Theoretical Studies of Reaction Mechanisms Relevant to Hydrocarbon Growth in Titan s Atmosphere

Transcription

1 Theoretical Studies of Reaction Mechanisms Relevant to Hydrocarbon Growth in Titan s Atmosphere Alexander M. Mebel, Adeel Jamal, Alexander Landera, and Ralf I. Kaiser Department of Chemistry and Biochemistry, Florida International University, Miami, Florida, and Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii 5 th Workshop on Titan Chemistry Observations, Experiments, Computations, and Modeling Kauai Island, Hawaii, April 11-14, 2011 This work is supported by the National Science Foundation within the CRC Program (Award No. CHE ).

2 1. Reactions of vinylacetylene with C, C 2, and C 2 H Vinylacetylene, H 2 C=CH-C CH, is the smallest hydrocarbon containing both a double and a triple C-C bonds. It is also believed to be one of significant gas phase hydrocarbon constituents on Titan. Although no direct evidence for its presence has been found, its formation has been identified in the reaction of C 2 H 4 with C 2 H. Additionally, vinylacetylene has been observed in acetylene ices irradiated by electrons. Here, we consider reaction mechanisms of vinylacetylene with C, C 2, and C 2 H in collaboration with crossed molecular beams studies by Ralf Kaiser s group. The goal to see whether these reactions can contribute to the formation of an aromatic ring or RSFR. 2. Reactions of C 2 H with C 4 H 6 isomers (1- and 2-butynes) We continue our systematic studies of C 2 H reactions with a variety of unsaturated hydrocarbons in order to understand their role in the growth of complex hydrocarbons in Titan s atmosphere.

3 Theoretical methods Ab initio methods: 1) Density functional (B3LYP/6-311G**) calculations for geometry optimization and vibrational frequencies (QCISD for some structures) 2) Coupled clusters CCSD(T) calculations with extrapolation to the complete basis set (CBS) limit: CCSD(T)/cc-pVDZ x = 2 Exponential fit of total energies: CCSD(T)/cc-pVTZ x = 3 E tot (x) = E tot ( ) + Be -Cx CCSD(T)/cc-pVQZ x = 4 E tot ( ) CCSD(T)/CBS total energy Statistical calculations of rate constants and product branching ratios: 1) RRKM theory (within harmonic approximation) unimolecular rate constants. 2) Solving kinetic equations for concentrations of various reaction intermediates and products.

4 C( 3 P) + HCCCHCH 2 (vinylacetylene) C 5 H 3 + H / C 3 H 2 + C 2 H 2 J. Phys. Chem. A 115, 593 (2011)

5 RRKM calculated product branching ratios (%) at different collision energies assuming a one to one entrance channel branching ratio toward i1/i3 initial intermediates E c, kj mol -1 products i-c 5 H 3 from i i-c 5 H 3 from i i-c 5 H 3 total n-c 5 H 3 from i n-c 5 H 3 from i n-c 5 H 3 total HCCCH + C 2 H The experimental crossed molecular beams study by Ralf Kaiser s group investigated the reaction dynamics of ground state carbon atoms, C( 3 P j ), with vinylacetylene at two collision energies of 18.8 kj mol 1 and 26.4 kj mol 1 and showed that the reaction leads to two resonantly stabilized free radicals. The reaction was found to be governed by indirect scattering dynamics and to proceed without an entrance barrier through a long-lived collision complex to reach the products, i- and n-c 5 H 3 isomers via tight exit transition states.

