Stereodynamical control of product branching in multi-channel barrierless hydrogen abstraction of CH3OH by F

Accurate fundamental invariant-neural network representation of ab initio potential energy surfaces

Highly accurate potential energy surfaces are critically important for chemical reaction dynamics. The large number of degrees of freedom and the intricate symmetry adaption pose a big challenge to accurately representing potential energy surfaces (PESs) for polyatomic reactions. Recently, our group has made substantial progress in this direction by developing the fundamental invariant-neural network (FI-NN) approach. Here, we review these advances, demonstrating that the FI-NN approach can represent highly accurate, global, full-dimensional PESs for reactive systems with even more than 10 atoms. These multi-channel reactions typically involve many intermediates, transition states, and products. The complexity and ruggedness of this potential energy landscape present even greater challenges for full-dimensional PES representation. These PESs exhibit a high level of complexity, molecular size, and accuracy of fit. Dynamics simulations based on these PESs have unveiled intriguing and novel reaction mechanisms, providing deep insights into the intricate dynamics involved in combustion, atmospheric, and organic chemistry.

Theoretical Insights into the Dynamics of Gas-Phase Bimolecular Reactions with Submerged Barriers

Much attention has been paid to the dynamics of both activated gas-phase bimolecular reactions, which feature monotonically increasing integral cross sections and Arrhenius kinetics, and their barrierless capture counterparts, which manifest monotonically decreasing integral cross sections and negative temperature dependence of the rate coefficients. In this Perspective, we focus on the dynamics of gas-phase bimolecular reactions with submerged barriers, which often involve radicals or ions and are prevalent in combustion, atmospheric chemistry, astrochemistry, and plasma chemistry. The temperature dependence of the rate coefficients for such reactions is often non-Arrhenius and complex, and the corresponding dynamics may also be quite different from those with significant barriers or those completely dominated by capture. Recent experimental and theoretical studies of such reactions, particularly at relatively low temperatures or collision energies, have revealed interesting dynamical behaviors, which are discussed here. The new knowledge enriches our understanding of the dynamics of these unusual reactions.

Benchmark ab initio potential energy surface mapping of the F + CH3NH2 reaction

This electronic structure study reveals four exothermic and two endothermic reaction pathways of the F + CH₃NH₂ system: the barrierless hydrogen abstraction from the methyl/amino group (HF + CH₂NH₂/CH₃NH), amino/methyl substitution (NH₂ + CH₃F and CH₃ + NH₂F) and hydrogen substitution from the two functional groups (H + CH₂FNH₂/CH₃NHF). The benchmark classical and adiabatic energies are obtained using a high-accuracy composite ab initio approach, where the gold-standard explicitly-correlated coupled cluster method (CCSD(T)-F12b) is applied with the correlation-consistent polarized valence quintuple-zeta F12 basis set (cc-pV5Z-F12) and further additive energy contributions. Considering indispensable post-(T) correlation, core correlation, scalar relativistic, spin-orbit and harmonic zero-point energy corrections, the obtained global minimum of the potential energy surface is the post-reaction CH₂NH₂⋯HF complex in the product channel. Although each substitution pathway has a high barrier, the energies of amino-substitution and methyl-hydrogen-substitution products are below the energy of the reactants, as well as the submerged-barrier hydrogen-abstraction pathways.

Reaction Pathway Control via Reactant Vibrational Excitation and Impact on Product Vibrational Distributions: The O + HO2 → OH + O2 Atmospheric Reaction

Chemical reactions often have multiple pathways, the control of which is of fundamental and practical importance. In this Letter, we examine the dynamics of the O + HO₂ → OH + O₂ reaction, which plays an important role in atmospheric chemistry, using quasi-classical trajectories on a recently developed full-dimensional potential energy surface (PES). This reaction has two pathways leading to the same products: the H abstraction pathway (O_a + HO_bO_c → O_aH + O_bO_c) and the O abstraction pathway (O_a + HO_bO_c → O_bH + O_aO_c). Under thermal conditions, the reaction is dominated by the latter channel, which is barrierless, leading to vibrational excitation of the O₂ product. However, we demonstrate that excitation of the HO₂ reactant in its O-H (v₁) vibrational mode results in dramatic switching of the reaction pathway to the activated H abstraction channel, which leads to a highly excited OH product vibrational state distribution. The implications of such dynamical effects in the atmospheric chemistry are discussed.

