Influence of collision energy and reagent vibrational excitation on the dynamics of the reaction H + LiH

We present a detailed quasiclassical trajectory (QCT) study of the dynamics corresponding to the reaction H + LiH proceeding via depletion and H‐exchange paths on a new potential energy surface of the electronic ground state. The effects of collision energy and reagent initial vibrational excitation on the reaction probability and cross sections are studied over a wide range of collision energies. The QCT‐calculated reaction probability and cross sections are in good agreement with previous time‐dependent wave packet results. More importantly, we found that the vibrational excitation of LiH molecule inhibits the LiH depletion reaction, whereas it promotes the H‐exchange reaction. In addition, the differential cross sections calculated for the depletion reaction at different collision energies and excitation states indicate a strong forward scattering of the product molecule H₂. © 2013 Wiley Periodicals, Inc.     

Quantum wave-packet dynamics of H+HLi scattering: reaction cross section and thermal rate constant

The channel specific and initial state-selected reaction cross section and temperature-dependent rate constant for the title system is calculated with the aid of a time-dependent wave-packet approach and using the ab initio potential energy surface of Dunne et al. [Chem. Phys. Lett. 336, 1 (2001)]. All partial-wave contributions up to the total angular momentum J=74 are explicitly calculated within the coupled states (CS) approximation. Companion calculations are also carried out employing the standard as well as the uniform J-shifting (JS) approximation. The overall variation of reaction cross sections corresponds well to the behavior of a barrierless reaction. The hydrogen exchange channel yielding HLi+H products is seen to be more favored over the HLi depletion channel yielding Li+H(2) products at low and moderate collision energies. Sharp resonance features are observed in the cross-section results for the HLi depletion channel at low energies. Resonance features in the reaction cross sections average out with various partial-wave contributions, when compared to the same observed in the individual reaction probability curve. Except near the onset of the reaction, the vibrational and rotational excitation of the reagent HLi, in general, does not dramatically influence the reactivity of either channel. The thermal rate constants calculated up to 4000 K show nearly Arrhenius type behavior. The rate constant decreases with vibrational excitation of the reagent HLi, indicating that the cold HLi molecules are efficiently depleted in the reactive encounter with H at relatively low temperatures. The results obtained from the JS approximation are found to agree well qualitatively with the CS results.     

Product distributions, rate constants, and mechanisms of LiH+H reactions

We present a quantum-mechanical investigation of the LiH depletion reaction LiH+H-->Li+H2 and of the H exchange reaction LiH+H'-->LiH'+H. We report product distributions, rate constant, and mechanism of the former, and rate constant and mechanism of the latter reaction. We use the potential-energy surface by Dunne et al. [Chem. Phys. Lett. 336, 1 (2001)], the real-wave-packet method by Gray and Balint-Kurti [J. Chem. Phys. 108, 950 (1998)], and the J-shifting approximation. The 1H2 nuclear-spin statistics and progressions of vib-rotational states (v',j') rule both initial-state-resolved and thermal product distributions, which have saw-toothed shapes with odd j' preferred with respect to even j'. At high collision energies and temperatures, we obtain a regular 3-to-1 intensity alternation of rotational states. At low collision energies and temperatures, the degeneracy and density of many H2 levels can, however, give more irregular distributions. During the collision, the energy flows from the reactant translational mode to the product vibration and recoil ones. The rate constants of both reactions are not Arrhenius type because the reactions are barrier-less. The low-temperature, LiH depletion rate constant is larger than the H exchange one, whereas the contrary holds at high temperature. The real-time mechanisms show the nuclear rearrangements of the nonreactive channel and of the reactive ones, and point out that the LiH depletion is preferred over the H exchange at short times. This confirms the rate-constant results.     

Cl+HD反应产物的极化与态分布

利用准经典轨线理论,在BW2和G3两个势能面上,研究了Cl+HD反应的动力学.计算结果表明,产物的转动取向对势能面及反应体系的质量因子非常敏感.在BW2势能面上,计算的两个产物的转动取向强于在G3势能面上计算的结果,而无论是在BW2势能面上还是在G3势能面上,DCl产物的取向都强于HCl产物的取向.计算结果还表明,在不同的势能面上反应物的转动激发对反应的影响有着显著的不同.在BW2势能面上,反应物的初始转动激发有利于Cl+HD反应的进行;而在G3势能面上,反应物的初始转动激发消弱了反应的反应性.     

