Collision Efficiency of Water in the Unimolecular Reaction CH₄ (+H₂O) ⇆ CH₃ + H (+H₂O): One-Dimensional and Two-Dimensional Solutions of the Low-Pressure-Limit Master Equation

The low-pressure-limit unimolecular decomposition of methane, CH₄ (+M) ⇆ CH₃ + H (+M), is characterized via low-order moments of the total energy, <i>E</i>, and angular momentum, <i>J</i>, transferred due to collisions. The low-order moments are calculated using ensembles of classical trajectories, with new direct dynamics results for M = H₂O and new results for M = O₂ compared with previous results for several typical atomic (M = He, Ne, Ar, Kr) and diatomic (M = H₂ and N₂) bath gases and one polyatomic bath gas, M = CH₄. The calculated moments are used to parametrize three different models of the energy transfer function, from which low-pressure-limit rate coefficients for dissociation, <i>k</i>₀, are calculated. Both one-dimensional and two-dimensional collisional energy transfer models are considered. The collision efficiency for M = H₂O relative to the other bath gases (defined as the ratio of low-pressure limit rate coefficients) is found to depend on temperature, with, e.g., <i>k</i>₀(H₂O)/<i>k</i>₀(Ar) = 7 at 2000 K but only 3 at 300 K. We also consider the <i>rotational</i> collision efficiency of the various baths. Water is the only bath gas found to fully equilibrate rotations, and only at temperatures below 1000 K. At elevated temperatures, the kinetic effect of “weak-collider-in-<i>J</i>” collisions is found to be small. At room temperature, however, the use of an explicitly two-dimensional master equation model that includes weak-collider-in-<i>J</i> effects predicts smaller rate coefficients by 50% relative to the use of a statistical model for rotations. The accuracies of several methods for predicting relative collision efficiencies that do not require solving the master equation and that are based on the calculated low-order moments are tested. Troe’s weak collider efficiency, β_c, includes the effect of saturation of collision outcomes above threshold and accurately predicts the relative collision efficiencies of the nine baths. Finally, a brief discussion is presented of mechanistic details of the energy transfer process, as inferred from the trajectories

The Role of Excited Electronic States in Hypervelocity Collisions: Enhancement of the O(³P) + HCl → OCl + H Reaction Channel

The role of excited electronic states in the O + HCl reaction was studied using the quasi-classical trajectory method for collision energies between 1 and 5.5 eV. Global potential energy surfaces were developed for the ground (³A′′) and first excited (³A′) electronic states of the OHCl system using an interpolating moving least-squares-based method for energies up to 6.5 eV above the reactant valley. High-accuracy ab initio data were computed at automatically selected points using an 18-electronic-state model and the generalized dynamically weighted multireference configuration interaction (GDW-MRCI) method extrapolated to the complete basis set limit. The results show significant dynamical differences between ground- and excited-state reactions. At high collision energies, over half of the total OCl reactive flux originates from reactions on the ³A′ state, whereas OH is produced almost exclusively by the ³A′′ state. Inclusion of the excited electronic state, therefore, dramatically alters the OCl/OH product branching ratio

Quasi-Classical Trajectory Calculation of Rate Constants Using an Ab Initio Trained Machine Learning Model (aML-MD) with Multifidelity Data

Machine learning (ML) provides a great opportunity for the construction of models with improved accuracy in classical molecular dynamics (MD). However, the accuracy of a ML trained model is limited by the quality and quantity of the training data. Generating large sets of accurate ab initio training data can require significant computational resources. Furthermore, inconsistent or incompatible data with different accuracies obtained using different methods may lead to biased or unreliable ML models that do not accurately represent the underlying physics. Recently, transfer learning showed its potential for avoiding these problems as well as for improving the accuracy, efficiency, and generalization of ML models using multifidelity data. In this work, ab initio trained ML-based MD (aML-MD) models are developed through transfer learning using DFT and multireference data from multiple sources with varying accuracy within the Deep Potential MD framework. The accuracy of the force field is demonstrated by calculating rate constants for the H + HO2 → H2 + 3O2 reaction using quasi-classical trajectories. We show that the aML-MD model with transfer learning can accurately predict the rate constants while reducing the computational cost by more than five times compared to the use of more expensive quantum chemistry training data sets. Hence, the aML-MD model with transfer learning shows great potential in using multifidelity data to reduce the computational cost involved in generating the training set for these potentials

Thermal Dissociation and Roaming Isomerization of Nitromethane: Experiment and Theory

