Accurate ab initio based potential energy surface and kinetics of the Cl + NH3 → HCl + NH2 reaction

The gas-phase reaction Cl + NH3 → HCl + NH2 is a prototypical hydrogen abstraction reaction, whose minimum energy path involves several intermediate complexes. In this work, a full-dimensional, spin-orbit corrected potential energy surface (SOC PES) is constructed for the ground electronic state of the Cl + NH3 reaction. About 52 000 energy points are sampled and calculated at the UCCSD(T)-F12a/aug-cc-pVTZ level, in which the data points located in the entrance channel are spin-orbit corrected. The spin-orbit corrections are predicted by a fitted three-dimensional energy surface from about 7520 energy points in the entrance channel at the level of CASSCF (15e, 11o)/aug-cc-pVTZ. The fundamental-invariant neural network method is utilized to fit the SOC PES, resulting in a total root mean square error of 0.12 kcal mol-1. The calculated thermal rate constants of the Cl + NH3 → HCl + NH2 reaction on the SOC PES with the soft-zero-point energy constraint agree reasonably well with the available experimental values.

Higher-Order Split Operator Schemes for Solving Tetratomic Reactions Using the Time-Dependent Wave Packet Method

In this work, using the time-dependent quantum wave packet method, quite a few typical higher-order split operators (HOSOs) were for the first time applied to calculate the tetratomic reactive scattering processes in the hyperspherical coordinate. It was found that the HOSOs were hardly efficient for a tetratomic reaction calculation, unlike those for a triatomic reactive scattering calculation. We proposed an efficient HOSO with a force gradient (denoted as 2G1 in the main text) for efficiently and accurately calculating a tetratomic reaction using the quantum wave packet method. Several typical tetratomic reactions, such as H2 + OH, HF + OH, and H2 + OH+, are calculated for demonstrating the effectiveness of the proposed 2G1 in terms of (product state-resolved) reaction probability and inelastic probability, by comparing with the performance of the previously reported various HOSOs. We suggest that the 2G1 propagator could be applied to efficiently calculate a general tetratomic reaction.

Accurate determination of reaction energetics and kinetics of the HO2˙ + O3 → OH˙ + 2O2 reaction

In the present work, we have studied the HO₂˙ + O₃ → HO˙ + 2O₂ reaction using chemical kinetics and quantum chemical calculations. We have employed the post-CCSD(T) method to estimate the barrier height and reaction energy for the title reaction. In the post-CCSD(T) method, we have included zero point energy corrections, contributions from full triple excitations and partial quadratic excitations at the coupled-cluster level, and core corrections. We have also computed the reaction rate in the temperature range of 197-450 K and found good agreement with all the available experimental results. In addition, we have also fitted the computed rate constants with the Arrhenius expression and obtained an activation energy of 1.0 ± 0.1 kcal mol^-1, almost identical to the value recommended by IUPAC and JPL.

Insights into the Diels-Alder Reaction of Furan with Maleic Anhydride from Its Prereactive Intermediate

The prereactive intermediate in the furan-maleic anhydride cycloaddition, a classical Diels-Alder reaction, has been captured and characterized in pulsed jets by Fourier transform microwave spectroscopy for the first time. The observed species is stabilized by the π-π* interaction between the two moieties, which connects to the endo channel of the cycloaddition. The secondary interactions between the C=C and C=O in the observed isomer are accountable for its lower energy with respect to the one with the exo channel. The present study tries to fill the significant void of the experimental information on prereactive intermediates as the first stage of Diels-Alder cycloadditions, by outlining the stability of the prereactive intermediate and its accurate molecular structure and by emphasizing the role of the π-π* interaction in governing the stereochemical outcome of the reaction.

Effect of microsolvation on the mode specificity of the OH˙(H2O) + HCl reaction

The present study investigates the mode specificity in the microsolvated OH˙(H₂O) + HCl reaction using on-the-fly direct dynamics simulation. To the best of our knowledge, this is the first study which aims to gain insights into the effect of microsolvation on the mode selectivity. Our investigation reveals that, similar to the gas phase OH˙ + HCl reaction, the microsolvated reaction is also predominantly affected by the vibrational excitation of the HCl mode, whereas the OH vibrational mode behaves as a spectator. Interestingly, in contrast to the behavior of the bare reaction, the integral cross section at the ground state of the microsolvated reaction decreases with an increase in translational energy. However, for the vibrational excited states, the reactivity of the microsolvated reaction is found to be higher than that of the bare reaction within the selected range of translational energies.

