Relative efficacy of vibrational vs. translational excitation in promoting atom-diatom reactivity: rigorous examination of Polanyi's rules and proposition of sudden vector projection (SVP) model

A critical comparison of neural network potentials for molecular reaction dynamics with exact permutation symmetry

Several symmetry strategies have been compared in fitting full dimensional accurate potentials for reactive systems based on a neural network approach.

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.

Vibrational control of selective bond cleavage in dissociative chemisorption of methanol on Cu(111)

Controlling product branching ratios in a chemical reaction represents a desired but difficult achievement in chemistry. In this work, we demonstrate the first example of altering the branching ratios in a multichannel reaction, i.e., methanol dissociative chemisorption on Cu(111), via selectively exciting specific vibrational modes. To this end, we develop a globally accurate full-dimensional potential energy surface for the CH3OH/Cu(111) system and perform extensive vibrational state-selected molecular dynamics simulations. Our results show that O-H/C-H/C-O stretching vibrational excitations substantially enhance the respective bond scission processes, representing extraordinary bond selectivity. At a given total energy, the branching ratio of C-O/C-H dissociation can increase by as large as 100 times by exciting the C-O stretching mode which possesses an unprecedentedly strong vibrational efficacy on reactivity. This vibrational control can be realized by the well-designed experiment using a linearly polarized laser.

Activation of Reactions in the Complex Region Using Microwave Irradiation

Many mode-specific behaviors in the gas phase and at the gas-surface interface have been reported in the past decades. Infrared activation of a reagent vibrational mode is often used to study these reactions. In this work, an inexpensive and easily applied scheme using microwave irradiation is proposed for activating complex-forming reactions by transferring populations between closely spaced resonances. The important combustion reaction of H + O₂ ↔ O + OH is used as a model system to demonstrate the feasibility of the proposed approach. The existence of a nonzero transition dipole moment matrix element between two highly excited resonance states separated by a small energy gap in the model system may allow one to use microwave irradiation to intervene and control the model reaction. The high energy resonance states of the model reaction can also release their energy by photon emission, which is in agreement with the experimentally observed chemiluminescence process.

Stretching vibration is a spectator in nucleophilic substitution

How chemical reactions are influenced by reactant vibrational excitation is a long-standing question at the core of chemical reaction dynamics. In reactions of polyatomic molecules, where the Polanyi rules are not directly applicable, certain vibrational modes can act as spectators. In nucleophilic substitution reactions, CH stretching vibrations have been considered to be such spectators. While this picture has been challenged by some theoretical studies, experimental insight has been lacking. We show that the nucleophilic substitution reaction of F^- with CH₃I is minimally influenced by an excitation of the symmetric CH stretching vibration. This contrasts with the strong vibrational enhancement of the proton transfer reaction measured in parallel. The spectator behavior of the stretching mode is supported by both quasi-classical trajectory simulations and the Sudden Vector Projection model.

Potential energy surface stationary points and dynamics of the F- + CH3I double inversion mechanism

Direct dynamics simulations were performed to study the S_N2 double inversion mechanism S_N2-DI, with retention of configuration, for the F^- + CH₃I reaction. Previous simulations identified a transition state (TS) structure, i.e. TS0, for the S_N2-DI mechanism, including a reaction path. However, intrinsic reaction coordinate (IRC) calculations from TS0 show it is a proton transfer (PT) TS connected to the F^-HCH₂I S_N2 pre-reaction complex and the FHCH₂I^- proton transfer post-reaction complex. Inclusion of TS0 in the S_N2-DI mechanism would thus involve non-IRC atomistic dynamics. Indeed, trajectories initiated at TS0, with random ensembles of energies as assumed by RRKM theory, preferentially form the S_N2-DI products and ∼70% follow the proposed S_N2-DI pathway from TS0 to the products. In addition, the Sudden Vector Projection (SVP) method was used to identify which CH₃I vibrational mode excitations promote access to TS0 and the S_N2-DI mechanism. Results of F^- + CH₃I simulations, with SVP specified mode excitations, are disappointing. With the CH₃ deformations of CH₃I excited, the S_N2 single inversion mechanism is the dominant pathway. If the CH stretch modes are also excited, proton transfer dominates the reaction. S_N2-DI occurs, but with a very small probability of ∼1%. The reasons behind these results are discussed.

