Adiabatic And Nonadiabatic Dynamics Strudies For The Na+XF(X=H,D) Reactions And The Construction Of A New Potential Energy Surface

The reactive scatterings of alkali or alkaline earth metals from hydrogen halides have long been the topics of extensive experimental and theoretical investigations.These studies played key roles in testing new experimental techniques or developing new concepts of reaction dynamics.A prominent example of them is the harpoon mechanism that arises from the electronic rearrangement during the course of reactions.Among these reactions,the title system is one of the few that are feasible for accurate theoretical characterizations thanks to the involvement of relatively light atoms and small number of electrons.The Na(3P)+HF reaction can only proceed via harpooning transition.The studies of this kind lead to better understanding of the internal mechanism.Accurate quantum dynamics calculations have been carried out for the Na(3S,3P)+XF reactions using the Chebyshev wave packet propagation method here.In addition,a new ab initio potential energy surface(PES)for the ground adiabatic state of Na(3S)+HF reactive system has been constructed at the MRCI/aug-cc-PV5Z level of theory.The main research contents are as follows:(1)Quantum dynamics studies of the Na(3S,3P)+HF?NaF+H reaction:Comprehensive Coriolis-coupled quantum wave packet calculations have been carried out for the Na(3S,3P)+HF?NaF+H reactions based on the nonadiabatic PES.In the Na(3S)+HF reaction,the nonadiabatic effect is found to be negligibly small,and the reactivity is significantly promoted by the initial vibrational excitations being in line with the available experimental result.Excellent quantitative agreement between theory and experiment is also achieved for the initial state specified integral cross sections(ICSs)and rate constants for v?3 vibrational states.However,the calculated rate constant for v=2 significantly underestimates the experimental result.The possible cause for the disagreement is discussed in detail in this paper.The effects of the initial rotational excitations for Na(3S)+HF are different between the process with threshold(v=2)and those without thresholds(v=3,4).The eigen-energy of the v=2 vibrational state lies below the barrier.The rotational excitations lead to the increasing of the internal energy of the v=2 state.Accordingly,the effective barrier height will decrease.So,the reactivity is significantly promoted by the initial rotational excitations.The theoretical results for the initial rotational state(associated v=3 and v=4)dependent rate constants(kv,j vs.j)at T=940 K are calculated.For kv=4,j.both the theory and the experiment show initially decreasing then increasing rotational dependence.For kv=3,j,the theoretical result decreases with j until j=9.For the Na(3P)+HF scattering,which can lead to the formation of either the ground state NaF+H product(reactive process)or Na(3S)+HF reactant(quenching process)via the harpooning process,the ICSs for the two competing processes are threshold-less and decrease sharply from large values and level off quickly.The reactive process is found to be favored at all the energies investigated.and the absolute values of the ICSs for the reactive process are in excellent agreement with the experimental result(2)Characteristics of the isotopic reaction Na(3S,3P)+DF:The isotopic substitution decreases the reactivity remarkably in both Na(3S)+HF and Na(3P)+HF.The results for v=3 vibrational state are influenced mostly because the isotopic substitution makes the v=3 state energetically lie below the reaction barrier from above it.As a result,the vibrational state dependent rates show sudden drop at v=3 from v=4.The isotopic effect of Na(3P)+DF may mean that the dissociation of the exciplex is statistical to a considerable degree.(3)A new ab initio potential energy surface for the ground state of Na(3S)+HF reactive system and quantum dynamics calculations for Na(3S)+HF(v=2-5)?NaF+H reaction:A new ab initio potential energy surface for the ground state of Na(3S)+HF system has been constructed by three-dimensional cubic spline interpolation of 37000 ab initio points computed at the MRCI+Q/aug-cc-pV5Z level of theory.The PES shows that the endoergicity of the reaction is 0.924 eV(with the zero point energy and the harmonic frequency is taken from the experiment),which is very close to the experimental data.The well depth is 0.195 eV and the barrier height is 0.943 eV(0.693 eV with the zero point energy).The dynamics calculations show that the initial state specified relative rate constants for v=4 and v=5 state(kv=4/kv=3 and kv=5/kv=3)show excellent quantitative agreement with the experimental data.For v=2,however,the agreement is still not satisfactory.The theory-experiment disparity needs further research.