Unveiling the\nSpectroscopy of Complex Organic Radicals\nby Exploiting Faraday Rotation at (Sub-)millimeter Wavelengths. Illustration\nwith the Acetonyl Radical

Radical species constitute the main reactants of numerous\nchemical\nreaction networks occurring in diverse environments. Rotationally\nresolved laboratory data, essential to undertake the detection of\nthese highly reactive species, remain difficult to obtain using conventional\nhigh-resolution spectroscopy techniques. In the present work, we exploit\na new experimental setup based on the Faraday rotation detection technique\nwhich allows us to study the gas phase spectra of relatively large\nradicals, such as dehydrogenated complex organic molecules (COMs).\nWe recorded 2086 pure rotational transitions of the acetonyl radical\n(CH<sub>3</sub>COCH<sub>2</sub>) in the 150–450 GHz range,\nfor which no rotational information was previously available. The\nradical exhibits relatively complex couplings of angular momenta,\ninvolving the overall rotation angular momentum, the spin of the unpaired\nelectron, and two large amplitude motions. The data set has been fit\nusing a semirigid Hamiltonian and shows the need for the development\nof specific theoretical models.