Ethyl + O₂ in Helium Nanodroplets: Infrared Spectroscopy of the Ethylperoxy Radical

Helium-solvated ethylperoxy radicals (CH₃CH₂OO^•) are formed via the in situ reaction between ²A′ ethyl radical and ³Σ_g^– dioxygen. The reactants are captured sequentially through the droplet pick-up technique. Helium droplets are doped with ethyl radical via pyrolysis of di-<i>tert</i>-amyl peroxide or <i>n</i>-propylnitrite in an effusive, low-pressure source. An infrared spectrum of ethylperoxy, in the CH stretching region, is recorded with species-selective droplet beam depletion spectroscopy. Spectral assignments are made via comparisons to second-order vibrational perturbation theory with resonances (VPT2 + K) based on coupled-cluster full quartic force fields. Cubic and quartic force constants, evaluated using a small basis set, are transformed into the normal coordinate system of the higher level quadratic force constants. This transformation procedure eliminates the mismatch between normal modes, which is a source of error whenever normal coordinate force constants from different levels of theory are combined. The spectrum shows signatures of both the <i>C</i>₁ <i>gauche</i> and <i>C</i>_<i>s</i> <i>trans</i> rotamers in an approximate 2:1 ratio; this is despite the prediction that the <i>gauche</i> rotamer lies 44 cm^–1 lower on the zero-Kelvin enthalpic potential surface for torsional interconversion. Helium droplets are 0.4 K at equilibrium; therefore, in situ ethylperoxy production is highly nonthermal.