Effects of Halogenations and Conformational Isomers on Positron Binding in Halogenated Hydrocarbons.

We studied isomeric conformer effects on the positron affinity (PA) of halogenated hydrocarbons using density functional theory combined with the electron-positron correlation-polarization potential (CPP) model. PA values are computed for 75 halogenated hydrocarbons, including fluorine (F), chlorine (Cl), and bromine (Br) derivatives of methane, ethylene, and ethane molecules. The positive PA values can be described by a linear combination of the dipole moment and polarizability of parent molecules.

Applications of the density functional method combined with the electron-positron correlation-polarization potential to positron binding to hydrocarbons and water clusters.

We present benchmark calculations using the electron-positron correlation-polarization potential (CPP) method with the atomic polarizability model to evaluate the positron affinities of key categories of polyatomic hydrocarbons and water microclusters. The universal model parameter of the generalized gradient approximation for CPP is optimized based on the experimentally measured positron affinities of the representative hydrocarbon molecules. Using this method, the positron affinities and positron binding features of water clusters up to hexamers are investigated.

Attractive Force-Induced Isotope Effects through Ring-Polymer Molecular Dynamics Simulations for the Barrierless Reaction between HNCO and H Isotopologues: H, HD, HD, and D.

We performed ring-polymer molecular dynamics (RPMD) simulations at temperatures of 100, 200, and 300 K to investigate the H + HNCO, HD + HNCO, HD + HNCO, and D + HNCO reactions on the recently developed potential energy surface. Thermal rate coefficients and branching fractions were obtained, showing a decrease in rate coefficients with lower temperatures, largely influenced by the Maxwell-Boltzmann averaged velocities and nuclear masses of the reactants. In the HD + HNCO and HD + HNCO reactions, the abstraction of the lighter proton was favored over that of the heavier deuteron at lower temperatures due to the attractive forces derived from the potential energy surface in the barrierless reaction.

Nuclear Quantum Effects in the Ionic Dissociation Dynamics of HCl on the Water Ice Cluster.

Nuclear quantum effects play a significant role in the dissociation dynamics of HCl ions during collisions with the (HO) ice cluster. These effects become particularly important when analyzing proton transfer, tunneling, and zero-point energy contributions during the dissociation process. In this study, we investigate the dissociation behavior of HCl when colliding with the (HO) ice cluster, focusing on the influence of the nuclear quantum effects on the proton transfer mechanism, ionic dissociation rates, and subsequent solvation dynamics.

Investigation of the Gas-Phase N + CHCN Reaction at Low Temperatures.

Rate coefficients for ion-polar-molecule reactions between acetonitrile molecules (CHCN) and nitrogen molecular ions (N), which are of importance to the upper atmospheric chemistry of Saturn's moon Titan, were measured for the first time at low translational temperatures. In the experiments, the reaction between sympathetically cooled N ions embedded in laser-cooled Ca Coulomb crystals and velocity-selected acetonitrile molecules generated using a wavy Stark velocity filter was studied to determine the reaction rate coefficients. Capture rate coefficients calculated by the Su-Chesnavich approach and by the perturbed rotational state theory considering the rotational state distribution of CHCN were compared to the experimental rate coefficients.

Temperature effects on the branching dynamics in the model ambimodal (6 + 4)/(4 + 2) intramolecular cycloaddition reaction.

CH (tetradecapentaene) is a simple model system exhibiting post transition-state behavior, wherein both the (6 + 4) and (4 + 2) cycloaddition products are formed from one ambimocal transition state structure. We studied the bifurcation dynamics starting from the two ambimodal transition state structures, the chair-form and boat-form, using the quasi-classical trajectory, classical molecular dynamics, and ring-polymer molecular dynamics methods on the parameter-optimized semiempirical GFN2-xTB potential energy surface. It was found that the calculated branching fractions differ between the chair-form and boat-form due to the different nature in the IRC pathways.

Computational study of the post-transition state dynamics for the OH + CHOH reaction probed by photodetachment of the CHO(HO) anion.

Dissociative photodetachment dynamics simulations were conducted to study the CHO(HO) → CHO + HO + e reaction using classical molecular dynamics (MD) and ring-polymer molecular dynamics (RPMD) techniques on two newly formulated neutral potential energy surfaces (PES1 and PES2) by different research groups. While the dissociation dynamics exhibited similarities between classical MD and RPMD, there were noticeable differences in the fluctuation of probability densities for the internal modes due to nuclear quantum effects. Upon comparison of our findings with experimental data concerning the electron binding energy distribution and photofragment relative energy, it suggests that the potential energy landscapes of PES2 are reasonably precise.

Theoretical Study of the Thermal Rate Coefficients of the H + CH Reaction: Dynamics Study on a Full-Dimensional Potential Energy Surface.

