The quantum dynamics of the reactions N+H2(HD,D2) and their vibrational excitation effect

The N(⁴S)+H₂ reaction and its isotopic variants have been investigated by means of time‐dependent quantum wave packet with split operator method on the ground state potential energy surface (Zhai and Han, J. Chem. Phys. 2011, 135, 104314). The reaction probabilities, integral cross sections, branching ratio of the integral cross sections, and effect of vibrational excitation of H₂, HD, and D₂ diatomic molecules are presented and discussed. The results reveal that the intramolecular isotopic effect is greater than the intermolecular one, and that the vibrational excitation of the diatomic molecules can promote the progress of this reaction. In addition, a limited number of rigorous Coriolis coupling calculations of the integral cross sections of the N(⁴S)+H₂ reaction have been carried out. Also shown is that since the Coriolis coupling plays a small part in this accurate quantum calculation, the cheaper centrifugal sudden calculations here reported are effective for this reactive system. © 2014 Wiley Periodicals, Inc.     

Collision theory thermal rate constants for H + H2 reactions on the SLTH potential surface

Rate constants for the reactions H + D₂ → HD + D and D + H₂ → HD + D were computed from collision theory using an excitation function derived assuming that reaction proceeds vibrationally adiabatically on the SLTH potential surface at fixed approach angles with reaction probability described by the line-of-centers model and an angle-dependent barrier. Most of the experimentally observed steric factor is accounted for by the excitation function of this model. The computed increase of activation energy with temperature can be understood as the consequence of opening up the cone of acceptance for the incoming atom with increasing relative translational energy.     

Direct MP2 molecular dynamics studies of H atom reaction with CD4 and CH4

The mechanism and dynamics of the H + CD₄ → CD₃ + HD (I) and H + CH₄ → CH₃ + H₂ (II) reactions have been investigated by electronic structure methods. The minimum‐energy path and vibrational frequencies along the intrinsic reaction coordinate are calculated at MP2/cc‐pVDZ level. Energy distributions of the products are also obtained by the direct classical trajectory calculations at the MP2/ cc‐pVDZ level. It is found that most of the available energy appears as product translational energy, and very little of the available energy is partitioned into internal excitation of the HD (H₂) product for reaction I (II), which is in agreement with the experimental evidence. The results indicate that the experimental results could be reproduced by the direct MP2 molecular dynamics calculations. The rotational state distributions of the products show the HD (H₂) products are formed with lower rotational quantum numbers than the CD₃ (CH₃) products. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Molecular beam scattering studies of the reaction D + H2 (v = 0) and D + H2 (v = 1) → HD + H

The reaction D + H₂ → HD + H has been investigated in two molecular beam scattering experiments. Angular and time-of-flight distributions have been measured for the initial vibrational ground state (v = 0) at a most probable collision energy of E_cm = 1.5 eV and for the first vibrational excited state (v = 1) at E_cm = 0.28 eV with the same apparatus. Results for the ground-state experiment are compared with quasiclassical trajectory calculations(QCT) on the LSTH-hypersurface transformed into the laboratory system and averaged over the apparatus distributions. The agreement isquite satisfactory. At this high collision energy the HD products are no longer scattered in a backward direction but in a wide angular region concentrated about θ = 90° in the center-of-mass system. The absolute reactive cross section has been determined and the agreement with the theoretical value from QCT calculations is within the experimental error. The high sensitivity of the experiment to different properties of the doubly differential cross section has also been demonstrated. A preliminary evaluation of the experiment with initial vibrational excitation (v = 1) shows that the HD-product molecules are preferably backward scattered and the change of internal energy is small supporting the concept of a reaction which is adiabatic with respect to the internal degrees of freedom.     

Quasiclassical trajectory study of the reaction NH(X~3∑~-)+H→N(~4S)+H_2

The dynamics of the NH + H→N+H2 reaction has been investigated by means of the 3D quasiclassical trajectory approach by using the LEPS potential energy surface.The calculated rate coefficient is in good agreement with the experimental value.The reaction was found to occur via a direct channel.The product H2 has a cold excitation of rotational state,but has a reverse distribution of the vibrational state with a peak at v=1.Based on the potential energy surface and the trajectory analysis,the reaction mechanism has been explained successfully.     

