Formation of interstellar 2,4-pentadiynylidyne, HCCCCC(XΠ2), via the neutral-neutral reaction of ground state carbon atom, C(P3), with diacetylene, HCCCCH(XΣg+1)

Abstract

The interstellar reaction of ground-state carbon atom with the simplest polyyne, diacetylene (HCCCCH), is investigated theoretically to explore probable routes to form hydrogen-deficient carbon clusters at ultralow temperature in cold molecular clouds. The isomerization and dissociation channels for each of the three collision complexes are characterized by utilizing the unrestricted B3LYP/6-311G(d,p) level of theory and the CCSD(T)/cc-pVTZ calculations. With facilitation of RRKM and variational RRKM rate constants at collision energies of 0-10 kcalmol, the most probable paths, thus reaction mechanism, are determined. Subsequently, the corresponding rate equations are solved that the evolutions of concentrations of collision complexes, intermediates, and products versus time are obtained. As a result, the final products and yields are identified. This study predicts that three collision complexes, c1, c2, and c3, would produce a single final product, 2,4-pentadiynylidyne, HCCCCC(X (2)Pi), C(5)H (p1)+H, via the most stable intermediate, carbon chain HC(5)H (i4). Our investigation indicates the title reaction is efficient to form astronomically observed 2,4-pentadiynylidyne in cold molecular clouds, where a typical translational temperature is 10 K, via a single bimolecular gas phase reaction.

A crossed molecular beam study on the synthesis of the interstellar 2,4-pentadiynylidyne radical (HCCCCC)

“…2͒: The most stable C ϱv symmetric linear C 5 H isomer ͓HCCCCC͑X 2 ⌸͔͒ ͑1͒, the C 2v symmetric cyclic HC 2 C 3 isomer of medium stability ͓+21-25 kJ mol −1 with respect to ͑1͔͒, holding a 2 B 2 ground state ͑2͒, and a second cyclic structure of C s symmetry, C 2 C 3 H, with a 2 AЈ electronic ground state ranging 100-113 kJ mol −1 above ͑1͒. The structure of these radicals has been confirmed recently by Haubrich et al 14 and Apponi et al 15 Subsequent computations by Mebel et al 16 and Sun et al 17 not only verified the stability sequence of ͑1͒-͑3͒, but also provided enthalpies of formation of the linear C 5 H isomer ͓HCCCCC͑X 2 ⌸͔͒ to be 923 and 931 kJ mol −1 , respectively. Based on these energetics, a reaction of ground state carbon atoms ͓C͑ 3 P͔͒ with diacetylene ͓C 4 H 2 ͑X 1 ⌺ g + ͔͒ should synthesize the linear C 5 H isomer ͓HCCCCC͑X 2 ⌸͔͒ plus atomic hydrogen.…”

“…To extract the chemical dynamics leading to the formation of the linear 2,4-pentadiynylidyne ͓HCCCCC͑X 2 ⌸͔͒ radical ͑1͒, we are merging our experimental findings with electronic structure calculations on the C 5 H 2 ͑intermediates͒ and C 5 H ͑products͒ potential energy surfaces. 13,16,17 A comparison of the structures of the diacetylene reactant and the linear 2,4-pentadiynylidyne ͓HCCCCC͑X 2 ⌸͔͒ molecule portrays that formally, the carbon chain is extended by one carbon atom at the expense of a hydrogen atom ͑Figs. 1, 2, and 6͒.…”

“…As a matter of fact, the existence of a tight exit transition state has been verified experimentally via the best-fit product translational energy distribution, P͑E T ͒, depicting a maximum of about 10 kJ mol −1 and also computationally predicting an exit transition state located 22 kJ mol −1 above the separated products. 17 Note that this intermediate, which is stabilized by 416 kJ mol −1 with respect to the separate reactants, 17 can only be excited to B-like rotations due to its linearity; these rotations interconvert the chemically equivalent hydrogen atoms in the HCCCCCH͑X 3 ⌺ g − ͒ molecule. Consequently, there exists an equal probability of a hydrogen atom to be released into and -; this always translates into a forward-backward symmetric flux contour map as extracted experimentally ͑Fig.…”

Synthesis of interstellar 1,3,5-heptatriynylidyne, C7H(X Π2), via the neutral-neutral reaction of ground state carbon atom, C(P3), with triacetylene, HC6H (X Σ1g+)

“…The reaction of carbon atom with the polyynes, HC 2n H, could also be viewed as a mechanism for the formation of C 2n+1 H and HC 2n+1 H isomers and thus the depletion of the polyynes. Together, the first two members seemingly open a reaction class of C͑ 3 49 exclusively long-chained particularly when n = 2. Recall that the reaction of ground state carbon atoms with acetylene also forms a cyclic C 3 H isomer; likewise, in this reaction, the tricarbon plus molecular hydrogen channel is open as well.…”

“…The prototype C͑ 3 P͒ +C 2 H 2 ͑acetylene͒ reaction 45,46 has been intensively studied experimentally and theoretically, since the neutral-neutral reactions 47,48 between atomic carbon and unsaturated hydrocarbons are considered an important synthetic route to complex molecules in interstellar medium. Recently, we examined 49,50 the reaction next in line, C͑ 3 P͒ +HC 4 H ͑di-acetylene͒. The reaction of carbon atom with the polyynes, HC 2n H, could also be viewed as a mechanism for the formation of C 2n+1 H and HC 2n+1 H isomers and thus the depletion of the polyynes.…”