The microwave spectrum of the thiomethoxy radical CH3S

The pure rotational spectrum of the thiomethoxy radical CH3S was observed for the first time in the millimeter-wave region. The radical was produced in a 0.5 m long free-space cell by the reaction of CH3SH with 2450 MHz microwave discharge products of CF4. The rotational transitions were observed to be split into doublets by spin-orbit interaction, and each component of the doublets further into K structures spread over several hundred MHz by vibronic Coriolis interaction and vibronic (2,2)- and (2,−1)-type interactions which are characteristic of a symmetric-top molecule in an orbitally degenerate 2E electronic state. The magnetic hyperfine structure due to the three protons was clearly resolved and was analyzed by taking into account the pseudodipolar interaction which had not been considered previously. Ground-state molecular parameters were determined by least-squares analyzing the observed spectrum, and the C–S bond length was estimated to be 1.791 Å from the observed B rotational constant while assuming an appropriate structure for the methyl group.