The microwave spectrum of the vinoxy radical

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Abstract
The pure rotational spectrum of the vinoxy radical, CH2CHO, was observed for the first time in the millimeter‐wave region. Both the C2H4+O and CH3CHO+F reactions yielded the radical, the latter being four to five times more efficient. Many a‐type R‐branch transitions were identified in the frequency region of 80 to 176 GHz with resolved fine structure and partially resolved hyperfine structure. An accidental degeneracy between the 624 J=6.5 and 716 J=6.5 levels enabled us to accurately determine the off‐diagonal components of the fine and hyperfine interaction constants. A small positive inertial defect Δ0=0.057 58 (16) amu Å2 calculated from the observed rotational constants indicates the radical to be planar in the ground electronic state. The observed hyperfine coupling constants of the protons clearly demonstrate that the formyl methyl type resonance form ⋅CH2CH=O makes dominant contribution to the ground electronic state, while the ethenyloxy form CH2=CHO⋅ is relatively unimportant.