Ion-molecular reactions play a significant role in molecular evolution within the interstellar medium. In this study, the entrance channel reaction, H + CH → H + CH, was investigated using classical molecular dynamic (classical MD) and ring polymer molecular dynamic (RPMD) simulation techniques. We developed an analytical potential energy surface function with a permutationally invariant polynomial basis, specifically employing the monomial symmetrized approach.

Possible Roles of Transition Metal Cations in the Formation of Interstellar Benzene via Catalytic Acetylene Cyclotrimerization.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous interstellar molecules. However, the formation mechanisms of PAHs and even the simplest cyclic aromatic hydrocarbon, benzene, are not yet fully understood. Recently, we reported the statistical and dynamical properties in the reaction mechanism of Fe-catalyzed acetylene cyclotrimerization, whereby three acetylene molecules are directly converted to benzene.

Dynamics study of the post-transition-state-bifurcation process of the (HCOOH)H → CO + HO/HCO + HO dissociation: application of machine-learning techniques.

The process of protonated formic acid dissociating from the transition state was studied using ring-polymer molecular dynamics (RPMD), classical MD, and quasi-classical trajectory (QCT) simulations. Temperature had a strong influence on the branching fractions for the HCO + HO and CO + HO dissociation channels. The RPMD and classical MD simulations showed similar behavior, but the QCT dynamics were significantly different owing to the excess energies in the quasi-classical trajectories.

Ring-polymer Molecular Dynamics Simulation for the Adsorption of H on Ice Clusters (H O) (n=8, 10, and 12).

In the interstellar medium, the H adsorption and desorption on the solid water ice are crucial for chemical and physical processes. We have recently investigated the probabilities of H sticking on the (H O) ice, which has quadrilateral surfaces. We have extended the previous work using classical MD and ring-polymer molecular dynamics (RPMD) simulations to the larger ice clusters, (H O) and (H O) , which have pentagonal and hexagonal surfaces, respectively.

Ring-Polymer Molecular Dynamics and Kinetics for the H + CH → H + CH Reaction Using the Full-Dimensional Potential Energy Surface.

The H + CH → H + CH reaction is important in understanding the production mechanisms of anionic molecules in interstellar environments. Herein, the rate coefficients for the H + CH → H + CH reaction were calculated using ring-polymer molecular dynamics (RPMD), classical molecular dynamics (MD), and quasi-classical trajectory (QCT) approaches on a newly developed ab initio potential energy surface (PES) in full dimensions. PES was constructed by fitting a large number of ab initio energy points and their gradients using the permutationally invariant polynomial basis set method.

Interstellar Benzene Formation Mechanisms via Acetylene Cyclotrimerization Catalyzed by Fe Attached to Water Ice Clusters: Quantum Chemistry Calculation Study.

Benzene is the simplest building block of polycyclic aromatic hydrocarbons and has previously been found in the interstellar medium. Several barrierless reaction mechanisms for interstellar benzene formation that may operate under low-temperature and low-pressure conditions in the gas phase have been proposed. In this work, we studied different mechanisms for interstellar benzene formation based on acetylene cyclotrimerization catalyzed by Fe bound to solid water clusters through quantum chemistry calculations.

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

The reaction between H and CO is important in understanding the H destruction mechanism in the interstellar medium. In this work, thermal rate coefficients for the H + CO and D + 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.

On-the-Fly Ring-Polymer Molecular Dynamics Calculations of the Dissociative Photodetachment Process of the Oxalate Anion.

The dissociative photodetachment dynamics of the oxalate anion, COH + → CO + HOCO + e, were theoretically studied using the on-the-fly path-integral and ring-polymer molecular dynamics methods, which can account for nuclear quantum effects at the density-functional theory level in order to compare with the recent experimental study using photoelectron-photofragment coincidence spectroscopy. To reduce computational time, the force acting on each bead of ring-polymer was approximately calculated from the first and second derivatives of the potential energy at the centroid position of the nuclei beads. We find that the calculated photoelectron spectrum qualitatively reproduces the experimental spectrum and that nuclear quantum effects are playing a role in determining spectral widths.

Theoretical study of the dissociative photodetachment dynamics of the hydrated superoxide anion cluster.

The dissociative photodetachment of the hydrated superoxide anion cluster, O2-·H2O + hν → O2 + H2O + e-, is theoretically investigated using path-integral and ring-polymer molecular dynamics simulation methods, which can account for nuclear quantum effects. Full-dimensional potential energy surfaces for the anionic and lowest two neutral states (triplet and singlet spin states) are constructed based on extensive density-functional theory calculations. The calculated photoelectron spectrum agrees well with the experimental spectra measured for different photodetachment laser wavelengths.

Application of Reaction Path Search Calculations to Potential Energy Surface Fits.

