Time-dependent quantum wave packet study of the Ar+H2+→ArH(+)+H reaction on a new ab initio potential energy surface for the ground electronic state (1(2)A')

Quantum and statistical state-to-state studies of cold Ar + H2+ collisions

In this work we present new state-to-state integral scattering cross sections and initial-state selected rate coefficients for the 36Ar (1S) + H2+ (X2Σg+,v = 0,j) reactive system for collision energies up to 0.1 eV (with respect to the 36Ar (1S) + H2+ (X2Σg+,v = 0,j = 0) channel). To the best of our knowledge, these cross sections are the first fully state resolved ones that were obtained by performing time-independent quantum mechanical and quantum statistical calculations. For this purpose a new full-dimensional ground state 2A' adiabatic electronic potential energy surface was calculated at the MRCI+Q/aug-cc-pVQZ level of theory, which was fitted by means of machine learning methods. We find that a statistical quantum method and a statistical adiabatic channel model reproduce quantum mechanical initial-state selected cross sections fairly well, thus suggesting that complex-forming mechanisms seem to be playing an important role in the reaction dynamics of the reaction that was studied.

Machine Learning Representations of the Three Lowest Adiabatic Electronic Potential Energy Surfaces for the ArH2+ Reactive System

In this work, we present Gaussian process regression machine learning representations of the three lowest coupled 2A' adiabatic electronic potential energy surfaces of the ArH2+ reactive system in full dimensionality. Additionally, the nonadiabatic coupling matrix elements were calculated. These adiabatic potentials and their nonadiabatic couplings are necessary ingredients in the theoretical investigation of the nonadiabatic reaction dynamics of the Ar + H2+ → ArH+ + H and Ar+ + H2 → ArH+ + H reactions, as well as the competing charge transfer process, Ar + H2+↔ Ar+ + H2. Accurate ab initio electronic structure calculations (ic-MRCI+Q/aug-cc-pVQZ), whereby the effect of spin-orbit coupling in Ar+ has been accounted for through the state interaction method, serve as input for the machine learning training process. The potential energy surfaces are fitted with high accuracies, with root-mean-square errors on the order of 10-7 eV for the three surfaces, which meet the requirements for chemical dynamics at low temperature. It was found that quite a large number of training points (of the order of 5000 ab initio points) are needed in order to achieve these accuracies due to the complex topography of these electronic surfaces.

Quantum and quasiclassical dynamical simulations for the Ar2H+ on a new global analytical potential energy surface

A new analytical potential energy surface (PES) has been constructed for the Ar₂H⁺ system from a dataset consisting of a large number of ab initio energies computed using the coupled-cluster singles, doubles and perturbative triples method and aug-cc-pVQZ basis set. The long-range interaction is added to the diatomic potentials using a standard long range expansion form to better describe the asymptotic regions. The vibrational states for the most stable structures of the Ar₂H⁺ system have been calculated, and few low lying states are assigned to quantum numbers. Reactive scattering studies have been performed for the Ar + Ar'H⁺ → Ar' + ArH⁺ proton exchange reaction on the newly generated PES. Reaction probability, cross sections, and rate constants are calculated for the Ar + Ar'H⁺(v = 0, j = 0) collisions within 0.01 eV-0.6 eV of relative translational energy using exact quantum dynamical simulations as well as quasiclassical trajectory (QCT) calculations. The effect of vibrational excitation of the reactants is also explored for the reaction. State averaged rate constants are calculated for the proton exchange reaction at different temperatures using the QCT method. The mechanistic pathways for the reaction are understood by analyzing the quasiclassical trajectories.

Experimental study of the proton-transfer reaction C + H2+ → CH+ + H and its isotopic variant (D2+)

We report absolute integral cross section (ICS) measurements using a dual-source merged-fast-beams apparatus to study the titular reactions over the relative translational energy range of Er ∼ 0.01-10 eV. We used photodetachment of C- to produce a pure beam of atomic C in the ground electronic 3P term, with statistically populated fine-structure levels. The H2+ and D2+ were formed in an electron impact ionization source, with well known vibrational and rotational distributions. The experimental work is complemented by a theoretical study of the CH2+ electronic system in the reactant and product channels, which helps to clarify the possible reaction mechanisms underlying the ICS measurements. Our measurements provide evidence that the reactions are barrierless and exoergic. They also indicate the apparent absence of an intermolecular isotope effect, to within the total experimental uncertainties. Capture models, taking into account either the charge-induced dipole interaction potential or the combined charge-quadrupole and charge-induced dipole interaction potentials, produce reaction cross sections that lie a factor of ∼4 above the experimental results. Based on our theoretical study, we hypothesize that the reaction is most likely to proceed adiabatically through the 14A' and 14A'' states of CH2+via the reaction C(3P) + H2+(2Σ+g) → CH+(3Π) + H(2S). We also hypothesize that at low collision energies only H2+(v ≤ 2) and D2+(v ≤ 3) contribute to the titular reactions, due to the onset of dissociative charge transfer for higher vibrational v levels. Incorporating these assumptions into the capture models brings them into better agreement with the experimental results. Still, for energies ⪅0.1 eV where capture models are most relevant, the modified charge-induced dipole model yields reaction cross sections with an incorrect energy dependence and lying ∼10% below the experimental results. The capture cross section obtained from the combined charge-quadrupole and charge-induced dipole model better matches the measured energy dependence but lies ∼30-50% above the experimental results. These findings provide important guidance for future quasiclassical trajectory and quantum mechanical treatments of this reaction.

