A chemical dynamics study of the HCl + HCl⁺ reaction

A recent guided ion beam study of the HCl + HCl⁺ reaction has revealed two different products [Phys. Chem. Chem. Phys. 2015, 17 (25), 16454–16461]. The first is the proton transfer product, H₂Cl⁺ + Cl, where the cross section of the reactions associated with this product, as predicted, monotonically decreases as the collision energy between the product increases. The second is the product HCl⁺ + HCl, where the cross section of the reaction shows a local maximum at the collision energy of 0.5 eV. The nature of this unusual behavior of the cross section is not clear. In this manuscript, state of the art ab initio molecular dynamics (AIMD) simulation is performed to study this bimolecular collision of HCl⁺ + HCl. The potential energy of profile of this system is first characterized with high-level ab initio methods, and then a computationally efficient method is selected for AIMD simulation. The cross sections from AIMD agree well with those from the experiments for both products. The AIMD trajectories reveal the complexity of this seemingly-simple reaction – a total of five different pathways that result in the aforementioned two products. The simulation also sheds light on the mystery of the local maximum of the cross section regarding the HCl⁺ + HCl product.