Radical-assisted polymerization in interstellar ice analogues: formyl radical and polyoxymethylene

Abstract

We present new laboratory experiments on the low-temperature formation of COMs such as polyoxymethylene (POM), glycolaldehyde (GA), ethylene glycol (EG) and possibly glyceraldehyde (GCA) and glycerol (GCO) through radical-induced reactivity from VUV photolysis of formaldehyde in Ar and Xe matrices. The radical reactivity and the endogenous formation of COMs were monitored in-situ via infrared spectroscopy in the solid state and post photolysis with temperature programmed desorption (TPD) using a quadripole mass spectrometer. Based on experimental finding and quantum calculations, we elaborate a formation pathway for formaldehyde polymerisation induced by radicals (HCO and CH 2 OH) that support the POM formation in cometary environments. In addition, fragmentation patterns obtained from the sublimation of short chain-length POM are consistent with data collected by the Ptolemy instrument on-board the Rosetta mission and strengthen the POM identification made by this instrument.

Detection of glyceraldehyde and glycerol in VUV processed interstellar ice analogues containing formaldehyde: a general formation route for sugars and polyols

“…Our experiments show that formaldehyde and water are not efficient precursors for the formation of MF as it has never been observed in such experiments (Butscher et al 2016(Butscher et al , 2019. Also, the clear formation of EG in exp.…”

“…6. This fragment can also be obtained with small three carbon-bearing POM chains initiated by HCO or CH 2 OH radicals as it can be seen in Butscher et al (2019). The fact that short-or long-chain POM molecules are formed whether the temperature is low or high is well represented in Fig.…”

“…The m/z = 61, m/z = 72 and m/z = 47 fragments are characteristic of GCO, GCA and POM and are displayed for 15, 50 and 90 K irradiated experiments. The m/z = 47 fragment is necessary to differentiate GCO and POM as these two species share the m/z = 61 fragment whereas only POM gives a fragment at m/z = 47 (Duvernay et al 2014;Butscher et al 2019). POM was reported to desorb in a wide range of temperatures (200 to 250 K) (Duvernay et al 2014;Butscher et al 2019) because of chain length distribution which confirms that high mass POM molecules are not formed in exp.…”

“…These two mechanisms may be inefficient at 15 K but they become interesting in high temperature irradiation conditions because there is more thermal energy, which makes possible radical-non radical reactions to occur. Furthermore, initiation of reactions (11) and ( 12) have relatively low calculated barriers at B3LYP/dev2-TZVP level of theory of 20 and 8 kJ mol −1 respectively (Butscher et al 2019) which make them very likely to occur in warmer environments. The temperature increase enhances the mobility of radical species and knowing that their reaction with formaldehyde is energetically feasible makes reactions (11) and ( 12) of main interest.…”

Revisiting the reactivity between HCO and CH3 on interstellar grain surfaces

“…Rubin et al (1971);Cazaux et al Kahane et al (2013); Mendoza et al (2014); López-Sepulcre et al (2015); Belloche et al (2017); Ligterink et al (2017); McGuire et al (2018)), in the circumstellar envelopes of AGB stars (Cernicharo et al (2000)), shocked regions (Arce et al (2008); Codella et al (2017); Lefloch et al (2017)) and even in external galaxies (Muller et al (2013)). Despite their presence has been known for decades, how iCOMs are synthesized is still an open question and under debate (Herbst & van Dishoeck (2009) ;Caselli & Ceccarelli (2012); Woods et al (2013); Balucani et al (2015); Butscher et al (2017); Butscher et al (2019); Fedoseev et al (2015); Enrique-Romero et al (2016); Enrique-Romero et al (2019); Gal et al (2017); Rivilla et al (2017); Vasyunin et al (2017); Rimola et al (2018); Lamberts et al (2019)). Two different paradigms have been proposed: i) on the surfaces of grains (either during the cold prestellar or warmer collapse phase (e.g.…”