibilities to extract incompletely characterized molecular parameters from experimental thermal rate constants for dissociation and recombination reactions are explored. The reaction system CF4 (+M) ⇄ CF3 + F (+M) is chosen as a representative example. A set of falloff curves is constructed and compared with the available experimental database. Agreement is achieved by minor (unfortunately not separable) adjustments of reaction enthalpy and collisional energy transfer parameters.

Characterization and Testing for Lowest Eutectic Mixture of TNBA/DNTF

AbstractIn this work, electrostatic spraying was used to prepare TNBA/DNTF eutectic mixtures with different mass ratios. The T−X phase diagram and the H−X phase diagram were drawn from the DSC curves of the eutectic mixtures. The morphology, composition, structure, thermal decomposition properties, mechanical sensitivity, thermal sensitivity, and detonation properties of the lowest eutectic were characterized by scanning electron microscope (SEM), energy dispersive X‐ray spectroscopy (EDS), X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (IR), X‐ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC) and thermogravimetric analysis‐mass spectrometry (TG‐MS), thermal sensitivity tests, mechanical sensitivity tests, and detonation performance calculations. The results showed that the best molar ratio of TNBA to DNTF was 59.41 : 40.59, whereas the best mass ratio was 60.17 : 39.83. The microscopic morphology was free of sharp edges and corners, the component proportions were the same as before electrostatic spraying, the surface elements were evenly distributed, and the crystal structure was essentially the same as those of the raw materials. The melting temperature was 352.0 K, which was 21.0 K and 33.1 K lower than those of the TNBA and DNTF raw materials, respectively. The thermal decomposition reaction rate constant (k), activation energy (EK), and preexponential factor (AK) of the lowest eutectic were 0.398 s−1, 81.34 kJ ⋅ mol−1, and 4.86×107 s−1, respectively. In the lowest eutectic, the component TNBA began to thermally decompose first, followed by DNTF. The main decomposition products of the lowest eutectic were CH4, H2O, H2, CO, N2, NH3, CO2, and N2O, with a small amount of C. The impact sensitivity (H50), friction sensitivity (P), and 5 s explosion temperature (T5s) of the lowest eutectic mixture were 39.7 cm, 28 %, and 563 K, respectively. Its detonation performance (OB=−36.52 %, QD=−4658 kJ ⋅ kg−1, vD=7598.37 m ⋅ s−1) was intermediate between those of TNBA and DNTF. The main detonation products were CO2, N2, CO, C(d), H2O, Br, and CH2O2.

Quantum Wave Packet Study of the H + Br2 → HBr + Br Reaction on a New Ab Initio Potential Energy Surface.

An accurate global potential energy surface (PES) for the HBr2 system has been constructed using the fundamental invariant neural network fitting method based upon 11 698 ab initio energies at the UCCSD(T)/CBS level of theory, with the spin-orbit coupling of the 2P3/2 orbit of the Br atom properly included. The time-dependent wave packet calculations have been performed to study the H + Br2 → HBr + Br reaction on the new PES. The total reaction probabilities for total angular momentum J = 0 for the ground initial state show no threshold due to the submerged barrier height (-0.351 kcal/mol) of the PES. The total integral cross sections (ICS) for reactant Br2 in ro-vibrational states (v0 = 0, j0 = 0, 10, 20, 30; v0 = 1-5, j0 = 0) were calculated for collision energy of up to 0.5 eV. It is found that the initial rotational excitation has a negligible effect on the ICS, and the initial vibrational excitation depresses the reactivity to some extent. The thermal rate constants for the title reaction in the temperature range of 100-1000 K were calculated from the Boltzmann averaging of the v0 = 0-5 rate constants, which overestimated the experimental results to some extent.

Simultaneous thermal analysis on the dynamical oxygen-lean combustion behaviors of coal in a O2/N2/CO2 atmosphere

This study aims at study the dynamical oxygen-lean combustion behaviors of coal in a multi-gas concomitant atmosphere. Two coal samples from Bulianta and Yuwu collieries were collected for simultaneous thermal analysis in O2/N2 (21 vol%/79 vol%) and O2/N2/CO2 (15 vol%/80 vol%/5 vol%) atmospheres. Based on the TG-DTG curves, five evolution stages were analyzed. A delay phenomenon occurred for coal lean-oxygen combustion in O2/N2/CO2, which was obvious at IV stage with a high staged temperature and a wide temperature range. Furthermore, the kinetic parameters were obtained after determining the most probable mechanism function, showing an increased apparent activation energy at III stage and a decreased one at IV stage in O2/N2/CO2 compared with in O2/N2, which indicated that the low heat conduction coefficient of CO2 played a dominant role on these changes. The study also showed the relationship between the thermal formation rate and reaction rate constant.

