Vibrational Excitation Hindering an Ion-Molecule Reaction: The c-C3H2+ - H2 Collision Complex

Experiments within a cryogenic 22-pole ion trap have revealed an interesting reaction dynamic phenomenon, where rovibrational excitation of an ionic molecule slows down a reaction with a neutral partner. This is demonstrated for the low-temperature hydrogen abstraction reaction c-C3H2+ + H-2, where excitation of the ion into the v(7) antisynunetric C-H stretching mode decreased the reaction rate coefficient toward the products c-C3H3+ + H. Supported by high-level quantum-chemical calculations, this observation is explained by the reaction proceeding through a C-C3H2+ - H2 collision complex in the entrance channel, in which the hydrogen molecule is loosely bound to the hydrogen atom of the c-C3H2+ ion. This discovery enables high-resolution vibrational action spectroscopy for c-C3H2+ and other molecular ions with similar reaction pathways. Moreover, a detailed kinetic model relating the extent of the observed product depletion signal to the rate coefficients of inelastic collisions reveals that rotational relaxation of the vibrationally excited ions is significantly faster than the rovibrational relaxation, allowing for a large fraction of the ions to be vibrationally excited. This result provides fundamental insight into the mechanism for an important class of chemical reactions, and is capable of probing the inelastic collisional dynamics of molecular ions.