Using isotopologues to probe the potential energy surface of reactions of C 2 H 2 + +C 3 H 4 .

Investigations into bimolecular reaction kinetics probe the details of the underlying potential energy surface (PES), which can help to validate high-level quantum chemical calculations. We utilize a combined linear Paul ion trap with a time-of-flight mass spectrometer to study isotopologue reactions between acetylene cations (C ₂ H ₂ ⁺ ) and two isomers of C ₃ H ₄ : propyne (HC ₃ H ₃ ) and allene (H ₂ C ₃ H ₂ ). In a previous study [Schmid et al., Phys. Chem. Chem. Phys. 22, 20303 (2020)], ¹ we showed that the two isomers of C ₃ H ₄ have fundamentally different reaction mechanisms. Here, we further explore the calculated PES by isotope substitution. While isotopic substitution of reactants is a standard experimental tool in the investigation of molecular reaction kinetics, the controlled environment of co-trapped, laser-cooled Ca ⁺ ions allows the different isotopic reaction pathways to be followed in greater detail. We report branching ratios for all of the primary products of the different isotopic species. The results validate the previously proposed mechanism: propyne forms a bound reaction complex with C ₂ H ₂ ⁺ , while allene and C ₂ H ₂ ⁺ perform long-range charge exchange only.