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Anharmonicity can greatly affect rate constants. One or even several orders of magnitude of deviation are found for obtaining rate constants using the standard rigid-rotor harmonic-oscillator model. In turn, reactive molecular dynamics (MD) simulations are a powerful way to explore chemical reaction networks and calculate rate constants from the fully anharmonic potential energy surface. However, the classical nature of the dynamics and the required numerical efficiency of the force field limit the accuracy of the resulting kinetics. We combine the best of both worlds by presenting an approximation that pairs anharmonic information intrinsic to classical MD with high-accuracy energies and frequencies from quantum-mechanical electronic structure calculations. The proposed scheme is applied to hydrogen abstractions in the methane system, which allows for the benchmarking of rate constants corrected by our approach against experimental rate constants. This comparison reveals a standard deviation of factor 2.6. Two archetypes of possible failure are identified in the course of a detailed investigation of the CH3• + H• → CH22• + H2 reaction. From this follows the application range of the method, within which the method shows a standard deviation of factor 2.1. The computational efficiency and beneficial scaling of the method allow for application to larger systems, as shown for hydrogen abstraction from 2-butanone by HO2•.

Biomass derived chemicals may offer sustainable alternatives to petroleum derived hydrocarbons, while also enhancing engine combustion performance with co-optimization of fuels and engines. This paper presents a numerical study on the oxidation and combustion of a novel biofuel compound, cyclopentanol. Its reaction kinetics and thermochemistry are first explored using ab initio quantum chemistry methods. Thermochemical properties are calculated for cyclopentanol and a set of its key oxidation intermediates. C-H bond dissociation energies of cyclopentanol are computed for different carbon sites. For the fuel radicals, the energy barriers of their ring-opening reactions and the potential energy surfaces of their oxidation reactions are determined. Based on the theoretical results, a chemical kinetic mechanism is proposed to describe the oxidation of cyclopentanol at low and high temperatures. The model is compared against data obtained from shock tube, rapid compression machine, combustion vessel, and counterflow burner experiments over a range of initial conditions. Furthermore, reaction pathway analysis is performed using the present mechanism to give insights into the underlying oxidation chemistry of cyclopentanol. It is found that the α-radical of cyclopentanol undergoes preferably an alcohol-specific HO2 elimination reaction to form stable cyclopentanone and this reaction can strongly retard reactivity. The major reaction pathways of β- and γ-radicals are similar to those of cyclopentyl radicals that are the sequential and formally direct reactions of fuel radicals with O2 to form cyclopentenols and HO2 radicals. The existence of the hydroxy moiety affects the bond dissociation energies and reaction barriers, slightly favoring the chain-branching channel for γ-radicals at low temperatures.

The kinetics of chemical reactions in the liquid phase are often strongly determined by the reaction solvent. Consequently, the choice of the optimal solvent is an important task in chemical proces...

In this study (Part II), the oxidation of dimethoxymethane (DMM) is investigated and a detailed chemical reaction model developed for a comprehensive description of both high- and low-temperature oxidation processes. The sub-mechanism of DMM is implemented using AramcoMech2.0 as the base mechanism. Rate coefficients are based on analogies with those for dimethyl ether, diethyl ether, and n-pentane oxidation. Furthermore, theoretical studies from recent works are also included in the present model and new calculations for the dissociation kinetics of Q ˙ OOH radicals have been carried out at the CCSD(T)/CBS(aug-cc-pVXZ; X = D, T) // B2PLYP-D3/6-311 + + G(d,p) level of theory. For validation, new ignition delay time experiments have been performed in a shock tube (ST), a rapid compression machine (RCM), and in a laminar flow reactor covering a wide range of conditions (p = 1–40 bar, T = 590–1215 K, φ = 1). In addition, the kinetic model is validated against laminar burning velocities, jet-stirred reactor, plug flow reactor and further ST and RCM experimental datasets from the literature. Pathway and sensitivity analyses were used to identify critical reaction pathways in the DMM oxidation mechanism. These show that the reactivity of DMM at intermediate temperatures is controlled by the branching between pathways initiated on the primary or secondary fuel radical. While primary fuel radicals eventually lead to chain branching, secondary fuel radical consumption is controlled by fast β-scission over a wide range of temperatures, which inhibits reactivity.