Cationic Noble-Gas Hydrides: From Ion Sources to Outer Space

Cationic species with noble gas (Ng)-hydrogen bonds play a major role in the gas-phase ion chemistry of the group 18 elements. These species first emerged more than 90 years ago, when the simplest HeH+ and HeH2 + were detected from ionized He/H2 mixtures. Over the years, the family has considerably expanded and currently includes various bonding motifs that are investigated with intense experimental and theoretical interest. Quite recently, the results of these studies acquired new and fascinating implications. The diatomic ArH+ and HeH+ were, in fact, detected in various galactic and extragalactic regions, and this stimulates intriguing questions concerning the actual role in the outer space of the Ng-H cations observed in the laboratory. The aim of this review is to briefly summarize the most relevant information currently available on the structure, stability, and routes of formation of these fascinating systems.

Vibrational frequencies and spectroscopic constants of three, stable noble gas molecules: NeCCH+, ArCCH+, and ArCN+

The search for possible, natural, noble gas molecules has led to quantum chemical, spectroscopic analysis of NeCCH⁺, ArCCH⁺, and ArCN⁺.

Scattering study of the Ne + NeH(+)(v0 = 0, j0 = 0) → NeH(+) + Ne reaction on an ab initio based analytical potential energy surface

Initial state selected dynamics of the Ne + NeH(+)(v0 = 0, j0 = 0) → NeH(+) + Ne reaction is investigated by quantum and statistical quantum mechanical (SQM) methods on the ground electronic state. The three-body ab initio energies on a set of suitably chosen grid points have been computed at CCSD(T)/aug-cc-PVQZ level and analytically fitted. The fitting of the diatomic potentials, computed at the same level of theory, is performed by spline interpolation. A collinear [NeHNe](+) structure lying 0.72 eV below the Ne + NeH(+) asymptote is found to be the most stable geometry for this system. Energies of low lying vibrational states have been computed for this stable complex. Reaction probabilities obtained from quantum calculations exhibit dense oscillatory structures, particularly in the low energy region and these get partially washed out in the integral cross section results. SQM predictions are devoid of oscillatory structures and remain close to 0.5 after the rise at the threshold thus giving a crude average description of the quantum probabilities. Statistical cross sections and rate constants are nevertheless in sufficiently good agreement with the quantum results to suggest an important role of a complex-forming dynamics for the title reaction.

State-resolved differential and integral cross sections for the Ne + H2 (+) (v = 0-2, j = 0) → NeH(+) + H reaction

State-to-state quantum dynamic calculations for the proton transfer reaction Ne + H2 (+) (v = 0-2, j = 0) are performed on the most accurate LZHH potential energy surface, with the product Jacobi coordinate based time-dependent wave packet method including the Coriolis coupling. The J = 0 reaction probabilities for the title reaction agree well with previous results in a wide range of collision energy of 0.2-1.2 eV. Total integral cross sections are in reasonable agreement with the available experiment data. Vibrational excitation of the reactant is much more efficient in enhancing the reaction cross sections than translational and rotational excitation. Total differential cross sections are found to be forward-backward peaked with strong oscillations, which is the indication of the complex-forming mechanism. As the collision energy increases, state-resolved differential cross section changes from forward-backward symmetric peaked to forward scattering biased. This forward bias can be attributed to the larger J partial waves, which makes the reaction like an abstraction process. Differential cross sections summed over two different sets of J partial waves for the v = 0 reaction at the collision energy of 1.2 eV are plotted to illustrate the importance of large J partial waves in the forward bias of the differential cross sections.

Quantum, Statistical, and Quasiclassical Trajectory Studies For the Ne + HeH(+) → NeH(+) + He Reaction on the Ground Electronic State

Real wave packet, statistical quantum, and quasiclassical trajectory methods were employed to study the dynamics of Ne + HeH(+)(v0,j0) → He + NeH(+) reaction on an ab initio potential energy surface [J. Phys. Chem. A 2013, 117, 13070-13078]. Quantum and statistical quantum calculations were performed within the centrifugal sudden (CS) approximation as well as including the Coriolis coupling (CC). Dense oscillatory structures of the quantum reaction probabilities and fair agreement between quantum and statistical cross sections suggest a complex forming mechanism for the reaction. No significant differences between cross sections obtained within the CS and CC approaches are observed. Quasiclassical trajectory results give an excellent average description of the quantum CC results. At low collision energies, there is a substantial decrease in reactivity for the reaction upon rovibrational excitation. Initial state selected rate constants for the title reaction are calculated between 20 and 1000 K, and the calculated value at 300 K agrees quite well with the available experimental result. Reaction cross sections and rate constants are also compared with those calculated via the Langevin capture model for exothermic reactions.

Wave packet and statistical quantum calculations for the He + NeH⁺ → HeH⁺ + Ne reaction on the ground electronic state

A real wave packet based time-dependent method and a statistical quantum method have been used to study the He + NeH(+) (v, j) reaction with the reactant in various ro-vibrational states, on a recently calculated ab initio ground state potential energy surface. Both the wave packet and statistical quantum calculations were carried out within the centrifugal sudden approximation as well as using the exact Hamiltonian. Quantum reaction probabilities exhibit dense oscillatory pattern for smaller total angular momentum values, which is a signature of resonances in a complex forming mechanism for the title reaction. Significant differences, found between exact and approximate quantum reaction cross sections, highlight the importance of inclusion of Coriolis coupling in the calculations. Statistical results are in fairly good agreement with the exact quantum results, for ground ro-vibrational states of the reactant. Vibrational excitation greatly enhances the reaction cross sections, whereas rotational excitation has relatively small effect on the reaction. The nature of the reaction cross section curves is dependent on the initial vibrational state of the reactant and is typical of a late barrier type potential energy profile.