The pure rotational spectrum of the methoxy radical in the2Eground electronic state was observed in the mmhyphen;wave region using a sourcehyphen;frequency modulation microwave spectrometer with a 1 m long free space absorption cell. The methoxy radical was produced in the cell by the reaction of methanol with 2450 MHz discharge products of CF4. Each rotational transition was found to split into fine and hyperfine components. A Hamiltonian was derived which includes the spinhyphen;orbit interaction, the vibronic Coriolis interaction, the (2,minus;1) and (2,2) rovibronic interactions, and the hyperfine interaction. It was successfully applied to explain the detailed feature of the observed spectrum and to derive molecular constants therefrom. The hyperfine coupling constants of the proton nuclei were determined for the first time. When the Cndash;H bond length was assumed to be 1.0937 Aring;, the observedAandBrotational constants led to the Cndash;O bond length of 1.376 Aring;, considerably shorter than the values hitherto accepted.
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