Ultra-low cross-linked (ULC) microgels are extremely soft polymers with advanced material properties exhibiting a colloid-to-polymer transition. Poly(N-isopropylacrylamide) (pNIPAM) microgels are commonly synthesized by radical precipitation polymerization of the fully protonated monomer N-isopropylacrylamide (NIPAM). Analogous deuterated monomers arouse interest regarding their varying scattering length densities in small-angle neutron scattering (SANS). An isotope substitution in the main monomer of a microgel changes its scattering length density, which enables the exploitation of the technique of contrast variation in SANS. Here, we demonstrate that the synthesis of the deuterated pNIPAM ULC microgels encounters challenges related to the self-cross-linking mechanism of the polymer chains. The location of the deuterium isotopes is crucial for the generation of deuterated pNIPAM ULC microgels: when the isopropyl group of NIPAM is deuterated, the cross-linking is strongly restrained and the formation of microgels is precluded. However, the deuteration of the vinyl group of NIPAM, ending as backbone in the pNIPAM chains, allows the self-cross-linking. Ab initio calculations of the bond dissociation enthalpies endorse that the cross-linking of the pNIPAM chains occurs via hydrogen atom abstraction at the tertiary carbon atom of the isopropyl group. Additionally, we show that the deuteration of the vinyl group barely shifts the transition temperature of the polymer compared to protonated pNIPAM, whereas deuteration of the isopropyl group results in a significant shift of the transition temperature toward higher temperatures. Finally, the deuterated ULC micro- and nanogels reveal a stronger cross-linked network with lower swelling ability compared to the protonated microgels.

Diethoxymethane (DEM) is a promising oxygenated fuel and fuel additive, which has similar positive combustion characteristics as dimethoxymethane. DEM contains C-C bonds and can form ethylene via beta-scission, which potentially increases its sooting tendency. Since DEM is rarely studied, however, kinetic modeling attempts are forced to rely on rate constant analogies. Therefore, we employ high level CCSD(T)/aug-cc-pV(T+D)Z//B2PLYPD3BJ/6-311++(d,p) theory along with transition state theory to predict reaction rate constants for H-abstraction by H and CH3 and the subsequent unimolecular reactions. We further prove that the DLPNO approximation to CCSD(T) leads to a deviation of less than 0.25 k/mol in barrier heights for the presently studied open-shell electronic structures and use it for the prediction of reaction rate constants for H-abstraction by C2H5 radicals. We find that H-abstraction by ethyl radicals might denote a significant pathway, which should not be neglected in kinetic modeling studies of DEM. It is also shown that reaction pathways leading to ethylene formation are of minor importance and give thereby a first insight into the fate of the C-C bonds. To the best of our knowledge, this study represents the first high-level ab-initio study of DEM, which makes the reaction kinetics and thermochemistry data provided by this study vital for future comprehensive kinetic modeling of DEM. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Functional microgels are novel materials, which can potentially be used for diverse applications such as drug delivery. Among other material properties, the radial cross-linker profile is of special importance to ensure desired product functionality. However, in the system under investigation, only a high cross-linker fraction in the center and a low cross-linker fraction in the shell is obtained in batch synthesis due to the fast reaction kinetics of the cross-linker. Other radial cross-linker fraction profiles can be synthesized in fed-batch, yet the determination of the required feeding profiles is not intuitively obvious. Based on recently developed quantitative nonlinear process models, a fed-batch synthesis can be dynamically optimized under constraints set by product quality and process limitations. Here, we apply dynamic optimization to microgel synthesis to obtain large microgels under constraints posed by the radial cross-linker profile and conversion. We present three different case studies to show in silico that a wide range of different cross-linker fraction profiles is feasible. The results show that the specified constraints are met and that various cross-linker profiles can be achieved while maintaining the microgel size.

The prediction of solvation free energies is essential for a variety of applications. Solvation free energies of neutral systems can be predicted quite accurately. The accuracy of predictions for solvation free energies of ionic solutes dissolved in neutral solvents, however, has been reported to be worse by at least 1 order of magnitude. In this study, the performance of three approaches for solvation free energy prediction of several hundred ions dissolved in neutral solvents is evaluated. The applied methods are COSMO-RS, cluster continuum model (CCM) together with COSMO-RS, and COSMO-RS-ES. It is emphasized that the reference data for model evaluation are subject to large uncertainties stemming from the impossibility to measure the so-called elusive absolute free energies of solvation of a single ion. Consequently, such uncertainty must be considered during the evaluation of prediction methods. Therefore, a straightforward approach to account for the underlying uncertainty is applied here. Hereby, it is revealed that the true performance of the method is better than what is often reported. The average absolute deviation (AAD) of COSMO-RS is calculated to be 2.3 kcal mol-1, while applying the CCM and COSMO-RS-ES each results in AADs of 2.0 kcal mol-1. This accuracy allows for qualitative assessment of solvation free energy-dependent quantities, such as reaction rate constants.

