Quantum dynamics of the Li+HF→H+LiF reaction at ultralow temperatures

Dynamical and Mechanical Insights into the Li(2 S)+ HCl( X 1 Σ + ${X^1 {\rm{\Sigma }}^ + }$ ) Reaction: A Detailed Quantum Wavepacket Study

Quantum wave packet dynamics of the Li(² S)+HCl( X 1 Σ + ${X^1 \Sigma ^ + }$ ) reaction in its electronic ground state is studied. The initial state-selected and energy-resolved dynamical attributes such as reaction probability, integral cross section, and thermal rate constant for the Cl-abstraction and H-abstraction pathways are reported. All partial wave contributions of J up to 120 were found to be necessary for the title reaction up to the collision energy of ∼1.0 eV. The dynamical results reveal that the Cl-abstraction is more favored over the H-abstraction for the different rovibrational (v, j) excitations. Due to the existence of an early barrier in the potential energy surface, the cross sections increase with increasing collision energy. The rate constants also monotonously increase with temperature for both channels. Resonances are identified and characterized in terms of eigenfunctions and lifetimes. Nearly 120 well-resolved eigenstates are reported for the LiHCl complex, and they are categorized as van der Waals (vdW), barrier and product states according to the nodal progressions along (R, r, γ). The vdW resonances reveal a local-mode behavior of quasibound type at low energies and extended progressions at high energies. Further, the single-quantized periodic orbit type is also observed in the barrier region, which decays very fast. Finally, the lifetime analysis reveals that the vdW resonances can survive as long as ∼2.2 ps, which is much longer than the lifetime of the resonances in the barrier region.

Quantum Wave Packet Treatment of Cold Nonadiabatic Reactive Scattering at the State-To-State Level

Cold and ultracold collisions are dominated by quantum effects, such as resonances, tunneling, and nonadiabatic transitions between different electronic states. Due to the extremely long de Broglie wavelength in such processes, quantum reactive scattering is most conveniently characterized using the time-independent close-coupling (TICC) methods. However, the TICC approach is difficult for systems with a large number of channels because of its steep numerical scaling laws. Here, a recently proposed quantum wave packet (WP) approach for solving adiabatic reactive scattering problems at low collision energies is extended to include nonadiabatic transitions. To impose the outgoing boundary conditions, the total scattering wavefunction is split into three parts, the interaction, the asymptotic, and the long-range regions. Each region is associated with a different set of basis functions, which could be optimized separately. In this way, an extremely long grid can be used to accommodate the characteristic long de Broglie wavelengths in the scattering coordinate. The better numerical scaling laws of the WP approach have the potential for handling larger nonadiabatic reactive systems at low temperatures in the future.

A new potential energy surface of LiHCl system and dynamic studies for the Li(2S) + HCl(X1Σ+) → LiCl(X1Σ+) + H(2S) reaction

A new global potential energy surface (PES) is constructed for the ground state of LiHCl system based on high-quality ab initio energy points calculated using multi-reference configuration interaction calculations with the Davidson correction. The AVQZ and WCVQZ basis sets are employed for H and Li atoms, respectively. To compensate the relativistic effects of heavy element, the AWCVQZ-DK basis set is employed for Cl atom. The neural network method is used for fitting the PES, and the root mean square error is small (1.36 × 10^-2 eV). The spectroscopic constants of the diatoms obtained from the new PES agree well with experimental data. The geometric characteristics of the transition state and the complex are examined and compared with the previous theoretical values. To study the reaction dynamics of the Li(²S) + HCl(X¹Σ⁺) → LiCl(X¹Σ⁺) + H(²S) reaction, quantum reactive scattering dynamics calculations using collection reactant-coordinate-based wave packet method are conducted based on the new PES. The results of the reaction probabilities indicate that a small barrier exists along the reaction path as observed from the PES. The integral cross section curves reveal that the product molecule LiCl is easily excited. In addition, the reaction is dominated by forward scattering, and similar pattern is observed from Becker's experiment.

EFFECTS ON THE CHEMICAL STEREODYNAMICS OF THE INITIAL VIBRATIONAL EXCITATION IN THE F + LiH (v = 0-2, j = 0) → LiF + H REACTION

In this work, the product polarization characteristics are reported for the reaction F + LiH (v = 0 - 2, j = 0) → LiF + H at the collision energy of 35 kcal/mol, by using the QCT method on the Aguado–Paniagua-potential energy surface (see Aguado et al.). The distribution of P(θr) which represents the K (reagent relative velocity vector) and J′ (product rotational angular momentum vector) correlation, the dihedral angle distribution of K-K′ (product relative velocity vector)-J′ P(φr), the angular distribution P(θr, φr) and the four polarization-dependent differential cross sections (2π/σ)(dσ00/dωt), (2π/σ)(dσ20/dωt), (2π/σ)(dσ22+/dωt), (2π/σ)(dσ21-/dωt) in each initial state are presented and discussed.

