Mechanisms In The Proton Transfer Reaction Of HOCO~+ With CO

Ion-molecule reactions are of essential in the field of astrochemistry.One of the most concerned interstellar isomers are HCO⁺/HOC⁺.The isomers,which generate several complex molecules found in space,such as alcohols,aldehydes,carboxylic acids,are thought to initiate different interstellar chemical reaction pathways,providing insight into their interactions and branching ratios,facilitating further proposition,construction,and optimization of precise astrochemistry network.In this paper,MP2 as well as six DFT functionals,M06,M06-2X,B3 LYP,Bhand H,PBE0 and B97-1,combined with aug-cc-p VDZ basis set,were employed to perform the geometry optimization for HOCO⁺ + CO reaction.Higher level coupled cluster theory CCSD?T?/aug-cc-p VTZ was carried out as a benchmark to calculate the standard deviation.The B97-1/aug-cc-p VDZ method with the smallest deviation is preferred as the best method for the following direct dynamics simulations.Two reaction channels are found,one generating HCO⁺ + CO₂ and the other generating HOC⁺ + CO₂.The more stable HCO⁺reaction is exothermic,and the metastable HOC⁺reaction is endothermic,with higher the relative energy of the product.When the collision energy is greater than 1.16 e V,the CO₂ catalytic isomer channel is allowed from a thermodynamic point of view.That is,the product HCO⁺can be isomerized to form another product HOC⁺ under the catalysis of CO₂.Direct dynamics simulation was used to study the HOCO⁺ + CO scattering angle distribution,energy distribution,HCO⁺/HOC⁺product branching,and explore the factors influencing the reaction probability at a collision energy of 2.35 e V.Three reaction mechanisms were obtained through visual analysis,including direct rebound,direct stripping and indirect mechanisms.Among them,the direct stripping mechanism predominates,leading to forward scattering.Due to the difficulty in distinguishing the two isomers at high collision energies,the HCO⁺/HOC⁺isomer branch ratio,which is highly desired,cannot obtain the accurate values in experiment.The exact HCO⁺product branching ratio is 98.15%.The HOC⁺ product branching ratio is 1.85%,which is consistent with the experimentally predicted upper limit of HOC⁺reaction of 2%..The dynamics simulations do not find the CO₂ catalytic isomer channel,which may be related to a CO half rotation required for isomerization.It is expected to occur at a timescale of tens of picoseconds without considering the sticking effect of the hydrogen bond on the CO rotation.At a given collision energy,the reaction speed is fast,making CO half rotation difficult to occur.It shows that the CO₂ catalytic isomer channel has little effect on the branch ratio of HCO⁺/HOC⁺.The simulation explores the factors thataffect the possibility of the reaction from two aspects,impact parameter?b value?and the reaction azimuth.Statistics show that the reaction azimuth has a certain effect on the reaction probability,and it is also a reason for the low overall probability of the reaction.