Infrared Laser Spectroscopy of the CH₃OO Radical Formed from the Reaction of CH₃ and O₂ within a Helium Nanodroplet

Helium nanodroplet isolation and infrared laser spectroscopy are used to investigate the CH₃ + O₂ reaction. Helium nanodroplets are doped with methyl radicals that are generated in an effusive pyrolysis source. Downstream from the introduction of CH₃, the droplets are doped with O₂ from a gas pick-up cell. The CH₃ + O₂ reaction therefore occurs between sequentially picked-up and presumably cold CH₃ and O₂ reactants. The reaction is known to lead barrierlessly to the methyl peroxy radical, CH₃OO. The ∼30 kcal/mol bond energy is dissipated by helium atom evaporation, and the infrared spectrum in the CH stretch region reveals a large abundance of droplets containing the cold, helium solvated CH₃OO radical. The CH₃OO infrared spectrum is assigned on the basis of comparisons to high-level ab initio calculations and to the gas phase band origins and rotational constants.