The Role of Zero-Point Vibration and Reactant Attraction in Exothermic Bimolecular Reactions with Submerged Potential Barriers: Theoretical Studies of the R + HBr → RH + Br (R = CH3, HO) Systems

The dynamics of the reactions CH₃ + HBr → CH₄ + Br and HO + HBr → H₂O + Br have been studied using the quasiclassical trajectory method to explore the interplay of the vibrational excitation of the breaking bond and the potential energy surface characterized by a prereaction van der Waals well and a submerged barrier to reaction. The attraction between the reactants is favorable for the reaction, because it brings together the reactants without any energy investment. The reaction can be thought to be controlled by capture. The trajectory calculations indeed provide excitation functions typical to capture: the reaction cross sections diverge when the collision energy is reduced toward zero. Excitation of reactant vibration accelerates both reactions. The barrier on the potential surface is so early that the coupling between the degrees of freedom at the saddle point geometry is negligible. However, the trajectory calculations show that when the breaking bond is stretched at the time of the encounter, an attractive force arises, as if the radical approached a HBr molecule whose bond is partially broken. As a result, the dynamics of the reaction are controlled more by the temporary "dynamical", vibrationally induced than by the "static" van der Waals attraction even when the reactants are in vibrational ground state. The cross sections are shown to drop to very small values when the amplitude of the breaking bond's vibration is artificially reduced, which provides an estimate of the reactivity due to the "static" attraction. Without zero-point vibration these reactions would be very slow, which is a manifestation of a unique quantum effect. Reactions where the reactivity is determined by dynamical factors such as the vibrationally enhanced attraction are found to be beyond the range of applicability of Polanyi's rules.

Feshbach Resonances in the Vibrationally Excited F + HOD(vOH/vOD = 1) Reaction Due to Chemical Bond Softening

Experiments and theories showed the ground-state reaction F + H₂O → HF + OH possesses Feshbach resonances trapped in the hydrogen bond well in the product region. However, it is not clear whether F + H₂O and its isotopic analogues have the same Feshbach resonances caused by chemical bond softening as those in the F + H₂/HD. Here, we reported state-to-state quantum dynamics studies of the F + HOD(v_OH = 1) → HF + OD and F + HOD(v_OD = 1) → DF + OH reactions on an accurate neural network potential energy surface. Detailed analysis reveals that the course of the title reactions is dominated by the Feshbach resonance states trapped in the peculiar HF(v'=3)-OD/DF(v'=4)-OH vibrationally adiabatic potential well created by the HF/DF bond softening, which can only be accessed via the HOD(v_OH = 1)/HOD(v_OD = 1) reaction pathway. Therefore, we confirm the wide existence of chemical bond softening resonances in reactions involving vibrationally excited molecules.

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.

Mode- and Bond-Selected Reaction of H with Local Mode Molecule HDS

Understanding mode- and bond-selected dynamics of elementary chemical reactions is of central importance in molecular reaction dynamics. The initial state-selected time-dependent wave packet method is employed to study the mode and bond selectivity, isotopic branching ratio, and temperature dependence of rate constants of the two-channel reaction of H with local mode molecule HDS. For the abstraction channel, fundamental excitation of the HS (DS) bond of the reactant HDS significantly enhances the H-abstraction (D-abstraction) reaction, whose efficacy is higher than the same amount of translational energy except at low energies just above the energy threshold. This is in sharp contrast to the prediction of Polanyi rules: translational energy is more efficient than vibrational energy in enhancing a reaction with an early barrier. The recent sudden vector projection model is then applied to rationalize the observed mode specificity, which, however, shows that the translational mode vector has a larger coupling with the reaction coordinate than the stretching vector of the active bond, implying a reversed relative efficacy on promoting the reaction as well. In contrast, the mode and bond specificity for the exchange channel is not as strong as for the abstraction channel due to the regulation of the shallow well along the reaction path.

