Frank Stahl

Reactions of ethynyl radicals as a source of C4 and C5 hydrocarbons in Titan's atmosphere

Abstract Crossed molecular beam experiments augmented by electronic structure computations of neutral–neutral reactions of the ethynyl radical (C2H, X2Σ+) with the unsaturated hydrocarbons acetylene (C2H2), methylacetylene (CH3CCH), and allene (H2CCCH2) are reviewed briefly. All reactions are characterized by a C2H versus H atom exchange and in the case of the C2H/C2H2 system by an additional molecular hydrogen (H2) elimination pathway. The attack of the ethynyl radical onto the π-electron density of the unsaturated hydrocarbons has no entrance barrier and initializes each reaction. Consecutive hydrogen atom migrations may precede the exit channels. Diacetylene (HCCCCH), the butadiynyl radical (HCCCC), methyldiacetylene (CH3CCCCH), ethynylallene (H2CCH(C2H)), and penta-4-diyne (HCC(CH2)C2H) were identified as products of which only diacetylene has yet been observed in Titans atmosphere. Our results, however, strongly suggest the presence of all these species on Titan, and the Cassini–Huygens mission is likely to detect these upon arrival in the Saturnian system in 2004.

A combined crossed molecular beam and ab initio investigation of C2 and C3 elementary reactions with unsaturated hydrocarbons—pathways to hydrogen deficient hydrocarbon radicals in combustion flames

Crossed molecular beam experiments on dicarbon and tricarbon reactions with unsaturated hydrocarbons acetylene, methylacetylene, and ethylene were performed to investigate the dynamics of channels leading to hydrogen-deficient hydrocarbon radicals. In the light of the results of new ab initio calculations, the experimental data suggest that these reactions are governed by an initial addition of C2/C3 to the pi molecular orbitals forming highly unsaturated cyclic structures. These intermediates are connected via various transition states and are suggested to ring open to chain isomers which decompose predominantly by displacement of atomic hydrogen, forming C4H, C5H, HCCCCCH2, HCCCCCCH3, H2CCCCH and H2CCCCCH. The C2(1 sigma g+) + C2H4 reaction has no entrance barrier and the channel leading to the H2CCCCH product is strongly exothermic. This is in strong contrast with the C3(1 sigma g+) + C2H4 reaction as this is characterized by a 26.4 kJ mol-1 threshold to form a HCCCCCH2 isomer. Analogous to the behavior with ethylene, preliminary results on the reactions of C2 and C3 with C2H2 and CH3CCH showed the H-displacement channels of these systems to share many similarities such as the absence/presence of an entrance barrier and the reaction mechanism. The explicit identification of the C2/C3 vs. hydrogen displacement demonstrates that hydrogen-deficient hydrocarbon radicals can be formed easily in environments like those of combustion processes. Our work is a first step towards a systematic database of the intermediates and the reaction products which are involved in this important class of reactions. These findings should be included in future models of PAH and soot formation in combustion flames.

Reaction of the ethynyl radical, C2H, with methylacetylene, CH3CCH, under single collision conditions: Implications for astrochemistry

The reaction between the ethynyl radical, C2Hu200a(Xu200a2Σ+), and methylacetylene (Xu200a1A1′), which yields ethynylallene, pentadiyne, and butadiyne, has been studied at the density functional (B3LYP/6-311+G**) and coupled cluster (coupled-cluster single double perturbative triple/cc-pVTZ) levels of theory. These results agree with data from crossed molecular beam experiments where ethynylallene (10) and pentadiyne (13) have been observed. The C2H(1) radical initially attacks the π system of methylacetylene (2) without an entrance barrier to form Z-1-ethynylpropen-2-yl (3) or Z-2-ethynylpropen-1-yl (4) in highly exothermic reactions. Geometric considerations as well as the computed enthalpies suggest Z-1-ethynylpropen-2-yl (3) to be the dominant initial intermediate. Assuming single collision conditions as found in cold molecular clouds in the interstellar medium and distinct planetary atmospheres, numerous rearrangements may ensue the initial reaction step before ejection of a hydrogen atom or a methyl group relea...