Quasi-classical trajectory calculations analyzing the dynamics of the C-H stretch mode excitation in the H+CHD3 reaction

A state-to-state dynamics study was performed at a collision energy of 1.53 eV to analyze the effect of the C-H stretch mode excitation on the dynamics of the gas-phase H+CHD3 reaction, which can evolve along two channels, H-abstraction, CD3+H2, and D-abstraction, CHD2+HD. Quasi-classical trajectory calculations were performed on an analytical potential energy surface constructed previously by our group. First, strong coupling between different vibrational modes in the entry channel was observed; i.e., the reaction is non-adiabatic. Second, we found that the C-H stretch mode excitation has little influence on the product rotational distributions for both channels, and on the vibrational distribution for the CD3+H2 channel. However, it has significant influence on the product vibrational distribution for the CHD2+HD channel, where the C-H stretch excitation is maintained in the products, i.e., the reaction shows mode selectivity, reproducing the experimental evidence. Third, the C-H stretch excitation by one quantum increases the reactivity of the vibrational ground-state, in agreement with experiment. Fourth, the state-to-state angular distributions of the CD3 and CHD2 products are reported, finding that for the reactant ground-state the products are practically sideways, whereas the C-H excitation yields a more forward scattering.     

Quasi-classical trajectory study of the HO + CO → H + CO2 reaction on a new ab initio based potential energy surface

We report extensive quasi-classical trajectory calculations of the HO + CO → H + CO(2) reaction on a newly developed potential energy surface based on a large number of UCCSD(T)-F12/AVTZ calculations. This complex-forming reaction is known for its unusual kinetics and dynamics because of its unique potential energy surface, which is dominated by the HOCO wells flanked by an entrance channel bottleneck and a transition state leading to the H + CO(2) products. It was found that the thermal rate coefficients are in reasonably good agreement with known experimental data in both low and high pressure limits. Excitation of the OH vibration is shown to enhance reactivity, due apparently to its promoting effect over the transition state between the HOCO intermediate and the H + CO(2) product. On the other hand, neither CO vibrational excitation nor rotational excitation in either CO or OH has a significant effect on reactivity, in agreement with experiment. However, significant discrepancies have been found between theory and the available molecular beam experiments. For example, the calculated translational energy distribution of the products substantially underestimates the experiment. In addition, the forward bias in the differential cross section observed in the experiment was not reproduced theoretically. While the origin of the discrepancies is still not clear, it is argued that a quantum mechanical treatment of the dynamics might be needed.     

Quasi-classical trajectory calculations analyzing the reactivity and dynamics of asymmetric stretch mode excitations of methane in the H + CH4 reaction

An exhaustive dynamics study was performed at two collision energies, 1.52 and 2.20 eV, analyzing the effects of the asymmetric (nu3) stretch mode excitation in the reactivity and dynamics of the gas-phase H + CH4 reaction. Quasi-classical trajectory (QCT) calculations, including corrections to avoid zero-point energy leakage along the trajectories, were performed on an analytical potential energy surface previously developed by our group. First, strong coupling between different vibrational modes in the entry channel was observed, indicating that energy can flow between these modes, and therefore that they do not preserve their adiabatic character along the reaction path; i.e., the reaction is nonadiabatic. Second, we found that the reactant vibrational excitation has a significant influence on the vibrational and rotational product distributions. With respect to the vibrational distribution, our results confirm the purely qualitative experimental evidence, although the theoretical results presented here are also quantitative. The rotational distributions are predictive, because no experimental data have been reported. Third, with respect to the reactivity, we found that the nu3 mode excitation by one quantum is more reactive than the ground state by a factor of about 2, independently of the collision energy, and in agreement with the experimental measurement of 3.0 +/- 1.5. Fourth, the state-to-state angular distributions of the products reproduce the experimental behavior at 1.52 eV, where the CH3 products scatter sideways and backward. At 2.20 eV this experimental information is not available, and therefore the results reported here are again predictive. The satisfactory reproduction of a great variety of experimental data by the present QCT study lends confidence to the potential energy surface constructed by our group and to those results whose accuracy cannot be checked by comparison with experiment.     