Feshbach resonances in the F + H2O → HF + OH reaction

Transiently trapped quantum states along the reaction coordinate in the transition-state region of a chemical reaction are normally called Feshbach resonances or dynamical resonances. Feshbach resonances trapped in the HF-OH interaction well have been discovered in an earlier photodetchment study of FH₂O^-; however, it is not clear whether these resonances are accessible by the F + H₂O reaction. Here we report an accurate state-to-state quantum dynamics study of the F + H₂O → HF + OH reaction on an accurate newly constructed potential energy surface. Pronounced oscillatory structures are observed in the total reaction probabilities, in particular at collision energies below 0.2 eV. Detailed analysis reveals that these oscillating structures originate from the Feshbach resonance states trapped in the hydrogen bond well on the HF(v' = 2)-OH vibrationally adiabatic potentials, producing mainly HF(v' = 1) product. Therefore, the resonances observed in the photodetchment study of FH₂O^- are accessible to the reaction.

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

Comprehensive dynamical simulations of a prototypical multi-channel reaction on a globally accurate potential energy surface show that the non-statistical product branching is dictated by unique stereodynamics in the entrance channels.

Hydrogen abstraction from methanol (CH₃OH) by F atoms presents an ideal proving ground to investigate dynamics of multi-channel reactions, because two types of hydrogen can be abstracted from the methanol molecule leading to the HF + CH₃O and HF + CH₂OH products. Using the quasi-classical trajectory approach on a globally accurate potential energy surface based on high-level ab initio calculations, this work reports a comprehensive dynamical investigation of this multi-channel reaction, yielding measurable attributes including integral and differential cross sections, as well as branching ratios. It is shown that while complex-forming and direct mechanisms coexist at low collision energies, these barrierless reaction channels are dominated at high energies by the direct mechanism, in which the reaction is only possible for trajectories entering into the respective dynamical cones of acceptance. Perhaps more importantly, the non-statistical product branching is found to be dictated by unique stereodynamics in the entrance channels.

Kinetics of the OH+HCl→H2 O+Cl reaction: Rate determining roles of stereodynamics and roaming and of quantum tunneling

The OH + HCl → H₂ O + Cl reaction is one of the most studied four-body systems, extensively investigated by both experimental and theoretical approaches. Here, as a continuation of our previous work on the OH + HBr and OH + HI reactions, which manifest an anti-Arrhenius behavior that was explained by stereodynamic and roaming effects, we extend the strategy to understand the transition to the sub-Arrhenius behavior occurring for the HCl case. As previously, we perform first-principles on-the-fly Born-Oppenheimer molecular dynamics calculations, thermalized at four temperatures (50, 200, 350, and 500 K), but this time we also apply a high-level transition-state-theory, modified to account for tunneling conditions. We find that the theoretical rate constants calculated with Bell tunneling corrections are in good agreement with extensive experimental data available for this reaction in the ample temperature range: (i) simulations show that the roles of molecular orientation in promoting this reaction and of roaming in finding the favorable path are minor than in the HBr and HI cases, and (ii) dominating is the effect of quantum mechanical penetration through the energy barrier along the reaction path on the potential energy surface. The discussion of these results provides clarification of the origin on different non-Arrhenius mechanisms observed along this series of reactions. © 2018 Wiley Periodicals, Inc.

Understanding mode-specific dynamics in the local mode representation

Mode specificity is a main characteristic of transition state control of reaction dynamics. The normal mode representation has been widely employed to describe the mode specificity in elementary chemical reactions. However, spectroscopists have demonstrated that the local mode representation has advantages in analyzing the overtone and combination band spectra. In this work, the mode-specific reaction dynamics between the hydrogen atom and the molecules H2S and H2O is studied using a full-dimensional quantum scattering model in the (2 + 1) Radau-Jacobi coordinates. The mode specificities in the reactions that violates our physical intuition in the normal mode representation are well rationalized in the local mode representation. The energy flow between different XH bonds resulting from the intramolecular interaction and/or intermolecular interaction is unveiled, together with its impacts on dynamics of the abstraction and exchange reactions.

Spiers Memorial Lecture : Introductory lecture: quantum dynamics of chemical reactions

This Spiers Memorial Lecture discusses quantum effects that can be calculated and observed in the chemical reactions of small molecules.

Effects of vibrational excitation on the F + H2O → HF + OH reaction: dissociative photodetachment of overtone-excited [F-H-OH]. Chem Sci 2017;8:7821-7833. [PMID: 29163919 PMCID: PMC5674243 DOI: 10.1039/c7sc03364h]

Photodetaching vibrationally excited FH₂O^– channels energy into the reaction coordinate of the F + H₂O reaction, as shown in this joint experimental-theoretical study.

