High vibrational excitation of the reagent transforms the late-barrier H + HOD reaction into an early-barrier reaction

Polanyi's rules predict that a late-barrier reaction yields vibrationally cold products; however, experimental studies showed that the H2 product from the late-barrier H + H2O(|04⟩-) and H + HOD(vOH = 4) reactions is vibrationally hot. Here, we report a potential-averaged five-dimensional state-to-state quantum dynamics study for the H + HOD(vOH = 0-4) → H2 + OD reactions on a highly accurate potential energy surface with the total angular momentum J = 0. It is found that with the HOD vibration excitation increasing from vOH = 1 to 4, the product H2 becomes increasingly vibrationally excited and manifests a typical characteristic of an early barrier reaction for vOH = 3 to 4. Analysis of the scattering wave functions revealed that vibrational excitation in the breaking OH bond moves the location of dynamical saddle point from product side to reactant side, transforming the reaction into an early barrier reaction. Interestingly, pronounced oscillatory structures in the total and product vibrational-state-resolved reaction probabilities were observed for the H + HOD(vOH = 3, 4) reactions, in particular at low collision energies, which originate from the Feshbach resonance states trapped in the bending/torsion excited vibrational adiabatic potential wells in the entrance region due to van der Waals interactions.

Feshbach resonances in the F + CHD3 → HF + CD3 reaction

The signature of dynamics resonances was observed in the benchmark polyatomic F + CH₄/CHD₃ reactions more than a decade ago; however, the dynamical origin of the resonances is still not clear due to the lack of reliable quantum dynamics studies on accurate potential energy surfaces. Here, we report a six-dimensional state-to-state quantum dynamics study on the F + CHD₃ → HF + CD₃ reaction on a highly accurate potential energy surface. Pronounced oscillatory structures are observed in the total and product rovibrational-state-resolved reaction probabilities. Detailed analysis reveals that these oscillating features originate from the Feshbach resonance states trapped in the peculiar well on the HF(v' = 3)-CD₃ vibrationally adiabatic potential caused by HF chemical bond softening. Most of the resonance structures on the reaction probabilities are washed out in the well converged integral cross sections (ICS), leaving only one distinct peak at low collision energy. The calculated HF vibrational state-resolved ICS for CD₃(v = 0) agrees quantitatively with the experimental results, especially the branching ratio, but the theoretical CD₃ umbrella vibration state distribution is found to be much hotter than the experiment.

MLIMC: Machine Learning-Based Implicit-Solvent Monte Carlo

Monte Carlo (MC) methods are important computational tools for molecular structure optimizations and predictions. When solvent effects are explicitly considered, MC methods become very expensive due to the large degree of freedom associated with the water molecules and mobile ions. Alternatively implicit-solvent MC can largely reduce the computational cost by applying a mean field approximation to solvent effects and meanwhile maintains the atomic detail of the target molecule. The two most popular implicit-solvent models are the Poisson-Boltzmann (PB) model and the Generalized Born (GB) model in a way such that the GB model is an approximation to the PB model but is much faster in simulation time. In this work, we develop a machine learning-based implicit-solvent Monte Carlo (MLIMC) method by combining the advantages of both implicit solvent models in accuracy and efficiency. Specifically, the MLIMC method uses a fast and accurate PB-based machine learning (PBML) scheme to compute the electrostatic solvation free energy at each step. We validate our MLIMC method by using a benzene-water system and a protein-water system. We show that the proposed MLIMC method has great advantages in speed and accuracy for molecular structure optimization and prediction.

Full-dimensional quantum dynamics study of isotope effects for the H2 + NH2/ND2/NHD and H2/D2/HD + NH2 reactions

A full-dimensional quantum dynamical study for the bimolecular reactions of hydrogen molecules with amino radicals for different isotopologues is reported. The nonreactive amino radical is described by two Radau vectors that are very close to the valence bond coordinates. Potential-optimized discrete variable representation basis is used for the vibrational coordinates of the amino radical. Starting from the reaction H₂ + NH₂, we study the isotope effects for the two reagents separately, i.e., H₂ + NH₂/ND₂/NHD and H₂/D₂/HD + NH₂. The effects of different vibrational mode excitations of the reagents on the reactivities are studied. Physical explanations about the isotope effects are also provided thoroughly including the influence of vibrational energy differences between the different isotopologues and the impact of the tunneling effect.

A neural network potential energy surface for the F + H2O ↔ HF + OH reaction and quantum dynamics study of the isotopic effect

We report here a global and full dimensional neural network potential energy surface for the F + CH₄ reaction and investigate the isotopic effect on the total reaction probabilities using the time-dependent wave packet method.