Role of the C-H stretch mode excitation in the dynamics of the Cl + CHD3 reaction: a quasi-classical trajectory calculation

To analyze the effect of the C-H stretch mode excitation on the dynamics of the Cl + CHD3 gas-phase abstraction reaction, an exhaustive state-to-state dynamics study was performed. This reaction can evolve along two channels: H-abstraction, CD3 + ClH, and D-abstraction, CHD2 + ClD. On an analytical potential energy surface constructed previously by our group, named PES-2005, quasi-classical trajectory calculations were performed at a collision energy of 0.18 eV, including corrections to avoid zero-point energy leakage along the trajectories. First, strong coupling between different vibrational modes in the entry valley was observed; i.e., the reaction is vibrationally nonadiabatic. Second, for the ground-state CHD3(nu=0) reaction, the diatomic fragments appeared in their ground states, and the H- and D-abstraction reactions showed similar reactivities. However, when the reactivity per atom is considered, the H is three times more reactive than the D atom. Third, when the C-H stretch mode is excited by one quantum, CHD3(nu1=1), the H-abstraction is strongly favored, and the C-H stretch excitation is maintained in the product CHD2(nu1=1) + ClD channel; i.e., the reaction shows mode selectivity, reproducing the experimental evidence, and also the reactivity of the vibrational ground state is increased, in agreement with experiment. Fourth, the state-to-state angular distributions of the CD3 and CHD2 products showed the products to be practically sideways for the reactant ground state, while the C-H excitation yielded a more forward scattering, reproducing the experimental data. The role of the zero-point energy correction was also analyzed, and we find that the dynamics results are very sensitive on how the ZPE issue is treated. Finally, a comparison is made with the similar H + CHD3(nu1=0,1) and Cl + CH4(nu1=0,1) reactions.     

Collaborative control of branching ratio in the O + HO2 → OH + O2 reaction via vibrational and rotational excitation

To understand the effect of different vibrational and rotational modes of reactant to enhance the reactivity of the O + HO₂ → OH + O₂ reaction, we revisited this important atmospheric reaction. We report here a quasi-classical trajectory (QCT) study of the reaction dynamics on a recently developed full-dimensional potential energy surface (PES). Our previous work has indicated that this reaction has two pathways, the H abstraction (HA) channel and the O abstraction (OA) channel, which lead to totally different product energy distribution. In this work, we identified that the vibrational excitation of the OH stretching (v₁) mode of HO₂ is the switch of the HA channel at low collision energy; meanwhile, the rotational excitation can also greatly change the branching ratio of the two pathways. With the excitation of v₁ mode, the original negligible HA channel controlled by the tight transition state becomes quite important. This work presents an approach to control the branching ratio via collaboration between vibrational and rotational excitation and will enrich the knowledge of the O + HO₂ reaction in atmospheric chemistry and physics.     

Quasiclassical trajectory calculations for Li(2(2)P(J)) + H2 → LiH(X(1)Σ+) + H: influence by vibrational excitation and translational energy

Ab initio potential energy surfaces and the corresponding analytical energy functions of the ground 1A' and excited 2A' states for the Li(2(2)P) plus H(2) reaction are constructed. Quasiclassical trajectory calculations on the fitted energy functions are performed to characterize the reactions of Li(2(2)P) with H(2)(v = 0, j = 1) and H(2)(v = 1, j = 1) as well as the reaction when the vibrational energy is replaced by collision energy. For simplicity, the transition probability is assumed to be unity when the trajectories go through the crossing seam region and change to the lower surface. The calculated rotational distributions of LiH(v = 0) for both H(2)(v = 0, j = 1) and H(2)(v = 1, j = 1) reactions are single-peaked with the maximum population at j' = 7, consistent with the previous observation. The vibrational excitation of H(2)(v = 1) may enhance the reaction cross section of LiH(v' = 0) by about 200 times, as compared to a result of 93-107 reported in the experimental measurements. In contrast, the enhancement is 3.1, if the same amount of energy is deposited in the translational states. This endothermic reaction can be considered as an analog of late barrier. According to the trajectory analysis, the vibrational excitation enlarges the H-H distance in the entrance channel to facilitate the reaction, but the excess energy may not open up additional reaction configuration.     

