As the most abundant carbon-containing compound in the universe, reactions of CO which lead to carbon-carbon bond formation are of critical importance for production of complex carbon-rich molecules in astronomical environments. Carbon-carbon bond formation has been routinely observed in laboratory analogs of astronomical CO ices, where bombardment with high-energy (10-100 keV) irradiation leads to production of a wide range of C_n, C_nO, and C_nO₂. However, the harsh conditions of such experiments make it difficult to develop a state-specific mechanism by which these molecules form.

Vibrational excitation of CO in the condensed phase is known to lead to vibrational energy pooling (VEP), where vibrational quanta are transferred between neighboring monomers through near-resonant processes described by
CO(v) + CO(w) → CO(v + 1) + CO(w – 1).
Here, we show that the levels of excitation achieved through VEP (v ≤ 37) are sufficient to induce chemical changes leading to production of equal amounts of CO₂ and C₃O₂, as evidenced by the clear signatures of these species in Fourier transform infrared absorption spectra. Exploration of the reaction pathway using density functional theory shows that the lowest-energy pathway to observed products requires transition to the triplet surface; trajectories remaining on the singlet surface are not productive, but lead back to reactants plus heat. Our results indicate that the required spin-flip occurs within a CO dimer that is consistent with the structural environment of the CO crystal as well as energetic thresholds for states populated through VEP. In addition to shedding light on the mechanism of C-C bond formation in CO ices, this work answers some key mysteries regarding the VEP dynamical framework, and represents an interesting case of vibrationally-driven chemistry.

Dr. Jessalyn A DeVine

UC Berkeley

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