Unveiling the Spectroscopy of Complex Organic Radicals by Exploiting Faraday Rotation at (Sub-)millimeter Wavelengths. Illustration with the Acetonyl Radical.

Radical species constitute the main reactants of numerous chemical reaction networks occurring in diverse environments. Rotationally resolved laboratory data, essential to undertake the detection of these highly reactive species, remain difficult to obtain using conventional high-resolution spectroscopy techniques. In the present work, we exploit a new experimental setup based on the Faraday rotation detection technique which allows us to study the gas phase spectra of relatively large radicals, such as dehydrogenated complex organic molecules (COMs). We recorded 2086 pure rotational transitions of the acetonyl radical (CH 3 COCH 2 ) in the 150-450 GHz range, for which no rotational information was previously available. The radical exhibits relatively complex couplings of angular momenta, involving the overall rotation angular momentum, the spin of the unpaired electron, and two large amplitude motions. The data set has been fit using a semirigid Hamiltonian and shows the need for the development of specific theoretical models.