Spin-Forbidden Carbon-Carbon Bond Formation in Vibrationally Excited α-CO

Fourier transform infrared spectroscopy of laser-irradiated cryogenic crystals shows that vibrational excitation of CO leads to the production of equal amounts of CO₂ and C₃O₂. The reaction mechanism is explored using electronic structure calculations, demonstrating that the lowest-energy pathway involves a spin-forbidden reaction of (CO)₂ yielding C(³P) + CO₂. C(³P) then undergoes barrierless recombination with two other CO molecules forming C₃O₂. Calculated intersystem crossing rates support the spin-forbidden mechanism, showing subpicosecond spin-flipping time scales for a (CO)₂ geometry that is energetically consistent with states accessed through vibrational energy pooling. This spin-flip occurs with an estimated ∼4% efficiency; on the singlet surface, (CO)₂ reconverts back to CO monomers, releasing heat which induces CO desorption. The discovery that vibrational excitation of condensed-phase CO leads to spin-forbidden C-C bond formation may be important to the development of accurate models of interstellar chemistry.