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 3Σg dioxygen. The reactants are captured sequentially through the droplet pick-up technique. Helium droplets are doped with ethyl radical via pyrolysis of di-tert-amyl peroxide or n-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 C1 gauche and Cs trans rotamers in an approximate 2:1 ratio; this is despite the prediction that the gauche 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.