Self-Reactions in the HBr⁺ (DBr⁺) + HBr System: A State-Selective Investigation of the Role of Rotation

Abstract

Self-reactions observed in the HBr + (DBr + ) + HBr system have been investigated using a guided ion-beam experiment under single-collision conditions. The reaction channels observed are proton transfer/hydrogen abstraction (PT/HA) in the case of HBr + and deuteron transfer/hydrogen abstraction (DT/HA) and charge transfer (CT) in the case of DBr + . HBr + /DBr + ions have been formed with rotational energies selected using the resonance-enhanced multiphoton ionization (REMPI) formation process. Cross sections have been measured as a function of the rotational energy of the ion, E rot , and of the center-of-mass collision energy, E cm . In the region of low rotational energies, the cross section for both PT/HA and DT/HA decreases with increasing ion rotation. In this region, the cross section for CT increases with increasing ion rotation. For higher rotational energies, the cross section increases with increasing ion rotation for PT/HA and less pronounced for DT/HA. The cross section for CT becomes independent of ion rotation for high rotational energies. Since all reaction channels are exothermic, all cross sections decrease with increasing E cm .

The Potential Energy Profile of the HBr+ + HCl Bimolecular Collision

“…The potential energy profile of the bimolecular collision HBr + + HCl is compared with reactions of similar sort, namely, the (self) HCl + + HCl and HBr + + HBr reactions. As stated in section , these two reactions have been studied with guided beam experiments and AIMD simulations, and they are reported to respond differently to rotational excitations. , The most obvious differences between the two self-reactions and the HBr + + HCl collisions are the simplification of the potential energy profiles. For example, the p2 and p3 in the self-reactions would become one single product, H 2 Cl + (H 2 Br + ) + Cl (Br).…”

“…11−14 For example, the collision of HBr + + HBr could form Br + H 2 Br + through proton transfer (PT) and hydrogen abstraction (HA) or HBr + HBr + through charge transfer (CT) and hydrogen exchange (HE). 14 It has been reported that as the rotational energy increases, the combined PT/HA reactivity monotonically decreases, while the combined CT/HE reactivity increases. 14 However, the impact of the rotational energy is quite different in a similar reaction, HCl + + HCl, whose PT/ HA reactivity demonstrates a local minimum.…”

“…have conducted a series of ion–molecule beam experiments to investigate the impact of various excitations on the dynamics of chemical reactions. − For example, the collision of HBr + + HBr could form Br + H 2 Br + through proton transfer (PT) and hydrogen abstraction (HA) or HBr + HBr + through charge transfer (CT) and hydrogen exchange (HE) . It has been reported that as the rotational energy increases, the combined PT/HA reactivity monotonically decreases, while the combined CT/HE reactivity increases . However, the impact of the rotational energy is quite different in a similar reaction, HCl + + HCl, whose PT/HA reactivity demonstrates a local minimum .…”

“…Although objective (1) is nondiscriminative with respect to potential energy, for objective (2), since the impact of the rotational energy is most profound when the total energy of the system is small, the focus is on the intermediates, TSs, and products that are energetically accessible (defined later). Under such conditions, a few products are expected such as HBr + HCl + , Cl + H 2 Br + , and Br + H 2 Cl + , which have seen a fair amount of study, ,, as well as another set of products: H 2 + BrCl + . The latter only has literature focused on its photodissociation and spectroscopy. − The only possible products excluded in this study due to high potential energy are the unstable protonated dihalogens, namely H + HClBr + and H + HBrCl + , for which only analogous species (HCl 2 + , HBr 2 + , HFCl + ) have had very limited experimental study. , …”

“…14 It has been reported that as the rotational energy increases, the combined PT/HA reactivity monotonically decreases, while the combined CT/HE reactivity increases. 14 However, the impact of the rotational energy is quite different in a similar reaction, HCl + + HCl, whose PT/ HA reactivity demonstrates a local minimum. 13 An ab initio molecular dynamics (AIMD) simulation of this reaction unraveled the mechanism: The fine balance between translational motion, rotational motion, and intermolecular interaction determines the distribution of the orientation between the reactant molecules at the moment of the collision, which governs the reactivity of the reaction.…”