Formation of Interstellar Propylene (CH3CHCH2)
Student Researcher Zhou Lin
(1)Detection of Interstellar Propylene
The discovery of propylene (CH3CHCH2) in the Taurus Molecular Cloud 1 (TMC-1) was reported in 2007.1 The observed column density is 4.0x1013 cm-2 and the derived fractional abundance to total hydrogen (n = n(H)+2n(H2)) is 4.0x10-9. Propylene is the most saturated hydrocarbon ever detected in TMC-1.
(2) Difficulty in Explanation
The chemical importance of interstellar propylene had been ignored for a long time, because it was difficult to detect its existence with radio astronomical techniques based on the weak dipole moment, and to propose an obvious formation pathway from highly unsaturated species. Propylene is produced from the dissociative recombination reaction of C3H7+ (CH3CHCH3+)2:
However, the production of protonated propylene from more unsaturated three-carbon hydrocarbon ions does not proceed via ordinary H atom-exchange ion-molecule reactions involving molecular hydrogen:
(1) Three-step Formation Pathway
One possibility to form propylene is via hydrogenation of C3H2 on interstellar dust grains, but there is no evidence for this process. In 2010, a new gas-phase formation pathway, which contains two radiative associations and one dissociative recombination, was proposed by Herbst et al3:
(2) Radiative Association Reactions
Radiative association (RA) is one important type of process in interstellar gas-phase chemistry, although few laboratory experiments exist.4 The rate coefficients for Rx #1 and #2 were obtained by theory.5 The proposed mechanism comprises two steps: equilibrium between the collision complex and the reactants, followed by relaxation of the complex by the emission of a photon:
The overall rate equation for this process is
Computation and Simulation
(1) Rate Coefficients
The radiative association rate coefficients were calculated with canonical ensemble theory and the steady-state approximation, which leads to the expression5,6
K(T) is the equilibrium constant between reactants and complex; kr is the rate of radiative emission; qint is the internal partition function of the complex, Evib is its vibrational energy, s is the number of its vibrational modes, and A is the Einstein emission coefficient. The formula for the radiative emission rate is expressed in terms of emission rates for the fundamental of each mode of the complex. Molecular geometries and energy levels were calculated using the coupled cluster method CCSD/6-311++G(d, p) via Gaussian.3
Unlike the case for radiative association, measured rate coefficients exist for dissociative recombination, although product branching fractions are less often measured. For the case of protonated propylene, the fraction leading to propylene and atomic hydrogen is known to be significant.2 The rate constants were fit to the empirical formula
Network osu_01_2009, a low-temperature gas-phase model with anions, and the source code Nahoon, were used in the chemical simulation of TMC-1.7 The network was updated by the addition of propylene and C3H7+ , and reactions leading to their formation and destruction. Important reactions in the network involve the following types:
•Interactions with grains
•Cosmic ray induced reactions
Nahoon was run at a temperature of T=10 K, a cosmic-ray ionization rate ζ=1.3x10-17s-1, and a visual extinction AV=10.
(1) Cation Geometries
The refined geometries of C3H3+ (CH2CCH+), C3H5+ (CH2CCH3+) and C3H7+ (CH3CHCH3+), calculated by Herbst et al.3, are listed in Fig 2. The three carbon atoms in C3H3+ and C3H5+ are collinear, while in C3H7+ the C-C-C angle is smaller than 180o.
(2) Parameters in Rate Coefficients
Parameters α, β, and γ for the radiative associations and dissociative recombination leading to the production of propylene are listed in Table 1 with rate coefficients at T=10 K. The inverse temperature dependence (β<0) derives from calculation for reactions (1) and (2), while γ=0 was indicated based on the barrier-free potential surfaces.
(3) Fractional Abundance of Propylene
The calculated fractional abundance of propylene is plotted versus time in Fig 3. At the standard “early-time” of 105 yr, where best agreement is obtained for TMC-1, the calculated abundance is 1.5 x 10-9.
(4) Important Reactions
The pathway proposed by Herbst et al.3 is the most important formation process for the production of interstellar propylene. Some other formation pathways shown below, are of lesser importance:
The destruction of propylene occurs via reactions with major ions but predominantly with atomic oxygen8:
(5) Comparison with Observation
The chemical age of TMC-1 is estimated to be 105 yr, a time that coincides with the second peak in Fig. 3. The derived fractional abundance of propylene from its observed column density is 4.0x10-9, which agrees well with the model value of 1.5 x 10-9.
The time evolution of interstellar propylene abundance is simulated using an updated gas-phase network, based on the proposed mechanism and calculated radiative association rate coefficients. Our results can account for the observed amount in TMC-1 (to the same order of magnitude).
•Search for other formation and destruction pathways for propylene in gas phase.
•Investigate grain-surface reactions involving propylene in gas-grain models.
•Run the model with uncertainties in rate coefficients9.
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2 A. I. Florescu-Mitchell and J. B. A. Mitchell. (2006). Physics Reports, 430: 277.
3 E. Herbst, E. Roueff, and D. Talbi. (2010). Molecular Physics, 108: 17, 2171.
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5 E. Herbst. (1980). The Astrophysical Journal, 237: 462
6 E. Herbst, (1982). Chemical Physics, 65: 185.
8 H. Sabbah, L. Biennier, I. R. Sims, Y. Georgievskii, S. J. Klippenstein, I. W. M. Smith. (2007). Science, 317: 102.
9 A. I. Vasyunin, D. Semenov, V. Wakelam, E. Herbst, A. M. Sobolev. (2008). The Astrophysical Journal, 672: 629