Combustion and flame 189, 433-442 (2018). doi:10.1016/j.combustflame.2017.07.037, Published by Elsevier Science, Amsterdam [u.a.]

A novel lignocellulosic biofuel 2-butyltetrahydrofuran (2-BTHF) is investigated experimentally and numerically in the present study. Laminar burning velocities were measured in a combustion vessel. Ignition delay times were experimentally determined in a rapid compression machine and in a shock tube at high pressures covering a wide temperature range. A detailed chemical kinetic mechanism is proposed to describe the oxidation of 2-BTHF at both low and high temperatures. It is demonstrated that the model gives satisfactory results for the various experimental configurations. For a better understanding of the oxidation kinetics, the chemical model is further used to elucidate the reaction pathways of 2-BTHF. A comparison of its ignition behavior with other C 8 hydrocarbons is further presented and the impact of the tetrahydrofuranic ring on the fuel ignition characteristics is highlighted and analyzed.

The journal of chemical physics 152(16), 164303 (2020). doi:10.1063/5.0004465, The site-specific first microsolvation step of furan and some of its derivatives with methanol is explored to benchmark the ability of quantum-chemical methods to describe the structure, energetics, and vibrational spectrum at low temperature. Infrared and microwave spectra in supersonic jet expansions are used to quantify the docking preference and some relevant quantum states of the model complexes. Microwave spectroscopy strictly rules out in-plane docking of methanol as opposed to the top coordination of the aromatic ring. Contrasting comparison strategies, which emphasize either the experimental or the theoretical input, are explored. Within the harmonic approximation, only a few composite computational approaches are able to achieve a satisfactory performance. Deuteration experiments suggest that the harmonic treatment itself is largely justified for the zero-point energy, likely and by design due to the systematic cancellation of important anharmonic contributions between the docking variants. Therefore, discrepancies between experiment and theory for the isomer abundance are tentatively assigned to electronic structure deficiencies, but uncertainties remain on the nuclear dynamics side. Attempts to include anharmonic contributions indicate that for systems of this size, a uniform treatment of anharmonicity with systematically improved performance is not yet in sight., Published by American Institute of Physics, Melville, NY

Often, the first step of fuel combustion is the formation of fuel radicals. These radicals can be formed via hydrogen abstraction and are typically rovibrationally excited due to the energy release from the reaction. These excitations increase the population at high energies, thus increase the probability for β -scission of the fuel radical. Hot β -scission of such non-thermal radicals can be interpreted as well-skipping reaction from the bimolecular hydrogen abstraction reactants to the dissociation products. In the present study, the impact of non-Boltzmann radical distributions on β -scission kinetics is screened for several hundred fuels. For that purpose, the free energy release between stable stationary points is predicted using group contribution theory. Based on that screening, fuel radicals with potentially pronounced β -scission of rovibrationally excited fuel radicals are picked for more detailed study. Hot β -scission is found to significantly contribute to the CO 2 loss reactions of formate radicals formed via hydrogen abstraction. Especially for methyl formate, β -scission of non-thermal radicals is found to dominate over a wide range of temperature and pressure. This reveals the need for including the origin of reactants of β -scission reactions.

2-butanone was recently identified as a promising gasoline biofuel. Its kinetic modeling requires high-level kinetic predictions of key reactions to reduce the uncertainty in the reaction model. The present work provides rate constants for hydrogen abstraction from 2-butanone by H ˙ and C ˙ H 3 . Subsequent unimolecular reactions on the 2-butanoyl radical potential energy surface were studied using RRKM/Master Equation. The updated rate constants deviate from previous predictions by up to two orders of magnitude. The potential energy surfaces computed for hydrogen abstraction and β-scission were connected by using rovibrationally excited 2-butanoyl radicals as reactants of the β-scission reaction network. The energy distributions of these excited radicals were computed from the preceding hydrogen abstraction reactions. Though at engine-relevant conditions rovibrationally excited 2-butanoyl radicals were found to be negligible, kinetic modeling of flames at very low pressures and high temperatures would require to include reactions of excited fuel radicals. The reported rate constants significantly change the ignition delay predictions, emphasizing the need for more accurate predictions for novel fuels.

