The quasi-unchanged gas-phase molecular structures of the atmospheric aerosol precursor β-pinene and its oxidation product nopinone

The rotational spectra of the two bicyclic molecules β-pinene and its oxidation product nopinone were investigated in the gas phase, using Fourier transform microwave spectroscopy coupled to a supersonic jet, in the 2-20 GHz range. The parent species and all heavy atom isotopologues have been observed in their natural abundance. The spectroscopic parameters of the ground states were determined from a Watson's Hamiltonian in the A reduction. The rotational constants were used together with geometrical parameters obtained from ab initio calculations to determine the r 0 and r structures of the skeletons, without any structural assumption in the fit concerning the heavy atoms. Comparison with solid phase and other bicyclic monoterpenes unveiled an unprecedented complete set of geometrical parameters for the rigid cages. The structures of β-pinene and nopinone are very close, except for the substituents at C 2 . In the gas phase C 2 is a centre of planarity in both molecules.