LAUSR

Context. Of the two structural isomers of CH 3 O, methoxy is the only radical whose astronomical detection has been reported through the observation of several rotational lines at 2 and 3 mm wavelengths. Although the hydroxymethyl radical, CH 2 OH, is known to be thermodynamically the most stable (by ~3300 cm -1 ), it has so far eluded rotational spectroscopy presumably because of its high chemical reactivity. Aims. Recent high-resolution (~10 MHz) sub-Doppler rovibrationally resolved infrared spectra of CH 2 OH (symmetric CH stretching a -type band) provided accurate ground vibrational state rotational constants, thus reviving the quest for its millimeter-wave spectrum in laboratory and subsequently in space. Methods. The search and assignment of the rotational spectrum of this fundamental species were guided by our quantum chemical calculations and by using rotational constants derived from high-resolution IR data. The hydroxymethyl radical was produced by hydrogen abstraction from methanol by atomic chlorine. Results. Ninety-six b -type rotational transitions between the v = 0 and v = 1 tunnelling sublevels involving 25 fine-structure components of Q branches (with K a = 1 ← 0) and 4 fine-structure components of R branches (assigned to K a = 0 ← 1) were measured below 402 GHz. Hyperfine structure alternations due to the two identical methylenic hydrogens were observed and analysed based on the symmetry and parity of the rotational levels. A global fit including infrared and millimeter-wave lines has been conducted using Pickett’s reduced axis system Hamiltonian. The recorded transitions (odd Δ K a ) did not allow us to evaluate the Coriolis tunnelling interaction term. The comparison of the experimentally determined constants for both tunnelling levels with their computed values secures the long-awaited first detection of the rotational-tunnelling spectrum of this radical. In particular, a tunnelling rate of 139.73 ± 0.10 MHz (4.6609(32) × 10 -3 cm -1 ) was obtained along with the rotational constants, electron spin-rotation interaction parameters and several hyperfine coupling terms. Conclusions. The laboratory characterization of CH 2 OH by millimeter-wave spectroscopy now offers the possibility for its astronomical detection for the first time.