C2H2···CO complex and its radiation-induced transformations: a building block for cold synthetic astrochemistry

Abstract

In this work we have examined the radiation-induced synthetic chemistry occurring in an astrochemically important C2H2–CO system at the molecular level using a matrix isolation approach. The 1:1 C2H2∙∙∙CO intermolecular complex of linear structure was obtained in the solid low-temperature (5 K) noble gas matrices by deposition of the C2H2/CO/Ng (Ng = Ar, Kr, Xe) gaseous mixtures and characterized by Fourier-transform infrared (FTIR) spectroscopy. It was found that the X-ray radiolysis of the C2H2∙∙∙CO complex resulted in formation of C3O (tricarbon monoxide), HCCCHO (propynal), c-H2C3O (cyclopropenone), H2CCCO (propadienone), and HC3O (oxoprorynyl radical). This means that the studied complex may be considered as the simplest building block (or minimal size of intermolecular reactor) for cold astrochemistry occurring in mixed interstellar ices. Remarkably, the discovered transformations of the complex actually represent synthetic routes leading to various C3 species, whereas the acetylenic C–H bond cleavage yielding ethynyl radical appears to be a minor process. Prolonged irradiation results in dehydrogenation, while the C3 skeleton is retained. The interpretation of the C2H2∙∙∙CO radiolysis mechanism (possible reactions pathways) is provided based on the analysis of kinetic curves and matrix effect. Astrochemical implications of the results are discussed.

Formation and Evolution of H2C3O+•Radical Cations: A Computational and Matrix Isolation Study

“…The C 2 H 2 /CO/Ar matrices deposited at 13–18 K contained a sufficient amount of matrix-isolated 1:1 C 2 H 2 ···CO complexes (besides C 2 H 2 and CO monomers as well as other aggregates), as evidenced by characteristic IR absorptions at 3281.5, 2143.1, 747.4, and 742.5 ( 12 C 2 H 2 ···CO) and 3274.4, 2143.1, 745.5, and 740.6 cm –1 ( 13 C 2 H 2 ···CO) described in our previous paper . FTIR spectroscopy identification of the complex relied on comparison to gas-phase data and quantum chemical computations at the CCSD­(T)/L3a_3 level of theory (predicted harmonic frequencies of C 2 H 2 ···CO, C 2 H 2 , and CO can be found in Table S7 of the SI).…”

“…FTIR spectroscopy identification of the complex relied on comparison to gas-phase data and quantum chemical computations at the CCSD­(T)/L3a_3 level of theory (predicted harmonic frequencies of C 2 H 2 ···CO, C 2 H 2 , and CO can be found in Table S7 of the SI). Irradiation of the C 2 H 2 /CO/Ar samples with X-rays results in decomposition of the C 2 H 2 ···CO complexes and formation of new molecular species as described in detail in our previous paper . According to these data, radiolysis of C 2 H 2 ···CO yields C 3 O (tricarbon monoxide), HC 3 O • (oxoprorynyl radical), HCCCHO (propynal), H 2 CCCO (propadienone), and c-H 2 C 3 O (cyclopropenone) .…”

“…This absorption appeared in the region of strongly blue-shifted CO vibrations and demonstrated rapid formation of the corresponding carrier at low conversions of the C 2 H 2 ···CO complex (below 10%) followed by its gradual decay at a high absorbed dose. On the basis of such kinetic behavior, we had tentatively assigned the band in focus to some intermediate species (presumably cationic) arising directly from the C 2 H 2 ···CO complex . The idea that this unknown species might be a cation comes from the observation of the increased yield of C 2 H – in the C 2 H 2 /CO/Ar matrices (as compared to C 2 H 2 /Ar ones), while the yield of Ar 2 H + had not been evidently affected by the presence of CO. To note, the Ar 2 H + cations most probably originate from deprotonation of extremely acidic H 2 O +• radical cations (a small amount of water is a typical impurity in matrix-isolation experiments) …”

“…Such behavior definitely supports the assignment of these absorptions to the CO stretching mode. It should be stressed that the intensities of other unassigned absorption bands did not increase in the presence of an electron scavenger. It is important to notice that the yields of all major neutral radiolysis products of the C 2 H 2 ···CO complex except c-H 2 C 3 O (namely, C 3 O, HCCCHO, and H 2 CCCO) dramatically decrease in the presence of SF 6 , and formation of these products demonstrates a well-defined induction period, which was not observed in the absence of scavenger (see Figure ).…”

“…In search of a suitable complex precursor for the radical cations of this family in the frame of the synthetic strategy outlined above, we have considered a C 2 H 2 /CO system, which is supposed to play a very important role in ice astrochemistry. − Indeed, the formal stoichiometry of the C 2 H 2 ···CO complex corresponds to the H 2 C 3 O formula. Furthermore, the reaction between C 2 H 2 +• and CO was suggested as a possible way for the formation of the H 2 C 3 O +• radical cation in the ISM. − In our recent study we demonstrated that the 1:1 C 2 H 2 ···CO intermolecular complex could be effectively stabilized in an Ar matrix by condensation of the C 2 H 2 /CO/Ar gaseous mixtures, and its X-ray irradiation at 5 K leads to the predominant formation of a number of synthetic produces containing a C 3 O moiety . Moreover, intermediate formation of the primary H 2 C 3 O +• radical cation was tentatively suggested on the basis of preliminary spectroscopic findings.…”