Destabilisation of the Li-N-H hydrogen storage system with elemental Si

A significant improvement in the dehydrogenation kinetics of the (LiNH(2) + LiH) system was obtained upon doping with elemental Si. Whilst, complete dehydrogenation of the (LiNH(2) + LiH) system requires more than 2 h, the time required for full dehydrogenation was reduced to less than 30 min by doping with elemental Si. It is observed that Si thermodynamically destabilises the system through the formation of novel intermediate phases resulting from the reaction of Si with both LiNH(2) and LiH. Such intermediate phases are also believed to enhance reaction kinetics by providing a path for accelerated dehydrogenation and the rapid release of hydrogen at the early stages of the reaction. It is believed that the dehydrogenation kinetics of the (LiNH(2) + LiH) system, which is controlled by the diffusion of H(-) from LiH and H(+) from LiNH(2), becomes independent of diffusion upon Si addition due to an enhanced concentration gradient in reactive ionic species.     

Correlations between dynamical properties and features of potential energy surfaces for the light atom transfer reactions O+HCl→OH+Cl AND Cl+HCl→ClH+Cl

A three-dimensional quasiclassical trajectory study of the dynamics of the light atom transfer reaction O(³P) + HCl(ν=0)→ OH + Cl was carried out employing two LEPS potential energy surfaces (I and II). Attention was focused mainly on three-dynamical properties; the oscillatory behavior of partial cross sections as a function of collision energy; the rotational excitation of the products; and the influence of reagent rotation on reactivity. Distinct differences were found between surfaces I and II with respect to these properties. The examination of individual trajectories indicated that there is a significant difference in the nature of these surfaces. While surface I is governed by weak repulsive forces, surface II is governed by strong attractive forces which tend to direct the reactants toward a collinear geometry. The present results confirm conclusions reached from an earlier study of the reaction Cl+HCl→ClH+Cl concerning correlations between dynamical properties and features of potential energy surfaces. For surfaces of the type that we termed HREP, since they are of repulsive nature and they lead to highly rotationally excited products, no significant oscillations of partial cross sections are obtained and reagent rotation promotes the reaction. On the other hand, for surfaces of the type that we termed COLD (collinearly directing), since they tend to direct the reactants toward a collinear geometry and form rotationally “cold” products, significant oscillations of partial cross sections are obtained and reagent rotation causes a decline in reactivity.     

The H + Li2 bimolecular exchange reaction: dynamical and kinetical properties at J = 0

For the first time in the literature, rigorous time-independent quantum scattering formalism was applied, by means of the ABC program, to the H + Li(2) → LiH + Li reaction. The state-to-state probabilities as a function of the total energy have been computed at zero total angular momentum (J = 0) allowing us to evaluate the effect of vibrational/rotational excitation on the reaction promotion/inhibition, the energetic distribution of products, and the temperature dependence of the J-shifting thermal rate coefficients.     

Cross sections and rate constants for OH + H2 reaction on three different potential energy surfaces for ro-vibrationally excited reagents

A systematic study of the reagent ro-vibrational excitations in H(2) + OH reaction is presented on three different potential energy surfaces using the multiconfiguration time-dependent Hartree method. An exact form of the kinetic energy operator including Coriolis coupling has been used. Coupled channel results on WDSE surface for vibrational excitation of H(2) produce very large cross sections in accordance with the previous approximate results. The rate constant obtained for H(2)(v = 1) at 300 K on the YZCL2 surface shows an excellent agreement with the most recent experimental result. Quantum dynamical results for ro-vibrational excitation of reagents obtained on the WSLFH surface show similar behavior to previous quasiclassical trajectory studies. The integral cross sections obtained for excited reagent rotations exhibit contrasting trends on the three surfaces. The effects are explained considering the different orientations of the transition state structure and the individual surface characteristics.     

