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

Fourier transform infrared spectroscopy of laser-irradiated cryogenic crystals shows thatvibrational excitation of CO leads to the production of equal amounts of CO2and C3O2. The reactionmechanism is explored using electronic structure calculations, demonstrating that the lowest-energypathway involves a spin-forbidden reaction of (CO)2yielding C(3P) + CO2.C(3P) then undergoesbarrierless recombination with two other CO molecules forming C3O2. Calculated intersystem crossingrates support the spin-forbidden mechanism, showing subpicosecond spin-flipping time scales for a(CO)2geometry that is energetically consistent with states accessed through vibrational energy pooling.This spin-flip occurs with an estimated similar to 4% efficiency; on the singlet surface, (CO)2reconverts back toCO monomers, releasing heat which induces CO desorption. The discovery that vibrational excitationof condensed-phase CO leads to spin-forbidden C-C bond formation may be important to thedevelopment of accurate models of interstellar chemistry.

Automated summary

This study investigates the vibrational excitation of CO and its reaction mechanism, leading to the production of CO2 and C3O2. The results reveal that the lowest-energy pathway involves a spin-forbidden reaction of (CO)2 yielding C(3P) and CO2, followed by barrierless recombination of C(3P) with two other CO molecules forming C3O2. The calculated spin-flipping time scales and efficiency support the spin-forbidden mechanism. This discovery has implications for accurate modeling of interstellar chemistry.