Magnetic properties and aromaticity of o-, m-, and p-benzyne

The relative aromaticities of the three singlet benzyne isomers, 1,2-, 1,3-, and 1,4-didehydrobenzenes have been evaluated with a series of aromaticity indicators, including magnetic susceptibility anisotropies and exaltations, nucleus-independent chemical shifts (NICS), and aromatic stabilization energies (all evaluated at the DFT level), as well as valence-bond Pauling resonance energies. Most of the criteria point to the o-benzyne<m-benzyne<p-benzyne aromaticity order, whereas the relative aromaticity of each isomer with respect to benzene depends on the aromaticity criterion. An additional aromaticity evaluation involved the transition state of the Bergman cyclization of (Z)-hexa-1,5-diyn-3-ene which yields p-benzyne. Dissected NICS calculations reveal an aromatic transition state with a larger total NICS but a smaller NICS(pi) component and thus lower aromaticity than benzene.

Chemical dynamics of d1-methyldiacetylene (CH3CCCCD; X 1A1) and d1-ethynylallene (H2CCCH(C2D); X 1A′) formation from reaction of C2D(X 2Σ+) with methylacetylene, CH3CCH(X 1A1)

The crossed beam reaction of the d1-ethynyl radical C2D(Xu200a2Σ+), with methylacetylene, CH3CCH(Xu200a1A1), was investigated at an average collision energy of 39.8 kJu200amol−1. Our experimental results were combined with electronic structure calculations. The chemical reaction dynamics are indirect, involve three distinct channels, and are initiated via a barrierless addition of C2D to the acetylenic bond through long lived cis and trans CH3CCH(C2D), 1-ethynylpropen-2-yl, intermediates. The reduced cone of acceptance of the carbon atom holding the methyl group favors a carbon–carbon σ bond formation at the carbon atom adjacent to the acetylenic hydrogen atom. A crossed beam experiment of C2D with partially deuterated methylacetylene, CD3CCH, shows explicitly that the reactive intermediates decompose to form both methyldiacetylene, CD3CCCCD (channel 1, 70%–90%), and to a minor amount ethynylallene, D2CCCH(C2D) (channel 2; 10%–30%), isomers through exit transition states located 7–15 kJu200amol−1 above the products. The ...

Atomic and molecular hydrogen elimination in the crossed beam reaction of d1-ethinyl radicals C2D(X 2Σ+) with acetylene, C2H2(X 1Σg+): Dynamics of d1-diacetylene (HCCCCD) and d1-butadiynyl (DCCCC) formation

The chemical reaction dynamics to form d1-diacetylene, DCCCCH (X 1Σ+), and the d1-butadiynyl radical, DCCCC, via the reaction of d1-ethinyl, C2D (X 2Σ+), with acetylene, C2H2 n(X 1Σg+), are explored in a crossed molecular beam experiment at an average collision energy of 26.1 kJ mol−1. The experiments show that the reaction follows indirect scattering dynamics via a C4H2D intermediate. The calculations confirm that the reaction has no entrance barrier and that it proceeds via an attack of the ethinyl radical on the π electron density of the acetylene molecule. The initially formed trans-1-d-ethinylvinyl-2 (HCCHC2D) intermediate rearranges to its cis form; the latter is found to fragment predominantly via nH atom emission to form d1-diacetylene, HCCCCD (X 1Σ+) and H (2S1/2) n(channel 1). A second involves a [1,2]-H shift in trans-HCCHC2D to yield a 1-d-ethinylvinyl-1 radical. The latter channel then shows two fragmentation pathways: a molecular hydrogen elimination to form the d1-butadiynyl radical (DCCCC) n(channel 2) and an atomic hydrogen loss to yield d1-diacetylene (HCCCCD) n(channel 3). Compared to the C4HD/H products (98–99%), the C4D/H2 channel presents only a minor pathway (1–2%). The solid identification of diacetylene under single collision conditions is the first experimental proof of a long-standing hypothesis that the title reaction can synthesize ndiacetylene in dark, molecular clouds, the outflow of dying carbon stars, hot molecular cores, as well as in the atmospheres of hydrocarbon rich planets and satellites such as the Saturnian moon Titan.