Pathways to Oxygen‐Bearing Molecules in the Interstellar Medium and in Planetary Atmospheres: Cyclopropenone (c‐C₃H₂O) and Propynal (HCCCHO)

Abstract

We investigated the formation of two C 3 H 2 O isomers, i.e., cyclopropenone (c-C 3 H 2 O) and propynal (HCCCHO), in binary ice mixtures of carbon monoxide (CO) and acetylene (C 2 H 2 ) at 10 K in an ultrahigh vacuum machine on highenergy electron irradiation. The chemical evolution of the ice samples was followed online and in situ via a Fourier transform infrared spectrometer and a quadrupole mass spectrometer. The temporal profiles of the cyclopropenone and propynal isomers suggest (pseudo-) first-order kinetics. The cyclic structure (c-C 3 H 2 O) is formed via an addition of triplet carbon monoxide to ground-state acetylene (or vice versa); propynal (HCCCHO) can be synthesized from a carbon monoxideYacetylene complex via a [HCO. . .CCH] radical pair inside the matrix cage. These laboratory studies showed for the first time that both C 3 H 2 O isomers can be formed in low-temperature ices via nonequilibrium chemistry initiated by energetic electrons as formed in the track of Galactic cosmic ray particles penetrating interstellar icy grains in cold molecular clouds. Our results can explain the hitherto unresolved gas phase abundances of cyclopropenone in star-forming regions via sublimation of c-C 3 H 2 O as formed on icy grains in the cold molecular cloud stage. Implications for the heterogeneous oxygen chemistry of Titan and icy terrestrial planets and satellites suggest that the production of oxygen-bearing molecules such as C 3 H 2 O may dominate on aerosol particles compared to pure gas phase chemistry.

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

“…The infrared spectra can also be exploited to identify functional groups such as those related to complex organic molecules (COMs) like carbonyl groups associated with aldehydes, ketones, and carboxylic acids (Socrates 2004). However, different molecules with the same functional groups will have similar vibrational frequencies differing by a few 10 cm −1 causing overlap in the infrared spectra observed; this limits the identification of individual molecules (Bennett et al 2005b;Zhou et al 2008). Due to these restrictions, FTIR aids in identifying small molecules and new functional groups of organics formed within the astrophysical ice analogs, but FTIR cannot always identify specific molecules of interest; therefore, the exclusive assignment of a single molecule to infrared bands in an unknown mixture of molecules is not advisable as many vibrational frequencies may overlap.…”

SPIN-FORBIDDEN AND SPIN-ALLOWED CYCLOPROPENONE (c-H2C3O) FORMATION IN INTERSTELLAR MEDIUM

“…Due to the high activation energy, the reaction of acetylene and carbon monoxide is unlikely to proceed in the conditions found within interstellar environment. However, this reaction could be driven by radiation exciting the reactants from the ground to an excited electronic state (Zhou et al 2008). In this work, we focused on the neutral-neutral reactions where c-C 3 H 2 detected in the same region of space as c-H 2 C 3 O is assumed to be the parent molecule of c-H 2 C 3 O.…”

“…Due to the high barrier, the value of the rate constant of this reaction is effectively zero, even at relatively high temperatures. Despite the high barrier for this gas phase reaction, Zhou and coworkers (Zhou et al 2008) were able to synthesize cyclopropenone from electronically excited acetylene and carbon monoxide on the surface of icy grains. They demonstrated that if either carbon monoxide or acetylene is excited to the first electronic excited state the reaction becomes barrierless.…”

“…The likelihood of reactions (1) and (2) to be the main paths to the cyclopropenone formation depends on their reaction barriers, and on the probability of triplet-singlet transition in reaction (1). Zho and coworkers (Zhou et al 2008) demonstrated that cyclopropenone could be also formed through reactions between carbon monoxide and acetylene if one of the reactants is excited to the triplet state by energetic electrons produced from cosmic rays penetrating icy grains of interstellar clouds. These laboratory studies were supported by electronic structure calculations used to determine the relative energies of reaction intermediates.…”

Investigating the "Minimum Energy Principle" in Searches for New Molecular Species - The Case of H2c3o Isomers

“…Zhou et al (2008) studied the non-equilibrium grain-ice chemistry in cold (∼10 K) regions of the ISM, finding that cyclopropenone is formed on grain surfaces via reactions between triplet CO and C 2 H 2 , i.e., and that propynal is formed in the bulk by carbon monoxide and acetylene through a radical pair reaction, i.e., Zhou et al (2008) found that cosmic ray protons (∼10 MeV) are likely the source of energy that drives these reactions, inducing a C-H cleavage in collisions with acetylene that initiates reactions (1) and (2). Experimental results from ultra-high vacuum irradiation of ice samples showed that the two metastable isomers of C 3 H 2 O were produced, with only a tentative detection of propadienone.…”