Growing environmental concern and strict regulations led to an increasing effort of the chemical industry to develop greener production pathways. To ensure that this development indeed improves environmental aspects requires an early-stage estimation of the environmental impact in early process design. An accepted method to evaluate the environmental impact is Life Cycle Assessment (LCA). However, LCA requires detailed data on mass and energy balances, which is usually limited in early process design. Therefore, predictive LCA approaches are required. Current predictive LCA approaches estimate the environmental impacts of chemicals only based on molecular descriptors. Thus, the predicted impacts are independent from the chosen production process. A potentially greener process cannot be distinguished from the conventional route. In this work, we propose a fully predictive, neural network-based framework, which utilizes both molecular and process descriptors to distinguish between production pathways. The framework is fully automatized and includes feature selection, setup of the network architecture, and predicts 17 environmental impact categories. The pathway-specific prediction is illustrated for two examples, comparing the CO2-based production of methanol and formic acid to their respective fossil production pathway. The presented framework is competitive to LCA predictions from literature but can now also distinguish between process alternatives. Thus, our framework can serve as initial screening tool to identify environmentally beneficial process alternatives.

The journal of chemical physics 148(1), 1-13 (2018). doi:10.1063/1.5009011, Published by American Institute of Physics, Melville, NY

Microgels based on poly(N-isopropylacrylamide)- and poly(N-vinylcaprolactam) can be synthesized by precipitation polymerization. To get a better insight into the kinetics of the microgel synthesis, a model-based approach is pursued. Herein, the approach proposed by Janssen et al. (2017) is extended to the terpolymerization system. The kinetic model for a two-phase terpolymerization is combined with parameter values from quantum mechanical calculations. The remaining unknown parameter values are estimated from experimental data using reaction calorimetry and Raman spectroscopy. Acceptable agreement of simulation and experimental measurements is obtained. The prediction of the gel phase growth is combined with the consumption of comonomers and the cross-linker N,N’-methylenebis(acrylamide) to predict the average distribution of comonomers, thus giving insight into the microgel structure. The resulting particle structures indicate uniform N-vinylcaprolactam and N-isopropylacrylamide compositions in the core and an outer layer with a high N-vinylcaprolactam fraction, while the cross-linker distribution decreases from core to shell.

Thermodynamic properties are often modeled by classical force fields which describe the interactions on the atomistic scale. Molecular simulations are used for retrieving thermodynamic data from such models, and many simulation techniques and computer codes are available for that purpose. In the present round robin study, the following fundamental question is addressed: Will different user groups working with different simulation codes obtain coinciding results within the statistical uncertainty of their data? A set of 24 simple simulation tasks is defined and solved by five user groups working with eight molecular simulation codes: DL_POLY, GROMACS, IMC, LAMMPS, ms2, NAMD, Tinker, and TOWHEE. Each task consists of the definition of (1) a pure fluid that is described by a force field and (2) the conditions under which that property is to be determined. The fluids are four simple alkanes: ethane, propane, n-butane, and iso-butane. All force fields consider internal degrees of freedom: OPLS, TraPPE, and a modified OPLS version with bond stretching vibrations. Density and potential energy are determined as a function of temperature and pressure on a grid which is specified such that all states are liquid. The user groups worked independently and reported their results to a central instance. The full set of results was disclosed to all user groups only at the end of the study. During the study, the central instance gave only qualitative feedback. The results reveal the challenges of carrying out molecular simulations. Several iterations were needed to eliminate gross errors. For most simulation tasks, the remaining deviations between the results of the different groups are acceptable from a practical standpoint, but they are often outside of the statistical errors of the individual simulation data. However, there are also cases where the deviations are unacceptable. This study highlights similarities between computer experiments and laboratory experiments, which are both subject not only to statistical error but also to systematic error.

Journal of chemical theory and computation : JCTC 13(9), 3955-3960 (2017). doi:10.1021/acs.jctc.7b00524, Published by American Chemical Society, Washington, DC

The journal of physical chemistry / B 121(13), 2887-2895 (2017). doi:10.1021/acs.jpcb.6b09147, Published by American Chemical Society, Washington, DC

Journal of chemical theory and computation 11(6), 2517-2524 (2015). doi:10.1021/acs.jctc.5b00201, Published by American Chemical Society (ACS), Washington, DC