State-to-State Dynamics Analysis of the F + CHD3 Reaction: A Quasiclassical Trajectory Study

Theoretical studies on the kinetics and dynamics of the BeH+ + H2O reaction: comparison with the experiment

The reaction of BeH+ with background gaseous H2O may play a role in qubit loss for quantum information processing with Be+ as trapped ions, and yet its reaction mechanism has not been well understood until now. In this work, a globally accurate, full-dimensional ground-state potential energy surface (PES) for the BeH+ + H2O reaction was constructed by fitting a total of 170 438 ab initio energy points at the level of RCCSD(T)-F12/aug-cc-pVTZ using the fundamental invariant-neural network method. The total root-mean-square error of the final PES was 0.178 kcal mol-1. For comparison, quasi-classical trajectory calculations were carried out on the PES at an experimental temperature of 150 K. The obtained thermal rate constant and product branching ratio of the BeD+ + H2O reaction agreed quite well with experimental results. In addition, the vibrational state distributions and energy disposals of the products were calculated and rationalized using the sudden vector projection model.

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.

Final-State-Resolved Dynamics of the H3+ + CO → H2 +HCO+/HOC+ Reaction: A Quasi-Classical Trajectory Study

The ion-molecule reaction H₃⁺ + CO → H₂ + HCO⁺/HOC⁺, which initiates the formation of crucial organic molecules, plays a key role in interstellar and circumstellar environments. In this work, the quasi-classical trajectory method is employed to study the reaction dynamics on a recently developed full-dimensional global potential energy surface (PES). The calculated product internal energy distributions and relative internal excited fractions agree reasonably well with the experimental measurements. For the two reaction channels, most of the available energy flows into the vibrational modes of HCO⁺ or HOC⁺ at low collision energies, followed by the translational mode and the rotational modes of HCO⁺ or HOC⁺. As the collision energy increases, the proportion of the product translational energy increases while the proportion of the product vibrational energy decreases. Furthermore, the CH and CO stretching modes and their combination bands are effectively excited for the product HCO⁺ while the bending mode is remarkably excited for the product HOC⁺.

Theoretical study of the F(2P) + NH3→ HF + NH2 reaction on an accurate potential energy surface: dynamics and kinetics

The highly exothermic hydrogen abstraction reaction of the F atom with NH₃ is investigated using the quasi-classical trajectory method on a newly developed potential energy surface (PES) for the ground electronic state. The full-dimensional PES is constructed by fitting 41 282 ab initio energy points at the level of UCCSD(T)-F12/aug-cc-pVTZ. The flexible fundamental invariant-neural network method is applied in the fitting, resulting in a total root mean square error of 0.13 kcal mol^-1. On one hand, the calculated differential cross sections agree reasonably well with the experimental results and indicate that the reaction is dominated by the direct abstraction and stripping mechanisms while a considerable amount of reaction takes place by the indirect "yo-yo" mechanism. The product energy partition also reproduces well the experimental result, which can be understood according to the geometry change along the minimum energy path. On the other hand, the obtained vibrational state distribution of the product HF follows P_νHF=2≈P_νHF=1 > P_νHF=0 > P_νHF=3, less consistent with the scattered experimental results. In addition, the calculated thermal rate coefficients have practically no temperature dependence within the statistical errors.

New Method To Extract Final-State Information of Polyatomic Reactions Based on Normal Mode Analysis

State-to-state reaction dynamics provides a comprehensive insight into reaction mechanisms of chemical reactions at the atomic level. A new scheme to extract final-state information based on normal mode analysis is proposed in this work. Different from the traditional scheme extracting the coordinates and momenta from the last step of each trajectory, they are taken in the new scheme from a specific step of each reactive trajectory within the last vibrational period of the product molecule by demanding the corresponding geometry of the step to have the minimum potential energy. Test calculations on the collisions between the atom H and the molecules H₂O, H₂S, and NH₃ show that the new scheme works much better than the traditional one. In addition, the new scheme is applied to calculate the vibrational state distribution of the product NH₂ in the reaction H + NH₃ → H₂ + NH₂.