Pilgrim: A thermal rate constant calculator and a chemical kinetics simulator

Pilgrim is a program written in Python and designed to use direct dynamics in the calculation of thermal rate constants of chemical reactions by the variational transition state theory (VTST), based on electronic structure calculations for the potential energy surface. Pilgrim can also simulate reaction mechanisms using kinetic Monte Carlo (KMC).For reaction processes with many elementary steps, the rate constant of each of these steps can be calculated by means of conventional transition state theory (TST) or by using VTST. In the current version, Pilgrim can evaluate thermal rates using the canonical version of reaction-path VTST, which requires the calculation of the minimum energy path (MEP) associated with each elementary step or transition structure. Multi-dimensional quantum effects can be incorporated through the small-curvature tunneling (SCT) approximation. These methodologies are available both for reactions involving a single structure of the reactants and the transition state and also for reactions involving flexible molecules with multiple conformations of the reactant and/or of the transition state. For systems with many conformers, the program can evaluate each of the elementary reaction rate constants by multipath canonical VTST or multi-structural VTST. Moreover, the reactant can be unimolecular or bimolecular.Torsional anharmonicity can be incorporated through either the MSTor or the Q2DTor programs. Dual-level calculations are also available in Pilgrim: automatic high-level single-point energies can be used to correct the energy of reactants, transition states, products, and MEP points using the interpolated single-point energies (ISPE) algorithm.When the rate constants of all the chemical processes of interest are known, by means of their calculation using Pilgrim or alternatively through analytical fits to the rate constants as functions of temperature, it is possible to simulate a multistep mechanism under specified laboratory conditions using KMC. This algorithm allows performing a kinetic simulation to monitor the evolution of each chemical species with time and obtain the product yields. Program summaryProgram Title: PilgrimCPC Library link to program files:http://dx.doi.org/10.17632/24cj4dwxvg.1Developer’s repository link:https://github.com/cathedralpkg/pilgrim/releases; https://comp.chem.umn.edu/pilgrim; https://conservancy.umn.edu/handle/11299/166578Licensing provisions: MITProgramming language: Python 3Nature of problem: Calculation of thermal rate constants for bimolecular and unimolecular chemical reactions and simulation of reaction mechanismsSolution method: The program uses variational transition state theory to calculate thermal rate constants and kinetic Monte Carlo to simulate reaction mechanisms.Restrictions and unusual features: The program cannot treat reactions without saddle points. Unimolecular reactions are calculated only in the high-pressure limit. Direct dynamics calculations with Pilgrim require an electronic structure package to be supplied by the user; currently, Pilgrim supports the Gaussian [Frisch et al. (2003), Frisch et al. (2016a), Frisch et al. (2016b)] and Orca [Neese (2011)] electronic structure packages. Pilgrim has an especially powerful suite of options for handling torsional anharmonicity and multistructural effects.

Preparation and Photochromic Performance of Homogeneous Phase Nitrocellulose Membrane Grafting Spirooxazine Moieties

The synthesis of 1,3,3-trimethyl-9′-acryloxyspiro[indoline-2,3′(3H)naphtho[2,1-b][l,4]-oxazine] (AISO) was carried out by catalytic esterification of 1,3,3-trimethyl-9′-hydroxyspiro-[indoline-2,3′(3H)naphtho[2,1-b][l,4]oxazine] (SO–OH) and acrylic acid in the presence of 1,3-dicyclohexylcarbodiimide (DCC) and N-dimethylaminopyridine (DMAP). Then, the synthesis of the target copolymer (NC-g-AISO) was was carried out by benzoyl peroxide (BPO)-induced graft copolymerization of the AISO monomer onto nitrocellulose (NC) in a homogeneous methyl isobutyl ketone medium. The structure of NC-g-AISO was characterized by Fourier transform infrared (IR) spectroscopy, 13C Nuclear Magnetic Resonance (NMR) spectra and thermogravimetric (TG) analysis. The photochromic properties of NC-g-AISO were investigated by examining UV–Vis spectra in ethyl acetate solution and solid membrane. Compared with the AISO monomer in ethyl acetate solution, the thermal color decay stability of the colored form of NC-g-AISO in ethyl acetate solution and in solid membrane improved significantly. The thermal color decay reaction rate constants in ethyl acetate solution and membrane at 25 °C were 1.77 × 10–2 and 1.36 × 10–3 s–1, respectively, fitted using the first-order reaction equation. After ten photochromic cycles, the relative absorption intensity of the colored form of NC-g-AISO decreased by 0.85%, indicating that the NC-g-AISO membrane has good reversible photochromic behavior.