Global potential energy surface and product pair-correlated distributions for the F(2P) + SiH4 reaction - comparison with experiments

In this paper we study the gas-phase hydrogen abstraction reaction between fluorine atoms and silane in a three-step process: potential energy surface, kinetics and dynamics. Firstly, we developed for the first time an analytical full-dimensional surface, named PES-2021, using high-level explicitly-correlated ab initio data as the input. PES-2021 represents a continuous and smooth potential with analytical gradients and includes intuitive concepts (stretching and bending nuclear motions). Based on the PES-2021 quasi-classical trajectory (QCT) calculations were performed to analyse the kinetics and dynamics. Secondly, in the kinetics study at room temperature we observed a very fast reaction with a rate constant of 3.90 × 10^-10 cm³ molecule^-1 s^-1, reproducing the scarce experimental evidence. Finally, the third step is the dynamics study, which was performed under two different conditions, a temperature of 77 K and a collision energy of 2.5 kcal mol^-1, for direct comparison with experiments. In the first case, we found the largest fraction, 44%, deposited as HF(v) vibration, where the most populated states were HF(v = 2, 3), both results reproducing the experimental evidence. The largest discrepancy with the experiment was found in the HF(j) rotational distribution, where hotter distributions were found, this discrepancy being associated with limitations of the QCT method. The second case, E = 2.5 kcal mol^-1, was a state-to-state correlated study and, therefore, more difficult. The theory overestimates (again) and consequently underestimates, respectively, the rotational and vibrational fractions of the HF(v,j) product as compared with experiments. While experimentally the SiH₃ product appears excited only in the umbrella mode, ν₂ = 0-5, correlated with the HF(v) co-product vibrational excited in v = 3 and 4, theoretically a wider vibrational distribution is found in both products, and these distributions have, obviously, an influence on the product correlated speed distributions. However, the product correlated angular distribution is well reproduced. In general, these results allowed us to test the capacity of PES-2021 + QCT tools to simulate the experimental evidence, revealing that agreement is better when average properties are compared, making the comparison worse when state-to-state properties are compared. Different causes of the theory/experiment discrepancies were analysed, and it was found that they are due, mainly, to limitations of the QCT method.

Theoretical study of the Cl(2P) + SiH4 reaction: global potential energy surface and product pair-correlated distributions. Comparison with experiment

For the theoretical study of the title reaction, an analytical full-dimensional potential energy surface named PES-2021 was developed for the first time, by fitting high-level explicitly-correlated ab initio data. This reaction presented high exothermicity, (298 K) = -11.6 kcal mol^-1, reproducing the experimental evidence; it is a barrierless reaction and no intermediate complexes were found. PES-2021 is a continuous and smooth potential energy surface, it includes intuitive concepts in its development and fitting, such as stretching and bending nuclear motions, and it presents analytical first energy derivatives. Based on PES-2021, kinetics and dynamics studies were carried out using quasi-classical trajectory calculations. In the kinetics study, over the temperature range 300-450 K, we observed that rate constants were practically independent of temperature, with an almost zero activation energy, as compared to 0.0 and -0.48 kcal mol^-1 experimentally reported. In this kinetics study the role of the spin-orbit effect on reactivity was analysed. In the dynamics study, different product pair-correlated dynamics properties were compared with the only experimental evidence: product energy partition, product vibrational distribution, product angular distribution and product speed distribution. We observed two mechanisms of reaction, a stripping mechanism associated with large impact parameters and forward scattering, and an indirect mechanism associated with sideways-backward scattering related with "nearly-trapped" trajectories due to the product rotation. In general, theoretical results reasonably simulate the experimental measurements when they consider some rotational and vibrational constraints as well as binning techniques to mimic a quantum-mechanical behaviour. Although the agreement is not quantitative, the present results shed light on the mechanism of this difficult polyatomic reactive system.

Kinetic and Dynamic Studies of the F(2P) + ND3 → DF + ND2 Reaction

The fast F reaction with NH₃ poses a big challenge to experimental studies because of secondary chemical and collisional reactions. The quasi-classical trajectory method is utilized to investigate the mode specificity, product energy disposal, and temperature dependence of the thermal rate coefficient of F + ND₃ → DF + ND₂ on a recently developed potential energy surface. The effect of isotopic substitution is explored by comparing the F + ND₃ reaction with the F + NH₃ reaction. The computed results permit a better understanding of the F + ammonia reaction. The DF vibrational state has a Λ-type distribution, in accordance with the experimental measurement by the fast flow reactor technique. The product ND₂ is dominantly populated in the ground state, and a considerable amount of ND₂ is produced in the fundamental states of the bending mode. The similar vibrational state distributions of HF and NH₂ in the F + NH₃ reaction indicate a weak isotopic substitution effect on the product energy disposal. Exciting the umbrella mode of ND₃ suppresses the reaction at low energies below 5 kcal mol^-1, in sharp contrast to the observation in the F + NH₃ reaction. These dynamical behaviors can be partially explained by the sudden vector projection model. In addition, the thermal rate coefficient of F + ND₃ shows no temperature dependence in the range between 150 and 2000 K. There exists an inverse kinetic isotope effect at temperatures from 150 to 1500 K.