A quantum reaction dynamics study of the translational, vibrational, and rotational motion effects on the HD + H3+ reaction

Time-dependent, quantum reaction dynamics wavepacket approach is employed to investigate the impacts of the translational, vibrational, and rotational motion on the HD+H(3)(+) → H(2)D(+) + H(2) reaction using the Xie-Braams-Bowman potential energy surface [Z. Xie, B. J. Braams, and J. M. Bowman, J. Chem. Phys. 122, 224307 (2005)]. We treat this five atom reaction with a seven-degree-of-freedom model by fixing one Jacobi and one torsion angle related to H(3) (+) at the lowest saddle point geometry of the potential energy surface. The initial state selected reaction probabilities show that the rotational excitations of H(+)-H(2) greatly enhance the reactivity with the reaction probabilities increased double at high rotational states compared to the ground state. However, the vibrational excitations of H(3) (+) hinder the reactivity. The ground state reaction probability shows no reaction threshold for this exoergic reaction, and as the translational energy increases, the reaction probability decreases. Furthermore, reactive resonances and zero point energy play very important roles on the reaction dynamics. The obtained integral cross section has the character of an exoergic reaction without a threshold: it decreases with the translational energy increasing. The calculated thermal rate constants using this seven-degree-of-freedom model are in agreement with a later experiment measurement.     

Ab initio study on the hydrogen desorption from MH-NH3 (M = Li, Na, K) hydrogen storage systems

The hydrogen storage system LiH + NH(3) ? LiNH(2) + H(2) is one of the most promising hydrogen storage systems, where the reaction yield can be increased by replacing Li in LiH with other alkali metals (Na or K) in order of Li < Na < K. In this paper, we have studied the alkali metal M (M = Li, Na, K) dependence of the reactivity of MH with NH(3) by calculating the potential barrier of the H(2) desorption process from the reaction of an M(2)H(2) cluster with an NH(3) molecule based on the ab initio structure optimization method. We have shown that the height of the potential barrier becomes lower in order of Li, Na, and K, where the difference of the potential barrier in Li and Na is relatively smaller than that in Na and K, and this tendency is consistent with the recent experimental results. We have also shown that the H-H distance of the H(2) dimer at the transition state takes larger distance and the change of the potential energy around the transition state becomes softer in order of Li, Na, and K. There are almost no M dependence in the charge of the H atom in NH(3) before the reaction, while that of the H atom in M(2)H(2) takes larger negative value in order of Li, Na, and K. We have also performed molecular dynamics simulations on the M(2)H(2)-NH(3) system and succeeded to reproduce the H(2) desorption from the reaction of Na(2)H(2) with NH(3).     

Quasi-classical trajectory study of the adiabatic reactions occurring on the two lowest-lying electronic states of the LiH2+ system

Quasi-classical trajectory calculations have been performed on the adiabatically allowed reactions taking place on the two lowest-lying electronic states of the LiH2+ system, using the ab initio potential energy surfaces of Martinazzo et al. (J. Chem. Phys., 2003, 119, 11 241). These reactions comprise: (i) the exoergic H2 and H2+ formation occurring through LiH+ + H and LiH + H+ collisions in the ground and in the first electronically excited state, respectively; (ii) the endoergic (ground state) LiH+ dissociation induced by collisions with H atoms; and (iii) the endoergic (excited state) Li + H2+ --> LiH + H+ reaction. The topic is of relevance for a better understanding of the lithium chemistry in the early universe. Thermal rate constants for the above reactions have been computed in the temperature range 10-5000 K and found in reasonably good agreement with estimates based on the capture model.     

Valence bond theoretical study for chemical reactivity

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Reaction paths between LiNH2 and LiH with effects of nitrides

The solid-state reaction between LiNH2 and LiH potentially offers an effective route for hydrogen storage if it can be tailored to meet all the requirements for practical applications. To date, there still exists large uncertainty on the mechanism of the reaction--whether it is mediated by a transient NH3 or directly between LiNH2 and LiH. In an effort to clarify this issue and improve the reactivity, the effects of selected nitrides were investigated here by temperature-programmed desorption, X-ray diffraction, in-situ infrared analysis, and hydrogen titration. The results show that the reaction of LiNH2 with LiH below 300 degrees C is a heterogeneous solid-state reaction controlled by Li+ diffusion from LiH to LiNH2 across the interface. At the LiNH2/LiH interface, an ammonium ion Li2NH2+ and a penta-coordinated nitrogen Li2NH3 could be the intermediate states leading to the production of hydrogen and the formation of lithium imide. In addition, it is identified that BN is an efficient "catalyst" that improves Li+ diffusion and hence the kinetics of the reaction between LiNH2 and LiH. Hydrogen is fully released within 7 h at 200 degrees C with BN addition, rather than several days without the modification.