The reaction F + H₂O → HF + OH is a four-atom system that provides an important benchmark for reaction dynamics. Hydrogen atom transfer at the transition state for this reaction is expected to exhibit a strong dependence on reactant vibrational excitation. In the present study, the vibrational effects are examined by photodetachment of vibrationally excited F^–(H₂O) precursor anions using photoelectron-photofragment coincidence (PPC) spectroscopy and compared with full six-dimensional quantum dynamical calculations on ab initio potential energy surfaces. Prior to photodetachment at hν_UV = 4.80 eV, the overtone of the ionic hydrogen bond mode in the precursor F^–(H₂O), 2ν_IHB at 2885 cm^–1, was excited using a tunable IR laser. Experiment and theory show that vibrational energy in the anion can be effectively carried away by the photoelectron upon a Franck–Condon photodetachment, and also show evidence for an increase of branching into the F + H₂O reactant channel. The experimental results suggest a greater role for product rotational excitation than theory. Improved potential energy surfaces and longer wavepacket propagation times would be helpful to further examine the nature of the discrepancy.

Dynamical importance of van der Waals saddle and excited potential surface in C(1D)+D2 complex-forming reaction

Encouraged by recent advances in revealing significant effects of van der Waals wells on reaction dynamics, many people assume that van der Waals wells are inevitable in chemical reactions. Here we find that the weak long-range forces cause van der Waals saddles in the prototypical C(¹D)+D₂ complex-forming reaction that have very different dynamical effects from van der Waals wells at low collision energies. Accurate quantum dynamics calculations on our highly accurate ab initio potential energy surfaces with van der Waals saddles yield cross-sections in close agreement with crossed-beam experiments, whereas the same calculations on an earlier surface with van der Waals wells produce much smaller cross-sections at low energies. Further trajectory calculations reveal that the van der Waals saddle leads to a torsion then sideways insertion reaction mechanism, whereas the well suppresses reactivity. Quantum diffraction oscillations and sharp resonances are also predicted based on our ground- and excited-state potential energy surfaces.

It is commonly held that van der Waals wells are inevitable in chemical reactions. Here, the authors show that weak van der Waals forces in the entrance channel of a prototypical complex-forming reaction cause a van der Waals saddle instead, with different dynamical effects from a well at low collision energies.

Dynamics of transient speciesviaanion photodetachment

Recent experimental and theoretical advances in transient reaction dynamics probed by photodetachment of polyatomic anions are reviewed.

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.

Reactive and Nonreactive Feshbach Resonances Accessed by Photodetachment of FH2O(-)

The photodetachment of the FH2O(-) anion is investigated quantum mechanically on accurate full-dimensional potential energy surfaces of the two lowest-lying electronic states of FH2O. The calculated photoelectron spectrum possesses both broad and sharp features, corresponding to reactive and nonreactive Feshbach resonances. The former extend to both reactant and product channels over the transition state, while the latter are supported by a hydrogen bonded HO-HF well in the product channel. Many of the resonances are assignable with quantum numbers for the stretching and bending modes of the HO-HF complex as well as the H-F vibration. The implications of these resonances in the F + H2O ↔ HF + HO reaction are discussed.

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.

Insights into the bond-selective reaction of Cl + HOD(nOH) → HCl + OD

The theoretical study of the title reaction reveals strong bond selectivity, vibrational enhancement, and unique product distributions, in agreement with the experiment.

Energy disposal and thermal rate constants for the OH + HBr and OH + DBr reactions: quasiclassical trajectory calculations on an accurate potential energy surface

We report reaction cross sections, energy disposal, and rate constants for the OH + HBr → Br + H2O and OH + DBr → Br + HDO reactions from quasiclassical trajectory calculations using an ab initio potential energy surface [ de Oliveira-Filho , A. G. S. ; Ornellas , F. R. ; Bowman , J. M. J. Phys. Chem. Lett. 2014 , 5 , 706 - 712 ]. Comparison with available experiments are made and generally show good agreement.

