First Experimental Confirmation of the CH₃O + H₂CO → CH₃OH + HCO Reaction: Expanding the CH₃OH Formation Mechanism in Interstellar Ices

Abstract

The successive addition of H atoms to CO in the solid phase has been hitherto regarded as the primary route to form methanol in dark molecular clouds. However, recent Monte Carlo simulations of interstellar ices alternatively suggested the radical-molecule H-atom abstraction reaction CH3O + H2CO → CH3OH + HCO, in addition to CH3O + H → CH3OH, as a very promising and possibly dominating (70%–90%) final step to form CH3OH in those environments. Here, we compare the contributions of these two steps leading to methanol by experimentally investigating hydrogenation reactions on H2CO and D2CO ices, which ensures comparable starting points between the two scenarios. The experiments are performed under ultrahigh vacuum conditions and astronomically relevant temperatures, with H:H2CO (or D2CO) flux ratios of 10:1 and 30:1. The radical-molecule route in the partially deuterated scenario, CHD2O + D2CO → CHD2OD + DCO, is significantly hampered by the isotope effect in the D-abstraction process, and can thus be used as an artifice to probe the efficiency of this step. We observe a significantly smaller yield of D2CO + H products in comparison to H2CO + H, implying that the CH3O-induced abstraction route must play an important role in the formation of methanol in interstellar ices. Reflection-absorption infrared spectroscopy and temperature-programmed desorption-quadrupole mass spectrometry analyses are used to quantify the species in the ice. Both analytical techniques indicate constant contributions of ∼80% for the abstraction route in the 10–16 K interval, which agrees well with the Monte Carlo calculations. Additional H2CO + D experiments confirm these conclusions.

The formation of CO2 through consumption of gas-phase CO on vacuum-UV irradiated water ice

“…It is thus unlikely that this CO is mixed with H 2 O. Mixtures of CO with CH 3 OH, however, do reproduce the red component in both peak position and width (Cuppen et al 2011). A mixture of CO and CH 3 OH is also more likely, as CH 3 OH is formed through hydrogenation of CO (Hiraoka et al 1994(Hiraoka et al , 2002Watanabe & Kouchi 2002a;Hidaka et al 2004;Watanabe et al 2004;Fuchs et al 2009;Santos et al 2022).…”

Searching for Propionamide (C2H5CONH2) toward Sagittarius B2 at Centimeter Wavelengths

“…As shown in Figure 5, by subsequent methyl and amine group additions, several common interstellar molecules are identified-specifically methanol (CH 3 OH) and dimethyl ether ((CH 3 ) 2 O). Yet, methyl group addition reactions are often not the preferred formation pathways for these species, such as CH 3 OH, where the route to form CH 3 OH is either via subsequent H-addition reactions to CO on the surfaces of interstellar dust grains (Chuang et al 2016) or more recently, through reactions of CH 3 O + H 2 CO (Santos et al 2022). Subsequent H-addition reactions were also proposed for the formation of both CH 2 CHCHO and CH 3 CH 2 CHO from HCCCHO (Hollis et al 2004b).…”

Methanol deuteration in high-mass protostars

“…3). Since CH 3 OH is formed through the hydrogenation of CO ice (e.g., Watanabe & Kouchi 2002;Fuchs et al 2009;Simons et al 2020;Santos et al 2022), this is a strong indication for inheritance of methanol and other COMs ices between low-mass prestellar phases and protostars. On the other hand, the average (D/H) CH 3 OH ratio for high-mass starless cores (5.9 ± 5.1 × 10 −3 ) seems to be about a factor of five higher than that for high-mass protostars (1.1 ± 0.7 × 10 −3 ).…”

“…Methanol forms on the surfaces of dust grains in dense prestellar phases through the hydrogenation of CO ice (e.g., Watanabe & Kouchi 2002;Fuchs et al 2009) and reactions between its grains-surface products (e.g., H 2 CO and CH 3 O Simons et al 2020;Santos et al 2022) and is therefore expected to exhibit a high D/H ratio. This is in strong contrast to, for example, water for which the bulk of the ice is formed in the warmer translucent cloud phase leading to a rather low overall HDO/H 2 O ratio ( 0.1%; Persson et al 2014;Furuya et al 2016;Jensen et al 2019;van't Hoff et al 2022).…”