Unimolecular Dissociation of the CH3OCO Radical: An Intermediate in the CH3O + CO Reaction †

This work investigates the unimolecular dissociation of the methoxycarbonyl, CH3OCO, radical. Photolysis of methyl chloroformate at 193 nm produces nascent CH3OCO radicals with a distribution of internal energies, determined by the velocities of the momentum-matched Cl atoms, that spans the theoretically predicted barriers to the CH3O + CO and CH3 + CO2 product channels. Both electronic ground- and excited-state radicals undergo competitive dissociation to both product channels. The experimental product branching to CH3 + CO2 from the ground-state radical, about 70%, is orders of magnitude larger than Rice−Ramsperger−Kassel−Marcus (RRKM)-predicted branching, suggesting that previously calculated barriers to the CH3OCO → CH3 + CO2 reaction are dramatically in error. Our electronic structure calculations reveal that the cis conformer of the transition state leading to the CH3 + CO2 product channel has a much lower barrier than the trans transition state. RRKM calculations using this cis transition state giv...