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
(CH3COCH2) 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.