Theoretical Study of the Energy Disposal Mechanism and the State-Resolved Quantum Dynamics of the H + LiH⁺ → H₂ + Li⁺ Reaction

Abstract

Despite several studies in the literature, the detailed quantum state-to-state level mechanism of the astrophysically important exoergic barrierless H + LiH+ → H2 + Li+ reaction is yet to be understood. In this work, we have investigated the energy disposal mechanism of the reaction in terms of integral reaction cross section, product internal state distributions, differential cross section, and rate constant. Fully converged and Coriolis coupled quantum mechanical calculations based on a time-dependent wave packet method have been performed at the state-to-state level on the ab initio electronic ground state potential energy surface (PES) constructed by Martinazzo et al. (J. Chem. Phys. 2003, 119, 11241–11248). The agreement between the present quantum mechanical and previous quasi-classical results is found even at very low relative translational energies of reagents. A non-statistical inverse Boltzmann vibrational distribution for the product is found. This is attributed to the “attractive” nature of the underlying PES, which facilitates the excess energy release mostly as product vibration (60–80%). The energy disposal in products is found to be unaffected by the rovibrational excitation of the reagent diatom due to the weak coupling between the vibrational modes of the reagent and the product. The mild effect of collision energy on the product energy disposal is ascribed to the effective coupling between the translational modes of the reagent and the product. It is found that the collisions lead to the formation of the product H2 in its rovibrationally excited levels.

Combined Quantum Mechanical and Quasi-Classical State-to-State Dynamical Study on the Isotopic Effect in H/D + LiH+/LiD+ → H2/HD/D2 + Li+ Reactions

“…The DCSs were reported up to 0.35 eV of collision energy, and QCT and QM-CC DCSs showed reasonably good agreement. The most recent and rigorous state-to-state dynamical study for the same was performed by Sahoo et al using the PES of Martinazzo et al The comparison of the QM-CC and previous QCT results for the total ICSs in the collision energy range of 0.001–1.0 eV was carried out, and an excellent agreement was found. The QCT and QM results were also compared for other exoergic triatomic reactions.…”

“…The average fractions of available energy entering into product vibration (⟨f V ′⟩), translation (⟨f T ′⟩), and rotation (⟨f R ′⟩) are shown in Figure 4 as a function of collision energy. The average fraction (⟨f ′⟩) values are calculated here from the state-to-state and total ICSs by using the eq 5−9 of ref 16. Panels (a) and (b) of Figure 4 show the QCT and QM-CC energy disposal for R1 and R2, respectively.…”

“…This behavior of energy disposal is typical for exoergic reactions occurring on "attractive" PES. 16,42,43 Interestingly, the amount of energy going into product vibration decreases roughly after 0.15 eV, and with this an increased amount of energy can be seen to flow to the product translation. However, the fraction of energy going into product rotation remains flat after collision energy of ≈0.5 eV.…”

“…A shallow minimum occurs at the C 2 v geometry having energy of ∼0.286 eV below the (H 2 + Li + ) product asymptote. This reaction is important in the astrophysical evolution of lithium chemistry and has been studied widely using quantum mechanical (QM) ,− and quasi-classical trajectory (QCT) − methods. Despite being a well-known system and a considerable amount of work carried out on this subject, only a few studies reported are quantum mechanically exact and performed at the state-to-state level.…”

Time-dependent wave packet dynamics study of the resonances in the H + LiH+(v = 0, j = 0) → Li+ + H2 reaction at low collision energies

“…Therefore, the title reaction is a good candidate for exploring the reaction resonances at low collision energies. Despite the fact that numerous quantum dynamics studies based on this PES have been devoted to the title reaction, [35][36][37][38] only a handful of studies have extended the calculations to low energies. In 2010, Brovino et al 33 performed the quantum mechanical (QM) calculations for the H + LiH + reaction by the reactive centrifugal sudden approximation (R-CSA) and reactive infinite-order sudden approximation (R-IOSA) methods to better understand the astrophysical importance on lithium chemistry.…”