Time-dependent Wave Packet Quantum Scattering Study of Reaction S(3P)+H2→HS+H on a New ab initio Potential Energy Surface 3A'

A new potential energy surface is presented for the triplet state ³A' of the chemical reaction S(³P)+H₂ from a set of accurate ab initio data. The single point energies are computed using highly correlated complete active space self-consistent-field and multi-reference config-uration interaction wave functions with a basis set of aug-cc-pV5Z. We have fitted the full set of energy values using many-body expansion method with an Aguado-Paniagua function. Based on the new potential energy surface, we carry out the time-dependent wave packet scattering calculations over the collision energy range of 0.8～2.2 eV. Both the centrifugal-sudden approximation and Coriolis Coupling cross sections are obtained. In addition, the total reaction probabilities are calculated for the reactant H₂ initially in the vibrational states v=0～3 (j=0). It is found that initial vibrational excitation enhances the title reaction.     

Effect of rotation on vibrational excitation of H2 by Li impact

Coupled channel calculations of integral cross sections for rotational and vibrational excitation of H₂(X¹Σ⁺_g by collision with Li⁺ are reported for 1.2 eV in the c.m. system employing an ab initio potential energy surface and numerical vibration—rotation functions of the Koo?s—Wolniewicz potential function including adiabatic correction. Pure rotational excitation is found to strongly dominate the inelastic scattering occurring at this energy. Preparation of H₂ in various allowed non-zero rotational states is seen to enhance the 0 → 1 vibrational cross section by approximately an order of magnitude.     

The exchange reaction H2 (ν = 1) + H = H + H2

The classical trajectory method is applied to calculate the total cross section for the exchange reaction H₂(ν = 1) + H = H + H₂. The vibrational excitation is shown to influence efficiently the threshold value. Partial reaction rate-constants calculated on the basis of the cross sections obtained are in good agreement with those measured in H-maser experiments.     

Thermal and state-selected rate constant calculations for O(3p) + H2 → OH + H and isotopic analogs

We present a new parametrization (based on ab initio calculations) of the bending potentials for the two lowest potential energy surfaces of the reaction O(³P) + H₂, and we use it for rate constant calculations by variational transition-state theory with multidimensional semiclassical tunneling corrections. We present results for the temperature range 250–2400 K for both the rate constants and the intermolecular kinetic isotope effects for the reactions of O(³P) with D₂ and HD. In general, the calculated rate constants for the thermal reactions are in excellent agreement with available experiments. We also calculate the enhancement effect for exciting H₂ to the first excited vibrational state. The calculations also provide information on which aspects of the potential energy surfaces are important for determining the predicted rate constants.     

Dynamics of the reaction H2+(He,H)HeH+. Influence of various forms of reactant energy on the total and differential cross section

Results of quasiclassical trajectory calculations of reactive processes between He atoms and H₂⁺ (υ, J) molecular ions in the collision energy interval 0.5–5.0 eV (c.m.) for a large number of selected υ, J combinations are analyzed with respect to the influence of the initial translational, vibrational, and rotational energy on the total and differential reaction cross sections. Vibrational energy is more effective in promoting the reaction than translational energy. Small rotational excitation has a negligible effect, whereas high rotational excitation has a similar influence on the reaction cross sections as the vibrational excitation of the same magnitude.     

Variational calculations on the vibrational level structure of S0 HDCO

We perform converged high precision variational calculations to determine the frequencies of the vibrational levels in S₀ HDCO, extending up to 5000 cm^?1 of vibrational excitation energy. For these calculations we use our specific vibrational method (recently employed for studies on H₂CO and D₂CO), consisting of a combination of a search/selection algorithm and a Lanczos iteration procedure and based on the Martin, Lee, Taylor potential energy surface for formaldehyde. The calculated level structure is compared to the recently measured frequencies by Ellsworth et al. in order to improve their assignments and further clarify the vibrational mixing pattern and vibrational resonances in HDCO that are very different from the other more symmetric formaldehyde species H₂CO and D₂CO studied recently.     

Reactions of fluorine atoms with iodides studied by crossed beam laser-induced fluorescence

The reactions of F atoms with C₂H₅I, C₂F₅I, and n-C₃H₅I were studied by the crossed beam laser-induced fluorescence techniques within the 570–620 nm wavelength region. The vibrational and rotational excitation spectra of the reaction product IF were measured. The relative vibrational population densities of v = 3,4, and 5 vibrational levels, and some of the relative detailed vibrational rate constants, the rotational temperatures, and the mean fractions of rotational energy in individual vibrational states of the reaction product IF were obtained. The reaction mechanism was discussed.     

Classical rotational cross sections for H2—HD and HD—HD collisions

Classical trajectory calculations of integral cross sections for rotationally inelastic collisions of HD-para H₂ and HD—HD were carried out for a wide variety of transitions over a wide range of initial relative translational energies. The results of the HD—H₂ calculations were compared with the quantum effective potential calculations of Chu. It was found that the classical method is in reasonably good agreement with the quantum method for the calculation of rotational transitions of HD at the higher initial translational energies, but the classical method is in poor agreement with quantum results for HD excitation at low energies and for H₂ excitations at all energies.     

