Infrared laser spectroscopy of the helium-solvated allyl and allyl peroxy radicals

Abstract

Infrared spectra in the C-H stretch region are reported for the allyl (CH2CHCH2) and allyl peroxy (CH2=CH-CH2OO·) radicals solvated in superfluid helium nanodroplets. Nine bands in the spectrum of the allyl radical have resolved rotational substructure. We have assigned three of these to the ν1 (a1), ν3 (a1), and ν13 (b2) C-H stretch bands and four others to the ν14/(ν15+2ν11) (b2) and ν2/(ν4+2ν11) (a1) Fermi dyads, and an unassigned resonant polyad is observed in the vicinity of the ν1 band. Experimental coupling constants associated with Fermi dyads are consistent with quartic force constants obtained from density functional theory computations. The peroxy radical was formed within the He droplet via the reaction between allyl and O2 following the sequential pick-up of the reactants. Five stable conformers are predicted for the allyl peroxy radical, and a computed two-dimensional potential surface for rotation about the CC-OO and CC-CO bonds reveals multiple isomerization barriers greater than ≈300 cm(-1). Nevertheless, the C-H stretch infrared spectrum is consistent with the presence of a single conformer following the allyl + O2 reaction within helium droplets.