There has been significant progress in recent years in the use of machine learning techniques to model high-dimensional reactive potential energy surfaces using large-scale data obtained from electronic structure calculations. In these methods, the strategy used to gather data becomes a key issue as the molecular size increases. In this work, we examine the applicability of the reaction path search algorithm implemented in the Global Reaction Route Mapping (GRRM) code as a data-gathering approach.

Quantum calculations of the photoelectron spectra of the OH·NH anion: implications for OH + NH→ HO + NH reaction dynamics.

We present the results of quantum dynamics calculations for analyzing the experimentally measured photoelectron spectra of the OH·NH anion complex. Detachment of an excess electron of OH·NH initially produces a molecular arrangement, which is close to the transition-state structure of the neutral OH + NH→ HO + NH hydrogen abstraction reaction due to the Franck-Condon principle, and thus finally leads to the OH + NH or HO + NH asymptotic channel. We used both the path integral method and the reduced-dimensionality quantum wave packet method to simulate the photoelectron spectra of the OH·NH anion.

Theoretical Analysis of the Formylmethylene Anion Photoelectron Spectrum: Importance of Wolff Rearrangement Dynamics.

Photoelectron spectroscopy of a molecular anion is very useful for investigating the transition state and intermediate regions on the reactive potential energy surfaces of a neutral system. In this work, we theoretically analyzed the previously measured photoelectron spectrum of the formylmethylene anion, HCCHO. We simulated the photoelectron spectra for both the singlet and triplet states using the semiclassical method with quantum nuclear densities and Franck-Condon factor calculations with harmonic vibrational analysis.

Franck-Condon simulations of transition-state spectra for the OH + HO and OD + DO reactions.

We present the results of quantum wave packet calculations performed to analyze the experimental transition-state spectra for the OH + H2O and OD + D2O reactions based on photodetachment of the H3O2- and D3O2- anions. We used a reduced-dimensionality model in which four normal-mode coordinates were considered for the neutral transition state. High-level ab initio potential energy surfaces were used for both the neutral and anionic states.

Quantum dynamics analysis of transition-state spectrum for the SH + HS → HS + SH reaction.

We present the results of quantum wave packet calculations analyzing the experimental transition-state spectrum for the SH + H2S hydrogen transfer reaction based on photodetachment of the H3S2- anion. We used a reduced-dimensionality model in which four normal-mode coordinates were considered for the dynamics of the neutral transition state. The four-dimensional potential energy surfaces for the anionic and neutral states were constructed using four different levels of theory, namely, MP2, B3LYP, CAM-B3LYP, and LC-BLYP, with the aug-cc-pVDZ basis set.

Spin-inversion mechanisms in O binding to a model heme compound: A perspective from nonadiabatic wave packet calculations.

Spin-inversion dynamics in O binding to a model heme complex, which consisted of Fe(II)-porphyrin and imidazole, were studied using nonadiabatic wave packet dynamics calculations. We considered three active nuclear degrees of freedom in the dynamics, including the motions along the Fe-O distance, Fe-O-O angle, and Fe out-of-plane distance. Spin-free potential energy surfaces for the singlet, triplet, quintet, and septet states were developed using density functional theory calculations, and spin-orbit coupling elements were obtained from CASSCF-level electronic structure calculations.

Positron-electron correlation-polarization potential model for positron binding in polyatomic molecules.

Positron binding energies (PBEs) of 41 polyatomic molecules were calculated using the positron-electron correlation-polarization potential (CPP) approach and compared with experimentally measured values. In this approach, the short-range positron-electron potential is modeled using the density-functional expression, whereas the long-range potential is approximated by the attractive polarization potential. The positron-electron CPP model based on local-density approximation yields larger PBEs than experimental values; however, the calculated values can be substantially improved by introducing generalized gradient approximation.

Reduced-Dimensionality Quantum Dynamics Study of the Fe(CO) + H FeH(CO) Spin-inversion Reaction.

Many chemical reactions of transition metal compounds involve a change in spin state via spin inversion, which is induced by relativistic spin-orbit coupling. In this work, we theoretically study the efficiency of a typical spin-inversion reaction, Fe(CO) + H FeH(CO). Structural and vibrational information on the spin-inversion point, obtained through the spin-coupled Hamiltonian approach, is used to construct three degree-of-freedom potential energy surfaces and to obtain singlet-triplet spin-orbit couplings.

Spin-inversion mechanisms in O binding to a model heme complex revisited by density function theory calculations.

Spin-inversion mechanisms in O binding to a model heme complex, consisting of Fe(II)-porphyrin and imidazole, were investigated using density-functional theory calculations. First, we applied the recently proposed mixed-spin Hamiltonian method to locate spin-inversion structures between different total spin multiplicities. Nine spin-inversion structures were successfully optimized for the singlet-triplet, singlet-quintet, triplet-quintet, and quintet-septet spin-inversion processes.