Quasiclassical Trajectory Calculations of the Rate Constant of the OH + HBr → Br + H2O Reaction Using a Full-Dimensional Ab Initio Potential Energy Surface Over the Temperature Range 5 to 500 K

We report a permutationally invariant, ab initio potential energy surface (PES) for the OH + HBr → Br + H2O reaction. The PES is a fit to roughly 26 000 spin-free UCCSD(T)/cc-pVDZ-F12a energies and has no classical barrier to reaction. It is used in quasiclassical trajectory calculations with a focus on the thermal rate constant, k(T), over the temperature range 5 to 500 K. Comparisons with available experimental data over the temperature range 23 to 416 K are made using three approaches to treat the OH rotational and associated electronic partition function. All display an inverse temperature dependence of k(T) below roughly 160 K and a nearly constant temperature dependence above 160 K, in agreement with experiment. The calculated rate constant with no treatment of spin-orbit coupling is overall in the best agreement with experiment, being (probably fortuitously) within 20% of it.

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

To provide a systematic and rigorous re-examination of the well-known Polanyi's rules, excitation functions of several A + BC(v = 0, 1) reactions are determined using the Chebyshev real wave packet method on accurate potential energy surfaces. Reactions with early (F + H2 and F + HCl), late (Cl + H2), and central (H∕D∕Mu + H2, where Mu is a short-lived light isotope of H) barriers are represented. Although Polanyi's rules are in general consistent with the quantum dynamical results, their predictions are strictly valid only in certain energy ranges divided by a cross-over point. In particular, vibrational excitation of the diatomic reactant typically enhances reactivity more effectively than translational excitation at high energies, while reverse is true at low energies. This feature persists irrespective of the barrier location. A sudden vector projection model is proposed as an alternative to Polanyi's rules. It is found to give similar, but more quantitative, predictions about mode selectivity in these reactions, and has the advantage to be extendible to reactions involving polyatomic molecules.

Dynamics study of the OH + NH3 hydrogen abstraction reaction using QCT calculations based on an analytical potential energy surface

To understand the reactivity and mechanism of the OH + NH3 → H2O + NH2 gas-phase reaction, which evolves through wells in the entrance and exit channels, a detailed dynamics study was carried out using quasi-classical trajectory calculations. The calculations were performed on an analytical potential energy surface (PES) recently developed by our group, PES-2012 [Monge-Palacios et al. J. Chem. Phys. 138, 084305 (2013)]. Most of the available energy appeared as H2O product vibrational energy (54%), reproducing the only experimental evidence, while only the 21% of this energy appeared as NH2 co-product vibrational energy. Both products appeared with cold and broad rotational distributions. The excitation function (constant collision energy in the range 1.0-14.0 kcal mol(-1)) increases smoothly with energy, contrasting with the only theoretical information (reduced-dimensional quantum scattering calculations based on a simplified PES), which presented a peak at low collision energies, related to quantized states. Analysis of the individual reactive trajectories showed that different mechanisms operate depending on the collision energy. Thus, while at high energies (E(coll) ≥ 6 kcal mol(-1)) all trajectories are direct, at low energies about 20%-30% of trajectories are indirect, i.e., with the mediation of a trapping complex, mainly in the product well. Finally, the effect of the zero-point energy constraint on the dynamics properties was analyzed.

A nine-dimensional global potential energy surface for NH4(X2A1) and kinetics studies on the H + NH3 ↔ H2 + NH2 reaction

A nine-dimensional global potential energy surface (PES) for the NH₄ system is developed from ∼10⁵ high-level ab initio points and the hydrogen abstraction kinetics on the PES agree with experiment.

SEVEN-DIMENSIONAL QUANTUM DYNAMICS STUDY OF THE H2 + NH2 → H + NH3 REACTION ON AN INTERPOLATED POTENTIAL ENERGY SURFACE

Initial-state-selected time-dependent wave packet dynamics studies have been performed for the H 2 + NH 2 → H + NH 3 reaction with a seven-dimensional model on a new interpolated ab initio potential energy surface (PES). The PES is constructed using modified Shepard interpolation Scheme and contains 1967 data points with ab initio calculations carried out on UCCSD(T)/aug-cc-pVTZ level. In the seven-dimensional model, NH 2 group keeps C2v symmetry and two NH bonds are fixed at their equilibrium values. The total reaction probabilities are calculated when (1) the two reactants are initially at their ground states; (2) NH 2 bending mode is excited, and (3) H 2 is on its first vibrational excited state. The integral cross sections are also reported for these initial states with centrifugal-sudden approximation. Thermal rate constants are calculated for the temperature range of 200–2000 K and compared with the previous calculated values and available experimental data. Good agreements between theory and experiments for the rate constants at intermediate temperature are achieved on this PES.