Abstract
Context. The hydroxymethyl radical (CH2OH) is one of two structural isomers, together with the methoxy radical (CH3O), that can be produced by abstraction of a hydrogen atom from methanol (CH3OH). In the interstellar medium (ISM), both CH2OH and CH3O are suspected to be intermediate species in many chemical reactions, including those of formation and destruction of methanol. The determination of the CH3O/CH2OH ratio in the ISM would bring important information concerning the formation processes of these species in the gas and solid phases. Interestingly, only CH3O has been detected in the ISM so far, despite the recent first laboratory measurement of the CH2OH rotation-tunneling spectrum. This lack of detection is possibly due to the non-observation in the laboratory of the most intense rotation-tunneling transitions at low temperatures. Aims. To support further searches for the hydroxymethyl radical in space, we present a thorough spectroscopic study of its rotation-tunneling spectrum, with a particular focus on transitions involving the lowest quantum numbers of the species. Methods. We recorded the rotation-tunneling spectrum of CH2OH at room temperature in the millimeter-wave domain using a frequency multiplication chain spectrometer. A fluorine-induced H-abstraction method from methanol was used to produce the radical. Results. About 180 transitions were observed, including those involving the lowest N and Ka quantum numbers, which are predicted to be intense under cold astrophysical conditions. These transitions were fitted together with available millimeter-wave lines from the literature. A systematic observation of all components of the rotational transitions yields a large improvement of the spectroscopic parameters allowing confident searches of the hydroxymethyl radical in cold to warm environments of the ISM.
Highlights
The hydroxymethyl radical, CH2OH, is suspected to participate in the formation and destruction of ubiquitous interstellar methanol and holds a considerable place in many astrochemistry models of the interstellar medium (ISM; e.g., Morisset et al 2019; Watanabe & Kouchi 2002; Hama & Watanabe 2013)
Because CH2OH has never been conclusively detected in the ISM – contrary to its isomer methoxy (CH3O; Cernicharo et al 2012) and closely related formaldehyde (H2CO; Snyder et al 1969) and formyl radical (HCO; Snyder et al 1976) – all models crucially lack molecular abundance for the CH2OH radical
The strongest transitions at low temperatures that have been measured in this work are indicated and labeled determined using using the the pfoarrtmitaiolinsmfun∆Kcati∆oNnsKaQ(N(30)0
Summary
The hydroxymethyl radical, CH2OH, is suspected to participate in the formation and destruction of ubiquitous interstellar methanol and holds a considerable place in many astrochemistry models of the interstellar medium (ISM; e.g., Morisset et al 2019; Watanabe & Kouchi 2002; Hama & Watanabe 2013). Theoretical and experimental surface chemistry studies tend to indicate that CH2OH should be one of the products of methanol desorption from ice analogues (Wakelam et al 2017; Bertin et al 2016; Öberg et al 2009) This species may be formed by gas-phase reactions (Balucani et al 2015; Acharyya et al 2015; Lucas et al 2015) or gas-grain reactions induced by cosmic rays (Garrod et al 2008). Several electronic and vibrational bands have been assigned using low-resolution matrix and gas-phase spectra, thereby allowing thermodynamic and structural properties to be estimated (Dixon & Norman 1963; Livingston & Zeldes 1966; Dulcey & Hudgens 1983; Johnson & Hudgens 1996; Dyke et al 1984; Jacox & Milligan 1973) Concurrent to these studies, many theoretical investigations on the radical have been undertaken, providing vibrational frequencies, A123, page 1 of 10
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