A new ab initio based global HOOH(13A″) potential energy surface for the O(3P) + H2O(X1A1) ↔ OH(X2Π) + OH(X2Π) reaction

Importance of Spin Channels from Radical-Radical Reactions in Hydrogen-Oxygen Combustion Mechanisms at High Temperatures

Radical-radical reactions can generate two channels with high and low spins. In this work, ten radical-radical reactions with different spin channels and four radical-molecule reactions in hydrogen-oxygen combustion were systematically investigated from a theoretical perspective. The potential energy surface (PES) of radical-radical reactions reveals that the high- and low-spin states of the reactant are energetically degenerate and the two channels are energetically feasible. The difference in rate constants between the high- and low-spin channels gradually decreases as the temperature increases. Then, the kinetic parameters of the 14 bimolecular reactions in the hydrogen-oxygen mechanism of the University of California, San Diego (UCSD), were replaced to simulate the ignition delay time and laminar flame speed. The simulation results agree well with the available experimental findings, indicating the necessity of considering both high- and low-spin channels for kinetic simulation.

Formation of Hydrogen Peroxide from O-(H2O)n Clusters

Hydrogen peroxide (H₂O₂) has recently received much attention as a safe and clean energy carrier for hydrogen molecules. In this study, based on direct ab initio molecular dynamics (AIMD) calculations, we demonstrated that H₂O₂ is directly formed via the photoelectron detachment of O^-(H₂O)_n (n = 1-6) (water clusters of an oxygen radical anion). Three electronic states of oxygen atoms were examined in the calculations: O(X)(H₂O)_n (X = ³P, ¹D, and ¹S states). After the photoelectron detachment of O^-(H₂O)_n (n = 1) to the ¹S state, a complex comprising O(¹S) and H₂O, O(¹S)-OH₂, was formed. A hydrogen atom of H₂O immediately transferred to O(¹S) during an intracluster reaction to form H₂O₂ as the final product. Simulations were run to obtain a total of 33 trajectories for n = 1 that all led to the formation of H₂O₂. The average reaction time of H₂O₂ formation was calculated to be 57.7 fs in the case of n = 1, indicating that the reaction was completed within 100 fs of electron detachment. All the reaction systems O(¹S)(H₂O)_n (n = 1-6) indicated the formation of H₂O₂ by the same mechanism. The reaction times for n = 2-6 were calculated to range between 80 and 180 fs, indicating that the reaction for n = 1 is faster than that of the larger clusters, that is, the larger the cluster size, the slower the reaction is. The reaction dynamics of the triplet O(³P) and singlet O(¹D) potential energy surfaces were calculated for comparison. All calculations yielded the dissociation product O(X)(H₂O)_n → O(X) + (H₂O)_n (X = ³P and ¹D), indicating that the O(¹S) state contributes to the formation of H₂O₂. The reaction mechanism was discussed based on the theoretical results.

A Full-Dimensional Potential Energy Surface and Dynamics of the Multichannel Reaction between H and HO2

In addition to its vital significance in combustion and atmospheric chemistry, the reaction between H' and HO₂ on the ground triplet state represents a prototype with multiple product channels, including H₂ + O₂, OH + OH, O + H₂O, and H + H'O₂. In this work, a full-dimensional accurate potential energy surface (PES) for the title reaction was developed to provide reliable descriptions for all dynamically relevant regions. Using this PES, we adopted the quasi-classical trajectory approach to study the corresponding reaction dynamics, including the reactivity of each product channel and the associated product branching ratio, the product energy distributions, product angular distributions, and associated microscopic mechanisms. For representing distributions of the product energies, such as product translational energy as well as product rotational and vibrational energies, both the traditional histogram and the kernel density estimation (KDE) methods were used and compared. It seems that the features of the resulting distributions in this work are very similar to each other among different methods. The KDE method is suggested for statistics, particularly for those populations with small oscillations in the histogram plot.

Mode- and Bond-Selected Reaction of H with Local Mode Molecule HDS

Understanding mode- and bond-selected dynamics of elementary chemical reactions is of central importance in molecular reaction dynamics. The initial state-selected time-dependent wave packet method is employed to study the mode and bond selectivity, isotopic branching ratio, and temperature dependence of rate constants of the two-channel reaction of H with local mode molecule HDS. For the abstraction channel, fundamental excitation of the HS (DS) bond of the reactant HDS significantly enhances the H-abstraction (D-abstraction) reaction, whose efficacy is higher than the same amount of translational energy except at low energies just above the energy threshold. This is in sharp contrast to the prediction of Polanyi rules: translational energy is more efficient than vibrational energy in enhancing a reaction with an early barrier. The recent sudden vector projection model is then applied to rationalize the observed mode specificity, which, however, shows that the translational mode vector has a larger coupling with the reaction coordinate than the stretching vector of the active bond, implying a reversed relative efficacy on promoting the reaction as well. In contrast, the mode and bond specificity for the exchange channel is not as strong as for the abstraction channel due to the regulation of the shallow well along the reaction path.

Recent advances in quantum scattering calculations on polyatomic bimolecular reactions

This review surveys quantum scattering calculations on chemical reactions of polyatomic molecules in the gas phase published in the last ten years.

An accurate multi-channel multi-reference full-dimensional global potential energy surface for the lowest triplet state of H2O2

A full dimensional potential energy surface for the lowest triplet state of H₂O₂ was developed at the MRCI-F12 level.

Aqueous production of oxygen atoms from hydroxyl radicals

Car–Parrinello MD simulations and advanced visualization techniques of OH*-pair encounters in water demonstrate the formation of the triplet oxygen atom.

A nine-dimensional global potential energy surface for NH4(X2A1) and kinetics studies on the H + NH3 ↔ H2 + NH2 reaction

A nine-dimensional global potential energy surface (PES) for the NH₄ system is developed from ∼10⁵ high-level ab initio points and the hydrogen abstraction kinetics on the PES agree with experiment.