Modulations of Transition-State Control of State-to-State Dynamics in the F + H₂O → HF + OH Reaction

The full-dimensional quantum dynamics of the F + H₂O → HF + OH reaction is investigated at the state-to-state level for the first time using a transition-state wave packet method on an accurate global potential energy surface. It is found that the H₂O rotation enhances the reactivity and the product-state distribution is dominated by HF vibrational excitation while the OH moiety serves effectively as a spectator. These observations underscore the transition-state control of the reaction dynamics, as both the H₂O rotational and HF vibrational modes are strongly coupled to the reaction coordinate at the transition state. It is also shown that the transition-state dominance of the reaction dynamics is modulated by other features on the potential energy surface, such as the prereaction well

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Control of Mode/Bond Selectivity and Product Energy Disposal by the Transition State: X + H₂O (X = H, F, O(³P), and Cl) Reactions

The ability to predict mode/bond selectivity and energy disposal is of central importance for controlling chemical reactions. We argue that the transition state plays a critical role in state-to-state reactivity and propose a simple sudden model based on coupling with the reaction coordinate at the transition state. The applicability of this so-called sudden vector projection (SVP) model is examined for several prototypical atom–triatom, namely, X + H₂O (X = H, F, O­(³P), and Cl) reactions. It is shown that the SVP model is capable of qualitatively predicting experimental and full-dimensional quantum dynamical results, including those reported in this work, for these polyatomic reactions. These results, and those for other reactions, suggest that the SVP model offers a general paradigm for understanding quantum state resolved reactivity in bimolecular reactions

Full-Dimensional Quantum Calculations of Vibrational Levels of NH₄⁺ and Isotopomers on An Accurate Ab Initio Potential Energy Surface

Vibrational energy levels of the ammonium cation (NH₄⁺) and its deuterated isotopomers are calculated using a numerically exact kinetic energy operator on a recently developed nine-dimensional permutation invariant semiglobal potential energy surface fitted to a large number of high-level ab initio points. Like CH₄, the vibrational levels of NH₄⁺ and ND₄⁺ exhibit a polyad structure, characterized by a collective quantum number <i>P</i> = 2­(<i>v</i>₁ + <i>v</i>₃) + <i>v</i>₂ + <i>v</i>₄. The low-lying vibrational levels of all isotopomers are assigned and the agreement with available experimental data is better than 1 cm^–1

UV Absorption Spectrum and Photodissociation Channels of the Simplest Criegee Intermediate (CH₂OO)

The lowest-lying singlet states of the simplest Criegee intermediate (CH₂OO) have been characterized along the O–O dissociation coordinate using explicitly correlated MRCI-F12 electronic structure theory and large active spaces. It is found that a high-level treatment of dynamic electron-correlation is essential to accurately describe these states. A significant well on the <i>B</i>-state is identified at the MRCI-F12 level with an equilibrium structure that differs substantially from that of the ground <i>X</i>-state. This well is presumably responsible for the apparent vibrational structure in some experimental UV absorption spectra, analogous to the structured Huggins band of the iso-electronic ozone. The <i>B</i>-state potential in the Franck–Condon region is sufficiently accurate that an absorption spectrum calculated with a one-dimensional model agrees remarkably well with experiment

First-Principles Insights into Ammonia Decomposition Catalyzed by Ru Clusters Anchored on Carbon Nanotubes: Size Dependence and Interfacial Effects

Ammonia decomposition catalyzed by Ru nanoparticles supported on carbon nanotubes offers an efficient way for CO_<i>x</i>-free hydrogen generation. To understand the catalytic mechanism, the two most important elementary steps of ammonia decomposition, namely the initial cleavage of the NH₂–H bond and the nitrogen recombination, have been studied using density functional theory on a carbon nanotube deposited with Ru_<i>x</i> (<i>x</i> = 1, 2, 6, and 13) clusters. The results indicate the reaction steps are catalyzed at Ru sites with barriers significantly lower than those on Ru(0001), but the barriers have a strong dependence on the size of the cluster. It is also found that Ru sites at the interface with the carbon nanotube are more active, showing a strong interfacial effect due apparently to facile charge transfer from the carbon nanotube to interfacial metal atoms

State-to-State Mode Specificity: Energy Sequestration and Flow Gated by Transition State

Energy flow and sequestration at the state-to-state level are investigated for a prototypical four-atom reaction, H₂ + OH → H + H₂O, using a transition-state wave packet (TSWP) method. The product state distribution is found to depend strongly on the reactant vibrational excitation, indicating mode specificity at the state-to-state level. From a local-mode perspective, it is shown that the vibrational excitation of the H₂O product derives from two different sources, one attributable to the energy flow along the reaction coordinate into the newly formed OH bond and the other due to the sequestration of the vibrational energy in the OH spectator moiety during the reaction. The analysis provided a unified interpretation of some seemingly contradicting experimental observations. It is further shown that the transfer of vibrational energy from the OH reactant to H₂O product is gated by the transition state, accomplished coherently by multiple TSWPs with the corresponding OH vibrational excitation

Reactant Vibrational Excitations Are More Effective than Translational Energy in Promoting an Early-Barrier Reaction F + H₂O → HF + OH

The exothermic F + H₂O → HF + OH reaction has a decidedly “early” or “reactant-like” barrier. According to a naïve interpretation of the Polanyi’s rules, translational energy would be more effective than vibrational energy in promoting such reactions. However, we demonstrate here using both quasi-classical trajectory and full-dimensional quantum wave packet methods on an accurate global potential energy surface that excitations in the H₂O vibrational degrees of freedom have higher efficacy in enhancing the reactivity of the title reaction than the same amount of translational energy, thus providing a counter-example to Polanyi’s rules. This enhancement of reactivity is analyzed using a vibrational adiabatic model, which sheds light on the surprising mode selectivity in this reaction

Quantum Dynamics of the HO + CO → H + CO₂ Reaction on an Accurate Potential Energy Surface

Full-dimensional quantum dynamics of the HO + CO → H + CO₂ reaction is investigated on a recent global potential energy surface based on a large number of ab initio points. The <i>J</i> = 0 reaction probability is small and essentially a monotonically increasing function with energy, superimposed by overlapping resonances. The reactivity is considerably enhanced by OH vibrational excitation while relatively insensitive to CO vibrational excitation. The rate constant estimated by the <i>J</i>-shifting approximation indicates a much better agreement with experiment than that obtained on a previous potential energy surface

Ring-Polymer Molecular Dynamics Rate Coefficient Calculations for Insertion Reactions: X + H₂ → HX + H (X = N, O)

The thermal rate constants of two prototypical insertion-type reactions, namely, N/O + H₂ → NH/OH + H, are investigated with ring polymer molecular dynamics (RPMD) on full-dimensional potential energy surfaces using recently developed RPMDrate code. It is shown that the unique ability of the RPMD approach among the existing theoretical methods to capture the quantum effects, e.g., tunneling and zero-point energy, as well as recrossing dynamics quantum mechanically with ring-polymer trajectories leads to excellent agreement with rigorous quantum dynamics calculations. The present result is encouraging for future applications of the RPMD method and the RPMDrate code to complex-forming chemical reactions involving polyatomic reactants