From Reactive Rainbow to Dynamic Resonance Well

Scattering resonance is a fascinating phenomenon which manifests as a peak or a dip in an observable as a function of collisional energy (E_c). Its occurrence requires a potential well to support the resonance states. In this regard, reactive resonance is unusual in that it can exist in a reaction with unbound Born-Oppenheimer potential energy surface, on which the quasi-bound states are dynamically trapped-meaning that some energy is temporarily tied to the other degrees of freedom than the reaction coordinate. The concept of vibrational adiabaticity has been the cornerstone in understanding this phenomenon, for which the vibrationally adiabatic well depth is of primary concern. Recent studies on the F + CH₃D reaction have accumulated compelling evidence for a dominant resonance-mediated pathway at low E_c as well as for a rainbow feature in pair-correlated angular distribution at higher E_c. Here, we report an in-depth study to not only substantiate both claims but also, more importantly, make the first attempt to quantify the vibrationally adiabatic well depth directly from the observed rainbow structure and then compare with the theoretical prediction.

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.

Simulating Solvation and Acidity in Complex Mixtures with First-Principles Accuracy: The Case of CH3SO3H and H2O2 in Phenol

We present a generally applicable computational framework for the efficient and accurate characterization of molecular structural patterns and acid properties in an explicit solvent using H₂O₂ and CH₃SO₃H in phenol as an example. To address the challenges posed by the complexity of the problem, we resort to a set of data-driven methods and enhanced sampling algorithms. The synergistic application of these techniques makes the first-principle estimation of the chemical properties feasible without renouncing to the use of explicit solvation, involving extensive statistical sampling. Ensembles of neural network (NN) potentials are trained on a set of configurations carefully selected out of preliminary simulations performed at a low-cost density functional tight-binding (DFTB) level. The energy and forces of these configurations are then recomputed at the hybrid density functional theory (DFT) level and used to train the neural networks. The stability of the NN model is enhanced by using DFTB energetics as a baseline, but the efficiency of the direct NN (i.e., baseline-free) is exploited via a multiple-time-step integrator. The neural network potentials are combined with enhanced sampling techniques, such as replica exchange and metadynamics, and used to characterize the relevant protonated species and dominant noncovalent interactions in the mixture, also considering nuclear quantum effects.

Full-dimensional quantum mechanical calculations of the reaction probability of the H + CH4 reaction based on a mixed Jacobi and Radau description

A full-dimensional time-dependent wave packet study using mixed polyspherical Jacobi and Radau coordinates for the title reaction has been reported. The non-reactive moiety CH₃ has been described using three Radau vectors, whereas two Jacobi vectors have been used for the bond breaking/formation process. A potential-optimized discrete variable representation basis has been employed to describe the vibrational coordinates of the reagent CH₄. About one hundred billion basis functions have been necessary to achieve converged results. The reaction probabilities for some initial vibrational states are given. A comparison between the present approach and other methods, including reduced and full-dimensional ones, is also presented.

Benchmark ab initio and dynamical characterization of the stationary points of reactive atom + alkane and SN2 potential energy surfaces

We review composite ab initio and dynamical methods and their applications to characterize stationary points of atom/ion + molecule reactions.

Two-state diabatic potential energy surfaces of ClH2 based on nonadiabatic couplings with neural networks

A neural network-fitting procedure based on nonadiabatic couplings is proposed to generate two-state diabatic PESs with conical intersections.

Detailed benchmark ab initio mapping of the potential energy surfaces of the X + C2H6 [X = F, Cl, Br, I] reactions

We provide benchmark relative energies for the stationary points of three different channels of the halogen atom + ethane reactions.

Mode-Specific Quasiclassical Dynamics of the F- + CH3I SN2 and Proton-Transfer Reactions

Mode-specific quasiclassical trajectory computations are performed for the F^- + CH₃I( v _k = 0, 1) S_N2 and proton-transfer reactions at nine different collision energies in the range of 1.0-35.3 kcal/mol using a full-dimensional high-level ab initio analytical potential energy surface with ground-state and excited CI stretching ( v₃), CH₃ rocking ( v₆), CH₃ umbrella ( v₂), CH₃ deformation ( v₅), CH symmetric stretching ( v₁), and CH asymmetric stretching ( v₄) initial vibrational modes. Millions of trajectories provide statistically definitive mode-specific cross sections, opacity functions, scattering angle distributions, and product internal energy distributions. The excitation functions reveal slight vibrational S_N2 inversion inhibition/enhancement at low/high collision energies ( E_coll), whereas large decaying-with- E_coll vibrational enhancement effects for the S_N2 retention (double inversion) and proton-transfer channels. The most efficient vibrational enhancement is found by exciting the CI stretching (high E_coll) for S_N2 inversion and the CH stretching modes (low E_coll) for double inversion and proton transfer. Mode-specific effects do not show up in the scattering angle distributions and do blue-shift the hot/cold S_N2/proton-transfer product internal energies.

F(2P) + C2H6 → HF + C2H5 kinetics study based on a new analytical potential energy surface

An exhaustive kinetics study was performed for the title reaction using two theoretical approaches: variational transition-state theory and quasi-classical trajectory calculations, based on an original new analytical full-dimensional potential energy surface, named PES-2018, which has been fitted to high-level ab initio calculations.