Infrared and reflectron time-of-flight mass spectroscopic analysis of methane (CH₄)–carbon monoxide (CO) ices exposed to ionization radiation – toward the formation of carbonyl-bearing molecules in extraterrestrial ices

Abstract

Ice mixtures of methane and carbon monoxide were exposed to ionizing radiation in the form of energetic electrons at 5.5 K to investigate the formation of carbonyl bearing molecules in extraterrestrial ices. The radiation induced chemical processing of the mixed ices along with their isotopically labeled counterparts was probed online and in situ via infrared spectroscopy (solid state) aided with reflectron time-of-flight mass spectrometry (ReTOFMS) coupled to single photon photoionization (PI) at 10.49 eV (gas phase). Deconvolution of the carbonyl absorption feature centered at 1727 cm(-1) in the processed ices and subsequent kinetic fitting to the temporal growth of the newly formed species suggests the formation of acetaldehyde (CH3CHO) together with four key classes of carbonyl-bearing molecules: (i) alkyl aldehydes, (ii) alkyl ketones, (iii) α,β-unsaturated ketones/aldehydes and (iv) α,β,γ,δ-unsaturated ketones/α,β-dicarbonyl compounds in keto-enol form. The mechanistical studies indicate that acetaldehyde acts as the key building block of higher aldehydes (i) and ketones (ii) with unsaturated ketones/aldehydes (iii) and/or α,β-dicarbonyl compounds (iv) formed from the latter. Upon sublimation of the newly synthesized molecules, ReTOFMS together with isotopic shifts of the mass-to-charge ratios was exploited to identify eleven product classes containing molecules with up to six carbon atoms, which can be formally derived from C1-C5 hydrocarbons incorporating up to three carbon monoxide building blocks. The classes are (i) saturated aldehydes/ketones, (ii) unsaturated aldehydes/ketones, (iii) doubly unsaturated aldehydes/ketones, (iv) saturated dicarbonyls (aldehydes/ketones), (v) unsaturated dicarbonyls (aldehydes/ketones), (vi) saturated tricarbonyls (aldehydes/ketones), molecules containing (vii) one carbonyl - one alcohol (viii), two carbonyls - one alcohol, (ix) one carbonyl - two alcohol groups along with (x) alcohols and (xi) diols. Reaction pathways to synthesize these classes were derived as well. The present experiments provide clear evidence for the formation of key organic molecules--acetaldehyde, acetone, and potentially vinylalcohol--which are among the 15 carbonyl containing organic molecules detected in the interstellar medium. Despite numerous previous experimental investigations probing the effect of ionizing radiation on simple astrophysical ice representatives, our results suggest that more complex organic molecules can be formed in extraterrestrial ices than previously suggested. An outlook on further identification of individual isomers is also presented.

COMPLEX HYDROCARBON CHEMISTRY IN INTERSTELLAR AND SOLAR SYSTEM ICES REVEALED: A COMBINED INFRARED SPECTROSCOPY AND REFLECTRON TIME-OF-FLIGHT MASS SPECTROMETRY ANALYSIS OF ETHANE (C2H6) AND D6-ETHANE (C2D6) ICES EXPOSED TO IONIZING RADIATION

“…Abplanalp et al (2016) recently unveiled a much more complex chemistry taking place within the ices than previously reported (Jones & Kaiser 2013;Kaiser et al 2014Kaiser et al , 2015Maity et al 2015). The previous ethane irradiation analyses (FTIR, QMS, GC-MS) were only able to determine that hydrocarbons as large as butane isomers (C 4 H 10 ) were produced from ethane irradiation.…”

“…The present study employs an alternative method of tunable photoionization coupled with reflectron time-of-flight mass spectrometry (PIReTOF-MS) to unravel the complex chemistry taking place in the irradiated ethane ices. The incorporation of fragment-free single photon soft photoionization utilizing tunable vacuum ultraviolet light (VUV) has previously demonstrated the feasibility of this approach (Jones & Kaiser 2013;Kaiser et al 2014Kaiser et al , 2015Maity et al 2014aMaity et al , 2014bMaity et al , 2015Förstel et al 2015;Maksyutenko et al 2015;Turner et al 2015Turner et al , 2016Abplanalp et al 2016). The products released into the gas phase are photoionized and sampled via a reflectron time-offlight mass spectrometer, which monitors the complete product spectrum simultaneously based on the mass-to-charge ratios of the neutral molecules after their ionization.…”

“…Furthermore, the concurrent monitoring of both the ice (FTIR) and gas phases (PI-ReTOF-MS) allows for a cross correlation of the data to extract more accurately which infrared peaks belong to which molecule based on their appearance in the PI-ReTOF-MS data and decline in the FTIR spectra. In summary, PI-ReTOF-MS coupled with FTIR has been established as a powerful tool fordetecting the complete inventory of molecules formed in laboratory astrophysics experiments (Jones & Kaiser 2013;Kaiser et al 2014Kaiser et al , 2015Maity et al 2014aMaity et al , 2014bMaity et al , 2015Förstel et al 2015;Maksyutenko et al 2015;Turner et al 2015Turner et al , 2016Abplanalp et al 2016). References.…”

Infrared and reflectron time-of-flight mass spectroscopic study on the synthesis of glycolaldehyde in methanol (CH3OH) and methanol–carbon monoxide (CH3OH–CO) ices exposed to ionization radiation

“…The n 14 band of glycolaldehyde (HOCH 2 CHO) at 1743 cm À1 is expected to show an isotopic shi of 26 cm À1 in CD 3 OD ices, 64 37 cm À1 in 13 CH 3 OH ices, 64 and 30 cm À1 in CH 3 18 OH ices. 65 In the present investigation, the corresponding isotopomers of glycolaldehyde, DOCD 2 CDO in CD 3 OD 3 18 OH ices, are assigned based on peak positions of 1711 cm À1 , 1703 cm À1 and 1713 cm À1 , respectively, with corresponding isotopic shis of 32 cm À1 , 40 cm À1 and 30 cm À1 (Table 1), in agreement with the expected frequency shis. 64,65 The formaldehyde identication was based on the n 4 band, observed at 1676, 1687 and 1693 cm À1 in the irradiated CD 3 OD, 13 OH ices, with isotopic shis of 38 cm À1 and 32 cm À1 respectively, in close agreement with typical shis of carbonyl stretching frequencies upon 13 C and 18 O substitution of about 40 cm À1 and 30 cm À1 , respectively.…”

A study of interstellar aldehydes and enols as tracers of a cosmic ray-driven nonequilibrium synthesis of complex organic molecules

“…Table S4). However, although possible fundamentals of acetaldehyde-vinyl alcohol and propanal-(E)/(Z)-1-propenol were observed, it should be stressed that these features and the carbonyl stretches in particular might also have contributions from higher molecular weight aldehydes with similar group frequencies as the molecules of interest (33).…”