Competition between dissociation and exchange processes in a collinear A + BC collision. I. Exact quantum results

Exact quantum results for collision-induced dissociation on a reactive surface are presented. A modified LEPS potential-energy surface modeling the H + HD → H₂ + D system has been used. HD and H₂ bearing respectively 7 and 6 bound states. This system has been chosen because it displays significant reactive scattering for total energies above the dissociation threshold. Calculations have been performed using the time-dependent wavepacket method for two initial vibrational quantum numbers of the HD molecule (v = 0, 2). For each vibrational quantum number, two wavepackets with overlapping energy distributions have been run, covering a total energy range up to more than three times the dissociation energy. Comparison with previous collision-induced dissociation calculations shows that the dissociation is greatly enhanced by the presence of concomitant reactive scattering. A vibrational enhancement effect is also observed above the dissociation threshold; for higher energies the system exhibits a pronounced vibrational inhibition effect.     

A model potential for vibrational excitation of diatomic molecules

A model interaction potential for the vibrational excitation of H₂ by Li⁺ of the form C exp(−αx+βy) is studied using non-iterative integral techniques. The results show that vibrational excitation is sensitive to small quantitative change in qualitatively similar surfaces.     

Semiclassical calculation of cross sections and rate constants for vibrational deactivation of HD (v = 1) colliding with 4He

A semiclassical collision model has been used to calculate the rate constant for vibrational relaxation in HD (v = 1, j = 0) colliding with ⁴He. The He + HD potential surface was obtained from an analytical He + H₂ surface previously used for similar calculations on He + H₂ and He + D₂. The theoretically calculated rate constant is about 50% below that experimentally determined in the temperature range 80–300 K.     

Quantum dynamics study of the Langmuir–Hinshelwood H + H recombination mechanism and H2 formation on a graphene model surface

We examine in this paper the associative desorption of two hydrogen atoms on a slab model that mimics a C(0 0 0 1) surface. Initially the two separated H atoms are physisorbed onto the graphene surface, then diffuse and recombine and H₂ gets desorbed into the gas phase. We use density functional theory (DFT) on a supercell model and apply periodic boundaries to build a potential energy surface (PES). The reaction is barrier less and exhibits a small H₂ vdW well on the global potential energy surface. We employ a two-dimensional quantum dynamics method exploiting the hyperspherical coordinates and report reaction probabilities for this mechanism. The calculations are performed for collision energies ranging from 1 to 15 meV relevant to the interstellar medium (ISM). The entrance channel dominates the reaction and the vibrational excitation of the desorbed H₂ is important and peaked at v′ = 8.     

Hydrogen fluoride vibrational energy distributions for the reactions of fluorine atoms with cyclanes

The hydrogen fluoride infrared chemiluminescence produced by the reactions of fluorine atoms with cyclopropane, cyclopentane, and cyclohexane have been studied. The emission data were used to determine the vibrational energy distributions for the abstraction of hydrogen from the secondary carbon–hydrogen bonds of these small cyclic hydrocarbons. The fraction of reaction exothermicity going into vibrational excitation of hydrogen fluoride was as follows: c-C₃H₆, 45%; c-C₅H₁₀, 53%; c-C₆H₁₂, 49%. The slightly lower fraction for the cyclopropane system may indicate that its radical reorganization energy is not completely available for excitation of product HF.     

Quantum dynamics calculation of reaction probability for H + Cl2 → HCl + Cl

We present in this paper a time-dependent quantum wave packet calculation of the initial state selected reaction probability for H + Cl₂ based on the GHNS potential energy surface with total angular momentumJ = 0. The effects of the translational, vibrational and rotational excitation of Cl₂ on the reaction probability have been investigated. In a broad region of the translational energy, the rotational excitation enhances the reaction probability while the vibrational excitation depresses the reaction probability. The theoretical results agree well with the fact that it is an early down-hill reaction.     

Quantum dynamics calculation of reaction probability for H + Cl2 → HCl + Cl

We present in this paper a time-dependent quantum wave packet calculation of the initial state selected reaction probability for H + Cl₂ based on the GHNS potential energy surface with total angular momentumJ = 0. The effects of the translational, vibrational and rotational excitation of Cl₂ on the reaction probability have been investigated. In a broad region of the translational energy, the rotational excitation enhances the reaction probability while the vibrational excitation depresses the reaction probability. The theoretical results agree well with the fact that it is an early down-hill reaction.