Investigating the Chemical Dynamics of the Reaction of Ground-State Carbon Atoms with Acetylene and Its Isotopomers

Abstract

We investigated the multichannel reaction of ground-state carbon atoms with acetylene, C2H2 (X1Sigmag+), to form the linear and cyclic C3H isomers (atomic hydrogen elimination pathway) as well as tricarbon plus molecular hydrogen. The experiments were conducted under single-collision conditions at three different collision energies between 8.0 kJ mol-1 and 31.0 kJ mol-1. Our studies were complemented by crossed molecular beam experiments of carbon with three isotopomers C2D2(X1Sigmag+), C2HD (X1Sigma+), and 13C2H2 (X1Sigmag+) to clarify a potential intersystem crossing (ISC), the effect of the symmetry of the reaction intermediates on the center-of-mass angular distributions, the collision energy-dependent branching ratios of the atomic versus molecular hydrogen elimination pathways, and deuterium-enrichment processes. The results are discussed in light of recent electronic structure and dynamics calculations.

NON-THRESHOLD, THRESHOLD, AND NONADIABATIC BEHAVIOR OF THE KEY INTERSTELLAR C + C2H2REACTION

“…The branching ratio R2 = σ (1c)/σ (1) takes values 0.82 and 0.87 at E T = 0.8 and 4.8 kJ mol −1 , respectively, which are higher than those given in the crossed-beam experiments of Leonori et al (2008) at E T 3.5 kJ mol −1 , the bulk study at 300 K of Bergeat & Loison (2001) and the calculations of Mebel et al (2007), and Park et al (2006). It is in line with the crossed-beam study of Gu et al (2007) since extrapolation of their curve based on data at E T 8.8 kJ mol −1 would result in R2 around 0.9 at low E T . Clearly, the title reaction is governed by indirect dynamics below E T 4.8 kJ mol −1 and the collision complex finds ample time to sample the whole phase space before dissociating into products.…”

“…Numerous theoretical and experimental studies have aimed at elucidating the dynamics of the C + C 2 H 2 reaction: determinations of the potential energy surfaces (Takahashi & Yamashita 1996;Ochsenfeld et al 1997;Guadagnini et al 1998;Buonomo & Clary 2001;Clary et al 2002;Takayanagi 2005;Mebel et al 2007) including calculations in the full nine dimensions (Park et al 2006), quantum (Buonomo & Clary 2001;Clary et al 2002;Takayanagi 2006), quasi-classical trajectory (Park et al 2006), and statistical calculations (Mebel et al 2007); measurements of differential cross sections and integral cross sections in crossed molecular beam experiments employing electron-impact ionization mass spectrometric time-of-flight analysis for C 3 H and C 3 detection with pulsed Gu et al 2007) or continuous beams (Clary et al 2002;Cartechini et al 2002;Costes et al 2006;Leonori et al 2008) and spectroscopic probing of the H-atom product (Clary et al 2002;Cartechini et al 2002;Costes et al 2006). Three channels have been identified: two H-elimination channels leading to cyclic and linear forms of the C 3 H radical, the former being slightly exoergic and the latter being almost thermoneutral Mebel et al 2007), and one strongly exoergic H 2 -elimination channel which is forbidden in the Born-Oppenheimer approximation, since it requires a nonadiabatic transition from the ground-state triplet potential energy surface to a singlet one: …”

Temperature dependent product yields for the spin forbidden singlet channel of the C(3P) + C2H2 reaction

“…As the translational energies of C 2 H 2 and C 2 H 4 were essentially the same (similar reduced masses and identical relative velocities) giving rise to the same collision energy, they were able to deconvolute the respective H-atom signals in the resulting Doppler-Fizeau spectra (see Figure 7 of Costes et al [7]) given the much lower exoergicity for H-atoms produced by the C + C 2 H 2 reaction. They derived values of 0.82 and 0.87 for the ratio (1c)/(1) at 0.8 and 4.8 kJ mol -1 , considerably larger than the ones obtained by previous work in the same energy range [6,8], or at equivalent temperatures [15]. Moreover, the ratio (1c)/(1) was seen to increase with increasing collision energy; an observation which is seemingly at odds with theoretical considerations based on the argument that intersystem crossing is promoted by an increased lifetime of the C 3 H 2 intermediate.…”

“…The H-atom yields listed in Table 2 Costes et al [7] seems to be erroneous, the energy (temperature) dependence of these results is nonetheless similar to the one determined in this work, indicating that the C 3 + H 2 product channel becomes less favourable as the temperature falls over the 300 -50 K range. In contrast, the earlier studies of Leonori et al [6] and Gu et al [8] …”

“…Rate constants for reaction (1) have been measured over a wide temperature range [3,4,15], proving that this process occurs over a barrierless potential energy surface for at least one of the three thermodynamically accessible pathways. One of the most interesting aspects of this reaction is the observation by several groups of large branching ratios for channel (1c) [6][7][8]; a process which can only take place by intersystem crossing between triplet and singlet potential energy surfaces of the intermediate C 3 H 2 molecule.…”