Dynamics of the O(3P) + H2 reaction at low temperatures: comparison of quasiclassical trajectory with quantum scattering calculations

Quasiclassical trajectory and quantum-mechanical scattering calculations are reported for the O((3)P) + H(2)(X (1)Sigma(g) (+);upsilon = 1-3,j = 0)-->OH(X (2)Pi) + H((2)S) reaction at energies close to the reaction threshold. The dynamics of the reaction have been investigated for zero total angular momentum using the lowest (3)A" potential-energy surface developed by Rogers et al. [J. Phys. Chem. A 104, 2308 (2000)] and its recent extensions by Brandao et al. [J. Chem. Phys. 121, 8861 (2004)] which provide an improved description of the van der Waals interaction. Good agreement is observed for this system between quasiclassical and quantal results for incident kinetic energies above the tunneling regime. Quantum-mechanical calculations also confirm recent theoretical predictions of a strong collision-energy dependence of the OH(v(') = 0)OH(v' = 1) product branching ratio in the O((3)P) + H(2)(v = 1) reaction, which explains the differences observed in OH vibrational populations between experiments using different O((3)P) sources.

Theories of reactive scattering

This paper is an overview of the theory of reactive scattering, with emphasis on fully quantum mechanical theories that have been developed to describe simple chemical reactions, especially atom-diatom reactions. We also describe related quasiclassical trajectory applications, and in all of this review the emphasis is on methods and applications concerned with state-resolved reaction dynamics. The review first provides an overview of the development of the theory, including a discussion of computational methods based on coupled channel calculations, variational methods, and wave packet methods. Choices of coordinates, including the use of hyperspherical coordinates are discussed, as are basis set and discrete variational representations. The review also summarizes a number of applications that have been performed, especially the two most comprehensively studied systems, H+H2 and F+H2, along with brief discussions of a large number of other systems, including other hydrogen atom transfer reactions, insertion reactions, electronically nonadiabatic reactions, and reactions involving four or more atoms. For each reaction we describe the method used and important new physical insight extracted from the results.

Quantum mechanical investigations of the N(S4)+O2(XΣg−3)→NO(XΠ2)+O(P3) reaction

The reaction between energetic nitrogen atoms and oxygen molecules has received important attention in connection with nitric oxide chemistry in the lower thermosphere. We report time-independent quantum mechanical calculations of the N(4S)+O2-->NO+O reaction employing the X 2A' and a 4A' electronic potential energy surfaces of Sayos et al. [J. Chem. Phys. 117, 670 (2002)]. We confirm the production of highly vibrationally excited NO molecules, consistent with previous semiclassical and more recent time-dependent quantum wave packet studies. Calculations are carried out for total angular momentum quantum number J=0 and cross sections and rate coefficients are extracted using the J-shifting approximation. The results are in good agreement with available experimental and theoretical data.

Reactivity enhancement of ultracold O(P3)+H2 collisions by van der Waals interactions

The role of van der Waals forces in O((3)P)+H(2)(upsilon=1,j=0) collisions is investigated theoretically at low and ultralow temperatures. Quantum scattering calculations have been performed for zero total angular momentum using the lowest London-Eyring-Polanyi-Sato double-polynomial (3)A(") potential-energy surface reported by [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)] and its recent BMS1 and BMS2 extensions developed by [Brandao et al., J. Chem. Phys. 121, 8861 (2004)] which provide a more accurate treatment of the van der Waals interaction. Our calculations show that van der Waals forces strongly influence chemical reactivity at ultracold translational energies. The presence of a zero-energy resonance for the BMS1 surface is found to enhance reactivity in the ultracold regime and shift the Wigner threshold to lower temperatures.

Heavy atom tunneling in chemical reactions: Study of H+LiF collisions

The H+LiF(X (1)sigma(+),upsilon=0-2,j=0)-->HF(X (1)sigma(+),upsilon',j')+Li(2S) bimolecular process is investigated by means of quantum scattering calculations on the chemically accurate X 2A' LiHF potential energy surface of Aguado et al. [A. Aguado, M. Paniagua, C. Sanz, and J. Roncero, J. Chem. Phys. 119, 10088 (2003)]. Calculations have been performed for zero total angular momentum for translational energies from 10(-7) to 10(-1) eV. Initial-state selected reaction probabilities and cross sections are characterized by resonances originating from the decay of metastable states of the H...F-Li and Li...F-H van der Waals complexes. Extensive assignment of the resonances has been carried out by performing quasibound states calculations in the entrance and exit channel wells. Chemical reactivity is found to be significantly enhanced by vibrational excitation at low temperatures, although reactivity appears much less favorable than nonreactive processes due to the inefficient tunneling of the relatively heavy fluorine atom strongly bound in van der Waals complexes.