Kinetic and dynamic studies of the H3++ CO → H2+ HCO+/HOC+reaction on a high-levelab initiopotential energy surface

Intramolecular Vibrational Energy Redistribution in the Reaction H3+ + CO → H2 + HCO. J Phys Chem A 2024. [PMID: 39374191 DOI: 10.1021/acs.jpca.4c03818] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

An ab initio direct molecular dynamics (MD) calculation at the RS2/aug-cc-pVQZ level, followed by vibration mapping, has been applied to the H3+ + CO → H2 + HCO+ reaction to elucidate the intramolecular vibrational energy redistribution (IVR) processes during the reaction. Direct MD calculations were carried out for 20 K (a typical temperature for interstellar dark clouds) and 330 K (a typical translational temperature for ions in a glow discharge). Under the Cs symmetry constraint, the approach of H3+ turned out to be the H-C stretching mode of the [H···CO]+ part, which invoked the C-O stretching and then the H-C-O bending modes. Under no symmetry constraint, a strong bending mode was first invoked, and the intensities of the subsequent H-C and C-O stretching modes were kept relatively small. The detailed analyses of the IVR during the reaction, in terms of vibration mixing, gave a clue to understanding experimentally observed anomalies in the bending modes, such as population inversion at some bending states. In the MD simulation at 20 K, less than two-thirds of the reaction energy was converted to the vibrational energy of the resultant HCO+ part and one-third to the translational and rotational energies of the leaving H2 molecule. These direct MD simulations, when combined with the experimental spectroscopy data, shed light on a clear understanding of the reaction mechanism, including the IVR during the reaction.

Validating experiments for the reaction H2 + NH2- by dynamical calculations on an accurate full-dimensional potential energy surface

Ion-molecule reactions play key roles in the field of ion related chemistry. As a prototypical multi-channel ion-molecule reaction, the reaction H₂ + NH₂^- → NH₃ + H^- has been studied for decades. In this work, we develop a new globally accurate potential energy surface (PES) for the title system based on hundreds of thousands of sampled points over a wide dynamically relevant region that covers long-range interacting configuration space. The permutational invariant polynomial-neural network (PIP-NN) method is used for fitting and the resulting total root mean squared error (RMSE) is extremely small, 0.026 kcal mol^-1. Extensive dynamical and kinetic calculations are carried out on this PIP-NN PES. Impressively, a unique phenomenon of significant reactivity suppression by exciting the rotational mode of H₂ is reported, supported by both the quasi-classical trajectory (QCT) and quantum dynamics (QD) calculations. Further analysis uncovers that exciting the H₂ rotational mode would prevent the formation of the reactant complex and thus suppress the reactivity. The calculated rate coefficients for H₂/D₂ + NH₂^- agree well with the experimental results, which show an inverse temperature dependence from 50 to 300 K, consistent with the capture nature of this barrierless reaction. The significant kinetic isotope effect observed by experiments is well reproduced by the QCT computations as well.

Energy- and Local-Gradient-Based Neural Network Method for Accurately Describing Long-Range Interaction: Application to the H2 + CO+ Reaction

The long-range interaction plays an important role in theoretically describing ion-molecule reaction. However, most energy-based neural network fitting methods usually introduce spurious long-range interactions. In this work, we propose an energy- and local-gradient-based neural network (ELGNN) method to fit potential energy surfaces (PESs). K-means clustering is employed to divide the whole configuration space into three regions: reactant asymptotic region, interaction region, and product asymptotic region. In the interaction region, only the energies of sampled points are computed, while in the asymptotic regions, the gradients of partially sampled configurations are calculated as well, and both the energies and energy gradients (if necessary) are used to fit long-range interactions. These regions are joined together by switching functions. The ELGNN method is first applied to fit the PES of the H₂ + CO⁺ reaction, which has significant long-range interactions. It is found that the ELGNN method works better than the energy-based NN method in describing long-range interactions. The dynamics and kinetics of the reaction are then investigated on the new PES.

Kinetic and dynamic studies of the NH2+ + H2 reaction on a high-level ab initio potential energy surface

The dynamics and kinetics of the NH2+ + H2 reaction are investigated on a newly developed ab initio potential energy surface using the quasi-classical trajectory method.

