Isomer specific ion-molecule reactions

Interactions between complex neutral molecules and ions play an important role in the chemical network of the interstellar medium and during combustion of hydrocarbons. To advance our understanding of the underlying fundamental physical and chemical processes, dedicated reaction experiments under controlled conditions are required. Unfortunately, such measurements at cold temperatures are very challenging and thus rare. We use tools borrowed from the cold atom community to measure such reactions under cold and well-controlled conditions. Here we study the influence of molecular structure in bi-molecular interactions. Specifically, we focus on the bi-molecular reaction between the acetylene cation, C₂H₂⁺, and the two isomers of C₃H₄ - propyne and allene. Measurements show a strong dependence on the structural configuration of the neutral reactant on the outcome of the reaction. We were able to identify multiple reactions pathways and measure their reaction rates and branching ratios. Together with high-level quantum-chemical calculations we can understand the distinct reaction mechanisms of the two isomers: For allene long-range charge exchange is favored, while an intermediate reaction complex is formed for propyne.