Rigorous close-coupling quantum dynamics calculation of thermal rate constants for the water formation reaction of H2 + OH on a high-level PES.

Thermal rate constants for the prototypical H2 + OH → H + H2O reaction are calculated using quantum dynamics simulations including all degrees of freedom and accurately accounting for overall rotation via close-coupling. Results are reported for a recent, highly accurate neural network potential [J. Chen et al., J. Chem. Phys. 138, 154301 (2013)] and compared to results obtained on a previous, semi-empirical potential. Thermal rate constants between 300 K and 1000 K are reported and very good agreement with experimental work is found. Additionally, reasonable agreement for the close-coupling simulations on both potentials is found. In contrast to previous work, we find that the J-shifting approximation works well for the title reaction given that a high-level PES is used for the dynamics calculation. Moreover, the importance of treating the spin-orbit coupling in the reactant partition function is discussed. The highly accurate results reported here will provide a benchmark for the development of approximate methods.

Investigation of coriolis coupling effect on the ND + D reaction

We present time dependent quantum wave packet calculations for the NDa1Δ+D'2S reaction on the modified NH2A∼2A1 potential energy surface using the real wave packet and flux analysis methods. Initial-state-resolved reaction probabilities, cross sections and rate constants were obtained taking into account the Coriolis Coupling (CC). The significance of including the CC quantum scattering calculations has been revealed by the comparison between the CC and the Centrifugal Sudden (CS) approximation calculation. The calculated thermal reaction rate constant is in a good agreement with the available experimental data and semiclassical results.

Mechanism and thermal rate constants for complete series reactions of bromochlorophenols with H

Formation of bromochlorophenoxy radicals (BCPRs) from the reaction of bromochlorophenols (BCPs) with H can play the most central role in the formation of mixed polybrominated and chlorinated dibenzo-p-dioxins and dibenzofurans (PBCDD/Fs).

Relevance of the Nuclear Quantum Effects on the Proton/Deuteron Transmission through Hexagonal Boron Nitride and Graphene Monolayers

According to recent experiments, atomically thin hexagonal boron nitride and graphene are permeable to protons and deuterons (and not to other atomic species), and the experimental estimates of the activation energy are lower than the theoretical values by about 0.5 eV for the isolated proton–membrane transfer model. Our analysis of the electronic potential energy surfaces along the normal to the transmission direction, obtained using correlated electronic structure methods, suggests that the aqueous environment is essential to stabilize the proton transmission, as opposed to the hydrogen atom. Therefore, the process is examined within a molecular model of H2O–H(D)+–material–H2O. Exact quantum-mechanical scattering calculations are performed to assess the relevance of the nuclear quantum effects, such as tunneling factors and the kinetic isotope effect (KIE). Deuteration is found to affect the thermal reaction rate constants (KIE of 3–4 for hexagonal boron nitride and 20–30 for the graphene) and to effect...

Potential energy construction in the diabatic picture for quantum mechanical rate constants of intermolecular proton transfer.

We propose a simple method for potential construction in the diabatic picture and the estimation of thermal rate constants for intermolecular proton transfer reactions using quantum dynamics simulations carried out on the constructed potentials. For symmetrical and asymmetrical proton transfer pairs, the obtained potentials and rate constants were in good agreement with the reference values. Furthermore, our method is used for the analysis of proton transfer in crystalline imidazolium succinate and discusses the proton conductivity in terms of intermolecular proton transfer. This approach can be used to estimate proton transfer rate constants for large molecular systems, even when the calculation of the transition state is impossible.

Effects of Lattice Motion on Dissociative Chemisorption: Toward a Rigorous Comparison of Theory with Molecular Beam Experiments.

The dissociative chemisorption of small molecules such as methane and water on metal surfaces is a key step in many important catalyzed reactions. However, it has only very recently become possible to directly compare theory with molecular beam studies of these reactions. For most experimental conditions, such a comparison requires accurate methods for introducing the effects of lattice motion into quantum reactive scattering calculations. We examine these methods and their recent application to methane and water dissociative chemisorption. New results are presented for CO2 chemisorption and methane dissociation at step edges. The type of molecule-lattice coupling that leads to a strong variation in the dissociative sticking of methane with temperature is shown to occur for many polyatomic-metal systems. Improvements to these models are discussed. The ability to accurately compare theory with molecular beam experiments should lead to improved density functionals and consequently more accurate thermal rate constants for these important reactions.