Fermi resonance controlled product branching in the H + HOD reaction

Excitation of the first overtone of bending mode results in a significant enhancement in the HD + OH channel due to the 1 : 2 Fermi resonance between the fundamental OD stretch and the first overtone of the bend.

Perspective: chemical dynamics simulations of non-statistical reaction dynamics

Non-statistical chemical dynamics are exemplified by disagreements with the transition state (TS), RRKM and phase space theories of chemical kinetics and dynamics. The intrinsic reaction coordinate (IRC) is often used for the former two theories, and non-statistical dynamics arising from non-IRC dynamics are often important. In this perspective, non-statistical dynamics are discussed for chemical reactions, with results primarily obtained from chemical dynamics simulations and to a lesser extent from experiment. The non-statistical dynamical properties discussed are: post-TS dynamics, including potential energy surface bifurcations, product energy partitioning in unimolecular dissociation and avoiding exit-channel potential energy minima; non-RRKM unimolecular decomposition; non-IRC dynamics; direct mechanisms for bimolecular reactions with pre- and/or post-reaction potential energy minima; non-TS theory barrier recrossings; and roaming dynamics.This article is part of the themed issue 'Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces'.

Kinetics study of the CN + CH4 hydrogen abstraction reaction based on a new ab initio analytical full-dimensional potential energy surface

We have developed an analytical full-dimensional potential energy surface, named PES-2017, for the gas-phase hydrogen abstraction reaction between the cyano radical and methane.

Desorption dynamics of CO2 from formate decomposition on Cu(111)

Based on the ab initio molecular dynamics simulations on formate decomposition into CO₂ and adsorbed H on Cu(111), we suggest that excitation of CO₂ bending mode can enhance formate synthesis.

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.

Control of chemical reactivity by transition-state and beyond

It has been long established that the transition state for an activated reaction controls the overall reactivity, serving as the bottleneck for reaction flux. However, the role of the transition state in regulating quantum state resolved reactivity has only been addressed more recently, thanks to advances in both experimental and theoretical techniques. In this perspective, we discuss some recent advances in understanding mode-specific reaction dynamics in bimolecular reactions, mainly focusing on the X + H2O/CH4 (X = H, F, Cl, and O(3P)) systems, extensively studied in our groups. These advances shed valuable light on the importance of the transition state in mode-specific and steric dynamics of these prototypical reactions. It is shown that many mode-specific phenomena can be understood in terms of a transition-state based model, which assumes in the sudden limit that the ability of a reactant mode for promoting the reaction stems from its coupling with the reaction coordinate at the transition state. Yet, in some cases the long-range anisotropic interactions in the entrance (or exit) valley, which govern how the trajectories reach (or leave) the transition state, also come into play, thus modifying the reactive outcomes.

First-principles quantum dynamical theory for the dissociative chemisorption of H2O on rigid Cu(111)

Despite significant progress made in the past decades, it remains extremely challenging to investigate the dissociative chemisorption dynamics of molecular species on surfaces at a full-dimensional quantum mechanical level, in particular for polyatomic-surface reactions. Here we report, to the best of our knowledge, the first full-dimensional quantum dynamics study for the dissociative chemisorption of H2O on rigid Cu(111) with all the nine molecular degrees of freedom fully coupled, based on an accurate full-dimensional potential energy surface. The full-dimensional quantum mechanical reactivity provides the dynamics features with the highest accuracy, revealing that the excitations in vibrational modes of H2O are more efficacious than increasing the translational energy in promoting the reaction. The enhancement of the excitation in asymmetric stretch is the largest, but that of symmetric stretch becomes comparable at very low energies. The full-dimensional characterization also allows the investigation of the validity of previous reduced-dimensional and approximate dynamical models.

Quantum Mechanical Investigation of Mode-Specific Tunneling upon Fundamental Excitation in Malonaldehyde

We present a quantum mechanical study of mode-specific tunneling upon fundamental excitation in malonaldehyde with a multidimensional theory that utilizes the saddle-point normal coordinates. We find that a ring-deformation normal mode is as essential as the well-known imaginary-frequency normal mode in the multidimensional investigation. The changes in tunneling splittings upon fundamental excitation are calculated. The results are competitive with those from a recently developed mixed classical-quantum method. Moreover, the results are qualitatively consistent with experiment for about half of all the modes.

