Inhibiting the Reaction H₂ + <i>c-</i>C₃H₂⁺ → H + <i>c-</i>C₃H₃⁺with Vibrational Excitation

Hydrogen abstraction of molecular hydrogen by carbocations is an important process in the chemical evolution of the interstellar medium. It contributes to the growth of hydrocarbon complexity in molecular clouds and could be responsible for the unexpectedly high abundances of species such as C₂H in photo-dissociation regions. Following the trend of ion-neutral reactions, hydrogen abstraction often occurs barrierlessly if the reaction is exothermic. A notable exception is the reaction H₂ + C₃H₂⁺ → H + C₃H₃⁺, which has a small barrier and proceeds slowly at low temperatures. This reaction was investigated in a cryogenic ion trap, where trapped C₃H₂⁺ ions were exposed to H₂ and subjected to mid-infrared laser radiation. Upon laser excitation of the ν₇ C-H asymmetric stretching mode of c-C₃H₂⁺, the reactivity was greatly reduced. Quantum chemical calculations provided insight into this unusual behavior, revealing a critical pre-reaction complex. In addition, this phenomenon enabled high-resolution vibrational action spectroscopy of c-C₃H₂⁺.