Quantum dynamics of (16)O + (36)O2 and (18)O + (32)O2 exchange reactions.

We present quantum dynamical investigations of (16)O + (36)O2 and (18)O + (32)O2 exchange reactions using a time-independent quantum mechanical method and an accurate global potential energy surface of ozone [Dawes et al., J. Chem. Phys. 135, 081102 (2011)]. Initial state-selected integral cross sections, rate constants, and Boltzmann averaged thermal rate constants are obtained and compared with earlier experimental and theoretical results. The computed thermal rate constants for the oxygen exchange reactions exhibit a negative temperature dependence, as found experimentally. They are in better agreement with the experiments than the previous studies on the same reactions.

Accurate ab Initio Thermal Rate Constants for Reaction of O(3P) with H2 and Isotopic Analogues

Semiclassical transition state theory, in combination with high accuracy quantum chemistry, is used to compute thermal rate constants from first principles for the O((3)P) + H2 reaction and its isotopic counterparts. In the temperature regime of 298-3500 K (which spans 8 orders of magnitude for rate constants), our theoretical results are in excellent agreement (within 5-15%) with all available experimental data from 298 to 2500 K but are somewhat too low (from 15 to 35%) at higher temperatures. Several possible reasons that might cause the degradation at high temperatures are discussed. Vibrational state-selected rate constants and their correlations with normal thermal rate constant are derived and are given in the Supporting Information.

The Influence of Collision Energy on the Reaction H+HS→H2+S

Quasi-classical trajectory calculations have been carried out for the reaction H+HS by using the newest triplet 3A potential energy surface (PES). The effects of the collision energy and reagent initial rotational excitation are studied. The cross sections and thermal rate constants for the title reaction are calculated. The results indicate that the integral cross sections (ICSs) are sensitive to the collision energy and almost independent to the initial rotational states. The ro-vibrational distributions for the product at different collision energies are presented. The investigations on the vector correlations are also performed. It is found that the collision energies play a postive role on the forward scatter of the product molecules. There is a negative influence on both the alignment and orientation of the product angular momentum for low collision energy at low energy region. Whereas the influence of collision energy is not obvious at high energy region.

Theoretical Study of Reaction of Ketene with Water in the Gas Phase: Formation of Acetic Acid?

Production of acetic acid via gas-phase hydration of ketene by water (uncatalyzed and in the presence of an additional water molecule) was theoretically characterized using high-level coupled-cluster methods, followed by a two-dimensional master equation analysis to compute thermal reaction rate constants. The results show that the formation of acetic acid quite likely occurs in high-temperature combustion of biomass, but that the rate of formation should be negligible under ambient atmospheric conditions.

Coupled Unimolecular Dissociation Kinetics of Bromotoluene Radical Cations

The unimolecular dissociations of o-, m-, and p-bromotoluene radical cations to C7H7(+) (benzylium and tropylium) are examined by considering the coupling of the three isomers in the dissociation pathways. The potential energy surface obtained from ab initio calculations suggests the interconversion of isomers through methylene and hydrogen migrations on the ring. The rate equations for each isomer are combined together to form a rate matrix for coupled reactions. The rate matrix contains the microcanonical rate constants for all elementary steps, which are calculated using Rice-Ramsperger-Kassel-Marcus theory based on the molecular parameters obtained from density functional theory. The unimolecular dissociation rates for coupled reactions are determined by numerically solving the matrix equation. As a result of reaction coupling, the product branching ratio becomes time-dependent and the reaction rates of three isomers become parallel to one another as the energy increases, although their initial rates differently vary with energy. The calculated rate-energy curves fall below the time-resolved photodissociation data in the energy range 2.2-2.7 eV but are in line with the photoelectron photoion coincidence data in the energy range 2.7-3.5 eV. The discrepancy between experiment and theory in the low-energy region is ascribed to the uncertainties of the potential energy surface as well as the contribution of the radiative relaxation rate that has not been taken into account in the theoretical calculations. The rate-energy curves are then used to calculate the thermal reaction rate constants, and the Arrhenius parameters are determined in the temperature range 700-1300 K. Comparison of the activation energy and entropy obtained from the Arrhenius plot with the calculated enthalpy and entropy changes between the reactant and the highest-lying transition state suggests that a series of [1,2] H-atom migrations occurring near the entrance comprise the rate-determining steps and the subsequent [1,2] H-atom migrations play an important role in increasing the activation energy and decreasing the entropy by reducing the net flux to the exit.

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.