Ring-Polymer Molecular Dynamics Calculations of Thermal Rate Coefficients and Branching Ratios for the Interstellar H3+ + CO → H2 + HCO+/HOC+ Reaction and Its Deuterated Analogue

The reaction between H₃⁺ and CO is important in understanding the H₃⁺ destruction mechanism in the interstellar medium. In this work, thermal rate coefficients for the H₃⁺ + CO and D₃⁺ + CO reactions are calculated using ring-polymer molecular dynamics (RPMD) on a high-level machine-learning potential energy surface. The RPMD results agree well with the classical molecular dynamics results, where nuclear quantum effects are completely ignored, whereas the agreement between the RPMD results and the previous quasi-classical trajectory is good only at low temperatures. The calculated [HCO⁺]/[HOC⁺] product branching ratios decrease as the temperature increases, and the product branching is exclusively determined by the initial collisional orientation, which governs the formation of an ion-dipole complex, H₃⁺···CO or H₃⁺···OC, that dissociates into H₂ + HCO⁺ or H₂ + HOC⁺, respectively, via a direct mechanism. However, the contribution of the indirect mechanism via the rearrangement between H₃⁺···CO and H₃⁺···OC increases as the temperature increases, although its absolute fraction is small.

Study on the kinetics and dynamics of the H2 + NH2- reaction on a high-level ab initio potential energy surface

Gas-phase ion-molecule reactions play major roles in many fields of chemistry and physics. The reaction of an amino radical anion with a hydrogen molecule is one of the simplest proton transfer reactions involving anions. A globally accurate full-dimensional potential energy surface (PES) for the NH₂^- + H₂ reaction is developed by the fundamental invariant-neural network method, resulting in a root mean square error of 0.116 kcal mol^-1. Quasi-classical trajectory calculations are then carried out on the newly developed PES to give integral cross sections, differential cross sections and thermal rate coefficients. This reaction has two reaction channels, proton transfer and hydrogen exchange. The reactivity of the proton transfer channel is about one or two orders of magnitude stronger than that of the hydrogen exchange channel in the energy range studied. Vibrational excitation of H₂ promotes the proton transfer reaction, while fundamental excitation of each vibrational mode of NH₂^- has a negligible effect. In addition, the theoretical rate coefficients of the proton transfer reaction on the PES show inverse temperature dependence from 150 to 750 K, in accordance with the available experimental results.

Hydrogen abstraction/addition reactions in soot surface growth

The hydrogen abstraction (HB) and addition reactions (HD) by H radicals are examined on a series of polycyclic aromatic hydrocarbon (PAH) monomers and models of quasi-surfaces using quasi-classical trajectory (QCT) method. QCT results reproduce the rate constants of HB reactions on PAH monomers from density functional theory (DFT) in the range of 1500-2700 K. The PAH size has a minor impact on the rates of HB reactions, especially at temperatures beyond 2100 K. In contrast, HD reactions have a clear size dependence, and a larger PAH yields a higher rate. It was also found that the preferred reaction pathway changes from HB to HD reactions at ∼1900 K. The rates of surface HB and HD reactions exceed those in the gas phase by nearly one factor of magnitude. Further analysis of the detailed trajectory of the QCT method reveals that about 50% of surface reactions can be attributed to the events of surface diffusion, which depends on the local energy transfer in gas-surface interactions. However, this phenomenon is not preferred in PAH monomers, as expected. Our finding here questions the treatment of the surface reactions of soot as the product of the first collision between the gaseous species and particle surface. The surface diffusion-induced reactions should be accounted for in the rates of the surface HB and HD reactions. The rate constants of HB and HD reactions on each reactive site (surface zig-zag, surface free-edge and pocket free-edge sites) were calculated by QCT method, and are recommended for the further development of surface chemistry models in soot formation.

Advances and New Challenges to Bimolecular Reaction Dynamics Theory

Dynamics of bimolecular reactions in the gas phase are of foundational importance in combustion, atmospheric chemistry, interstellar chemistry, and plasma chemistry. These collision-induced chemical transformations are a sensitive probe of the underlying potential energy surface(s). Despite tremendous progress in past decades, our understanding is still not complete. In this Perspective, we survey the recent advances in theoretical characterization of bimolecular reaction dynamics, stimulated by new experimental observations, and identify key new challenges.

Final-State-Resolved Dynamics of the H3+ + CO → H2 +HCO+/HOC+ Reaction: A Quasi-Classical Trajectory Study

The ion-molecule reaction H₃⁺ + CO → H₂ + HCO⁺/HOC⁺, which initiates the formation of crucial organic molecules, plays a key role in interstellar and circumstellar environments. In this work, the quasi-classical trajectory method is employed to study the reaction dynamics on a recently developed full-dimensional global potential energy surface (PES). The calculated product internal energy distributions and relative internal excited fractions agree reasonably well with the experimental measurements. For the two reaction channels, most of the available energy flows into the vibrational modes of HCO⁺ or HOC⁺ at low collision energies, followed by the translational mode and the rotational modes of HCO⁺ or HOC⁺. As the collision energy increases, the proportion of the product translational energy increases while the proportion of the product vibrational energy decreases. Furthermore, the CH and CO stretching modes and their combination bands are effectively excited for the product HCO⁺ while the bending mode is remarkably excited for the product HOC⁺.