Full-dimensional quantum stereodynamics of the non-adiabatic quenching of OH(A2Σ+) by H2

The Born–Oppenheimer approximation, assuming separable nuclear and electronic motion, is widely adopted for characterizing chemical reactions in a single electronic state. However, the breakdown of the Born–Oppenheimer approximation is omnipresent in chemistry, and a detailed understanding of the non-adiabatic dynamics is still incomplete. Here we investigate the non-adiabatic quenching of electronically excited OH(A²Σ⁺) molecules by H₂ molecules using full-dimensional quantum dynamics calculations for zero total nuclear angular momentum using a high-quality diabatic-potential-energy matrix. Good agreement with experimental observations is found for the OH(X²Π) ro-vibrational distribution, and the non-adiabatic dynamics are shown to be controlled by stereodynamics, namely the relative orientation of the two reactants. The uncovering of a major (in)elastic channel, neglected in a previous analysis but confirmed by a recent experiment, resolves a long-standing experiment–theory disagreement concerning the branching ratio of the two electronic quenching channels.

The breakdown of the Born–Oppenheimer approximation is omnipresent in chemistry and detailed understanding of non-adiabatic dynamics is still incomplete. Now, the non-adiabatic quenching of electronically excited OH(A²Σ⁺) molecules by H₂ has been investigated using full-dimensional quantum dynamics calculations and a high-quality diabatic-potential-energy matrix, providing insight into the branching ratio of the two electronic quenching channels.

Searching Configurations in Uncertainty Space: Active Learning of High-Dimensional Neural Network Reactive Potentials

Neural network (NN) potential energy surfaces (PESs) have been widely used in atomistic simulations with ab initio accuracy. While constructing NN PESs, their training data points are often sampled by molecular dynamics trajectories. This strategy can be however inefficient for reactive systems involving rare events. Here, we develop an uncertainty-driven active learning strategy to automatically and efficiently generate high-dimensional NN-based reactive potentials, taking a gas-surface reaction as an example. The difference between two independent NN models is used as a simple and differentiable uncertainty metric, allowing us to quickly search in the uncertainty space and place new samples at which the PES is less reliable. By interfacing this algorithm with the first-principles simulation package, we demonstrate that a globally accurate NN potential of the H₂ + Ag(111) system can be constructed with merely ∼150 data points. This PES can be further refined to describe H₂ dissociation on Ag(100) by adding ∼130 more configurations on this facet. The entire process is completely automatic and self-terminated once the relative error criterion is fulfilled. Impressively, data points sampled by this uncertainty-driven strategy are substantially fewer than by the traditional trajectory-based sampling. The final NN PES not only converges well the quantum dissociation probability of the molecule but also well-reproduces the phonon properties of the substrate and is capable of describing surface temperature effects. These results show the potential of this active learning approach in developing high-dimensional NN reactive potentials in gas and condensed phases.

An accurate full-dimensional potential energy surface for the reaction OH + SO → H + SO2

An accurate full-dimensional PES for the OH + SO ↔ H + SO₂ reaction is developed by the permutation invariant polynomial-neural network approach.

Benchmark ab initio stationary-point characterization of the complex potential energy surface of the multi-channel Cl + CH3NH2 reaction

We characterize the exothermic low/submerged-barrier hydrogen-abstraction (HCl + CH2NH2/CH3NH) as well as, for the first time, the endothermic high-barrier amino-substitution (CH3Cl + NH2), methyl-substitution (NH2Cl + CH3), and hydrogen-substitution (CH2ClNH2/CH3NHCl + H) pathways of the Cl + CH3NH2 reaction using an accurate composite ab initio approach. The computations reveal a CH3NH2Cl complex in the entrance channel, nine transition states corresponding to different abstractions, Walden-inversion substitution, and configuration-retaining front-side attack substitution pathways, as well as nine post-reaction complexes. The global minima of the electronic and vibrationally adiabatic potential energy surfaces correspond to the pre-reaction CH3NH2Cl and post-reaction CH2NH2HCl complexes, respectively. The benchmark composite energies of the stationary points are obtained by considering basis-set effects up to the correlation-consistent polarized valence quadruple-zeta basis augmented with diffuse functions (aug-cc-pVQZ) using the explicitly-correlated coupled-cluster singles, doubles, and perturbative triples CCSD(T)-F12b method, post-(T) correlation up to CCSDT(Q) including full triples and perturbative quadruples, core correlation, and scalar relativistic and spin-orbit effects, as well as harmonic zero-point energy corrections.

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.