Influence of vibration in the reactive scattering of D + MuH: the effect of dynamical bonding

The dynamics of the D + MuH(v = 1) reaction has been investigated using time-independent quantum mechanical calculations. The total reaction cross sections and rate coefficients have been calculated for the two exit channels of the reaction leading, respectively, to DMu + H and DH + Mu. Over the 100-1000 K temperature range investigated the rate coefficients for the DMu + H channel are of the order of 10(-10) cm(3) s(-1) and those for the DH + Mu channel vary between 1 × 10(-12) and 8 × 10(-11) cm(3) s(-1). These results point to a virtually barrierless reaction for the DMu + H channel and to the presence of a comparatively small barrier for the DH + Mu channel and are consistent with the profiles of their respective collinear vibrationally adiabatic potentials (VAPs). The effective barrier in the VAP of the DH + Mu channel is located in the reactant valley and, consequently, translation is found to be more efficient than vibration for the promotion of the reaction over a large energy interval in the post threshold region. Below this barrier, the DH + Mu channel can be accessible through an indirect mechanism implying crossing from the DMu + H pathway. The most salient feature found in the present study is revealed in the total reaction cross section for the DMu + H channel, which shows a sharp resonance caused by the presence of a deep well in the vibrationally adiabatic potential. This well has a dynamical origin, reminiscent of that found recently in the vibrationally bonded BrMuBr complex [Fleming, et al., Angew. Chem., Int. Ed., 2014, 53, 1], and is due to the stabilizing effect of the light Mu atom oscillating between the heavier H and D isotopes and to the bond softening associated with vibrational excitation of MuH.

Recent Advances in Quantum Dynamics of Bimolecular Reactions

In this review, we survey the latest advances in theoretical understanding of bimolecular reaction dynamics in the past decade. The remarkable recent progress in this field has been driven by more accurate and efficient ab initio electronic structure theory, effective potential-energy surface fitting techniques, and novel quantum scattering algorithms. Quantum mechanical characterization of bimolecular reactions continues to uncover interesting dynamical phenomena in atom-diatom reactions and beyond, reaching an unprecedented level of sophistication. In tandem with experimental explorations, these theoretical developments have greatly advanced our understanding of key issues in reaction dynamics, such as microscopic reaction mechanisms, mode specificity, product energy disposal, influence of reactive resonances, and nonadiabatic effects.

Mode specificity for the dissociative chemisorption of H2O on Cu(111): a quantum dynamics study on an accurately fitted potential energy surface

The mode-specific dynamics for the dissociative chemisorption of H₂O on Cu(111) is first investigated by seven-dimensional quantum dynamics calculations, based on an accurately fitted potential energy surface (PES) recently developed by neural network fitting to DFT energy points.

Mode specific dynamics in the H2 + SH → H + H2S reaction

Full-dimensional quantum dynamics and quasi-classical trajectory studies indicate strong mode selectivity in the H₂ + SH reaction.

Quantum dynamics of polyatomic dissociative chemisorption on transition metal surfaces: mode specificity and bond selectivity

Recent advances in quantum dynamical characterization of polyatomic dissociative chemisorption on accurate global potential energy surfaces are critically reviewed.

Towards an accurate specific reaction parameter density functional for water dissociation on Ni(111): RPBE versus PW91

Approximated nine dimensional quantum dynamics on a new potential energy surface for water dissociation on Ni(111) computed using the RPBE functional.

Influence of the Reactants Rotational Excitation on the H + D2(v = 0, j) Reactivity

We have analyzed the influence of the rotational excitation on the H + D2(v = 0, j) reaction through quantum mechanical (QM) and quasiclassical trajectories (QCT) calculations at a wide range of total energies. The agreement between both types of calculations is excellent. We have found that the rotational excitation largely increases the reactivity at large values of the total energy. Such an increase cannot be attributed to a stereodynamical effect but to the existence of recrossing trajectories that become reactive as the target molecule gets rotationally excited. At low total energies, however, recrossing is not significant and the reactivity evolution is dominated by changes in the collision energy; the reactivity decreases with the collision energy as it shrinks the acceptance cone. When state-to-state results are considered, rotational excitation leads to cold product's rovibrational distributions, so that most of the energy is released as recoil energy.