The thermal decomposition of nitromethane provides a classic example of the competition between roaming mediated isomerization and simple bond fission. A recent theoretical analysis suggests that as the pressure is increased from 2 to 200 Torr the product distribution undergoes a sharp transition from roaming dominated to bond-fission dominated. Laser schlieren densitometry is used to explore the variation in the effect of roaming on the density gradients for CH₃NO₂ decomposition in a shock tube for pressures of 30, 60, and 120 Torr at temperatures ranging from 1200 to 1860 K. A complementary theoretical analysis provides a novel exploration of the effects of roaming on the thermal decomposition kinetics. The analysis focuses on the roaming dynamics in a reduced dimensional space consisting of the rigid-body motions of the CH₃ and NO₂ radicals. A high-level reduced-dimensionality potential energy surface is developed from fits to large-scale multireference ab initio calculations. Rigid body trajectory simulations coupled with master equation kinetics calculations provide high-level a priori predictions for the thermal branching between roaming and dissociation. A statistical model provides a qualitative/semiquantitative interpretation of the results. Modeling efforts explore the relation between the predicted roaming branching and the observed gradients. Overall, the experiments are found to be fairly consistent with the theoretically proposed branching ratio, but they are also consistent with a no-roaming scenario and the underlying reasons are discussed. The theoretical predictions are also compared with prior theoretical predictions, with a related statistical model, and with the extant experimental data for the decomposition of CH₃NO₂, and for the reaction of CH₃ with NO₂

Comment on “When Rate Constants Are Not Enough”

Comment on “When Rate Constants Are Not Enough

Radical–Radical Reactions in Molecular Weight Growth: The Phenyl + Propargyl Reaction

The mechanism for hydrocarbon ring growth in sooting environments is still the subject of considerable debate. The reaction of phenyl radical (C6H5) with propargyl radical (H2CCCH) provides an important prototype for radical–radical ring-growth pathways. We studied this reaction experimentally over the temperature range of 300–1000 K and pressure range of 4–10 Torr using time-resolved multiplexed photoionization mass spectrometry. We detect both the C9H8 and C9H7 + H product channels and report experimental isomer-resolved product branching fractions for the C9H8 product. We compare these experiments to theoretical kinetics predictions from a recently published study augmented by new calculations. These ab initio transition state theory-based master equation calculations employ high-quality potential energy surfaces, conventional transition state theory for the tight transition states, and direct CASPT2-based variable reaction coordinate transition state theory (VRC-TST) for the barrierless channels. At 300 K only the direct adducts from radical–radical addition are observed, with good agreement between experimental and theoretical branching fractions, supporting the VRC-TST calculations of the barrierless entrance channel. As the temperature is increased to 1000 K we observe two additional isomers, including indene, a two-ring polycyclic aromatic hydrocarbon, and a small amount of bimolecular products C9H7 + H. Our calculated branching fractions for the phenyl + propargyl reaction predict significantly less indene than observed experimentally. We present further calculations and experimental evidence that the most likely cause of this discrepancy is the contribution of H atom reactions, both H + indenyl (C9H7) recombination to indene and H-assisted isomerization that converts less stable C9H8 isomers into indene. Especially at low pressures typical of laboratory investigations, H-atom-assisted isomerization needs to be considered. Regardless, the experimental observation of indene demonstrates that the title reaction leads, either directly or indirectly, to the formation of the second ring in polycyclic aromatic hydrocarbons

Detection and Identification of the Keto-Hydroperoxide (HOOCH₂OCHO) and Other Intermediates during Low-Temperature Oxidation of Dimethyl Ether

In this paper we report the detection and identification of the keto-hydroperoxide (hydroperoxymethyl formate, HPMF, HOOCH₂OCHO) and other partially oxidized intermediate species arising from the low-temperature (540 K) oxidation of dimethyl ether (DME). These observations were made possible by coupling a jet-stirred reactor with molecular-beam sampling capabilities, operated near atmospheric pressure, to a reflectron time-of-flight mass spectrometer that employs single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation. On the basis of experimentally observed ionization thresholds and fragmentation appearance energies, interpreted with the aid of <i>ab initio</i> calculations, we have identified HPMF and its conceivable decomposition products HC­(O)­O­(O)­CH (formic acid anhydride), HC­(O)­OOH (performic acid), and HOC­(O)­OH (carbonic acid). Other intermediates that were detected and identified include HC­(O)­OCH₃ (methyl formate), <i>cycl</i>-CH₂–O–CH₂–O– (1,3-dioxetane), CH₃OOH (methyl hydroperoxide), HC­(O)­OH (formic acid), and H₂O₂ (hydrogen peroxide). We show that the theoretical characterization of multiple conformeric structures of some intermediates is required when interpreting the experimentally observed ionization thresholds, and a simple method is presented for estimating the importance of multiple conformers at the estimated temperature (∼100 K) of the present molecular beam. We also discuss possible formation pathways of the detected species: for example, supported by potential energy surface calculations, we show that performic acid may be a minor channel of the O₂ + ĊH₂OCH₂OOH reaction, resulting from the decomposition of the HOOCH₂OĊHOOH intermediate, which predominantly leads to the HPMF