Tracking the energy flow in the hydrogen exchange reaction OH + H2O → H2O + OH

The prototypical hydrogen exchange reaction OH + H2O → H2O + OH has attracted considerable interest due to its importance in a wide range of chemically active environments. In this work, an accurate global potential energy surface (PES) for the ground electronic state was developed based on ∼44 000 ab initio points at the level of UCCSD(T)-F12a/aug-cc-pVTZ. The PES was fitted using the fundamental invariant-neural network method with a root mean squared error of 4.37 meV. The mode specific dynamics was then studied by the quasi-classical trajectory method on the PES. Furthermore, the normal mode analysis approach was employed to calculate the final vibrational state distribution of the product H2O, in which a new scheme to acquire the Cartesian coordinates and momenta of each atom in the product molecule from the trajectories was proposed. It was found that, on one hand, excitation of either the symmetric stretching mode or the asymmetric stretching mode of the reactant H2O promotes the reaction more than the translational energy, which can be rationalized by the sudden vector projection model. On the other hand, the relatively higher efficacy of exciting the symmetric stretching mode than that of the asymmetric stretching mode is caused by the prevalence of the indirect mechanism at low collision energies and the stripping mechanism at high collision energies. In addition, the initial collision energy turns ineffectively into the vibrational energy of the products H2O and OH while a fraction of the energy transforms into the rotational energy of the product H2O. Fundamental excitation of the stretching modes of H2O results in the product H2O having the highest population in the fundamental state of the asymmetric stretching mode, followed by the ground state and the fundamental state of the symmetric stretching mode.

Dynamics and kinetics of the reaction OH + H2S → H2O + SH on an accurate potential energy surface

Mode specificity and product energy disposal are unveiled in the reaction OH + H₂S → H₂O + SH.

A neural network potential energy surface for the F + CH4reaction including multiple channels based on coupled cluster theory

We report here a new global and full dimensional potential energy surface (PES) for the F + CH₄reaction.

Quasiclassical trajectory calculations of correlated product distributions for the F + CHD3(v1 = 0, 1) reactions using an ab initio potential energy surface

We report quasiclassical trajectory (QCT) calculations of the correlated product distributions and branching ratios of the reactions F+CHD(3)(v(1)=0,1)-->HF(v)+CD(3)(v) and DF(v)+CHD(2)(v) using a recently published ab initio-based full-dimensional global potential energy surface [G. Czako et al., J. Chem. Phys. 130, 084301 (2009)]. Harmonic normal mode analysis is done for the methyl products to determine the classical actions of each normal mode and then standard histogram binning and Gaussian binning (GB) methods are employed to obtain quantum state-resolved probabilities of the products. QCT calculations have been performed for both the vibrationally ground state and the CH stretching excited F+CHD(3)(v(1)=0,1) reactions at eight different collision energies in the 0.5-7.0 kcal/mol range. HF and DF vibrationally state-resolved rotational and angular distributions, CD(3) and CHD(2) mode-specific vibrational distributions, and correlated vibrationally state-specific distributions for the product pairs have been obtained and the correlated results were compared to the experiment. We find that the use of GB can be advantageous especially in the threshold regions. The CH stretching excitation in the reactant does not change the CD(3) vibrational distributions significantly, whereas the HF molecules become vibrationally and rotationally hotter. On the other hand in the case of the DF+CHD(2) channel the initially excited CH stretch appears mainly "intact" in the CHD(2) product and the DF distributions are virtually the same as formed from the ground state CHD(3) reaction. The computed results qualitatively agree with recent crossed molecular beam experiment [W. Zhang et al., Science 325, 303 (2009)] that (a) CHD(2)(v(1)=1) is the most populated product state of the F+CHD(3)(v(1)=1) reaction and this reaction produces much less CHD(2)(v=0) compared to the reaction F+CHD(3)(v=0); (b) the cross section ratio of CHD(2)(v(1)=1):CHD(2)(v=0) formed from the reactions F+CHD(3)(v(1)=1):F+CHD(3)(v=0) is less than 1 and shows little collision energy dependency; (c) the reactant CH stretch excitation increases the DF:HF ratio at low collision energies; (d) the correlated vibrational and angular distributions for DF(v)+CHD(2)(v(1)=0,1) from the ground state and stretch-excited reactions, respectively, are almost identical.