6 C 2 ( 1 g+ ) + HCCCHCH 2 (vinylacetylene) C 6 H 3 + H / C 4 H 2 + C 2 H 2 C 2 (X 1 g + ) + C 4 H 4 1,2,4-tridehydrobenzene 1,2,3-tridehydrobenzene ,3,5-tridehydrobenzene H i2_s H + H + H P i1_s i3_s C 4 H 2 + C 2 H i4_s i7_s i5_s i6_s (see Alex Landera s poster for more detail)

7 C 2 ( 3 u ) + HCCCHCH 2 (vinylacetylene) C 6 H 3 + H

8 Based on our RRKM results, the major product of C 2 + C 4 H 4 is l-c 6 H 3 (H 2 CCCCCCH, P1). It is formed in excess of 90% from all possible entrance routes. Cyclic structures are only formed as minor products (up to 4%), and on the singlet surface C 2 H 2 + C 4 H 2 is the second most abundant product at about 7-8%. Experimentally, it would be difficult to distinguish the linear C 6 H 3 product from tridehydrobenzenes exclusively from the kinetic energy distribution because l-c 6 H 3, 1,2,3-and 1,2,4-tridehydrobenzenes lie close in energy (within 2 kcal/mol).

9 C 2 H + C 4 H 4 (vinylacetylene): C 2 H addition to acetylenic C

10 C 2 H + C 4 H 4 (vinylacetylene): C 2 H addition to terminal CH 2 RRKM calculated product branching ratios (%) E col, kcal/mol o-benzyne (Z)-hexa-3- ene-1,5-diyne

11 C 2 H + C 4 H 4 (vinylacetylene): C 2 H addition to middle carbons

12 RRKM calculated product branching ratios (%) at different collision energies assuming equal probabilities of the four different entrance channels collision energy, kj mol -1 Products vinyldiacetylene hexa-1,2,3-triene-5-yne m-benzyne o-benzyne (Z)-hexa-3-ene-1,5-diyne diethynylethene C 2 H 3 +C 4 H Flux contour map of the reaction of the deuterated ethynyl radical with vinylacetylene to form C 6 H 3 D and H at a collision energy of 40.9 kj mol 1 Astrophys J. 728, 141 (2011): The elementary reaction of the D1-ethynyl radical (C 2 D) with vinylacetylene (C 4 H 4 ) was studied under single collision conditions via crossed molecular beam experiments and electronic structure calculations. The results suggested that besides two acyclic isomers as predicted computationally, the ortho-benzyne (o-c 6 H 4 ) in its singly deuterated form which is considered as an important intermediate in the formation of polycyclic aromatic hydrocarbons, can be formed in this process.

13 C 2 H + C 4 H 6 (1-butyne) products Soorkia, S.; Trevitt, A.; Selby, A.; Osborn, D.; Wilson, K.; Leone, S.; J. Phys. Chem. A. 2010, 114, 3340.

14 Potential energy diagram for the terminal C 2 H addition to 1-butyne Adeel Jamal and A. M. Mebel J. Phys. Chem. A 115, 2196 (2011)

15 Potential energy diagram for the central C 2 H addition to 1-butyne

16 Potential energy map of the channels leading to the formation of fulvene and DMCB

17 Product branching ratios in the C 2 H + 1-butyne reaction calculated for different collision energies and with various C 6 H 7 initial adducts INT A, INT C-T, and INT 1 Products Initial Adduct E col, kcal/mol C 6 H 6 (isomers) + H 2-ethynyl-1,3-butadiene INT A INT C-T INT ,3-hexadiene-5-yne INT A INT C-T INT ,4-hexadiene-1-yne INT A INT C-T INT ,3-hexadiyne INT A INT C-T INT ,1-ethynylmethylallene INT A INT C-T INT Fulvene INT A INT C-T INT DMCB INT A INT C-T INT C 5 H 4 (isomers) + CH 3 Ethynylallene INT A INT C-T INT methyldiacetylene INT A INT C-T INT C 4 H 2 + C 2 H 5 Diacetylene INT A INT C-T INT

18 Product branching ratios in the C 2 H + 1-butyne reaction calculated for different collision energies assuming equal probabilities of the three different entrance channels Products E col, kcal/mol C 6 H 6 (isomers) + H 2-ethynyl-1,3-butadiene ,3-hexadiene-5-yne ,4-hexadiene-1-yne ,3-hexadiyne ,1-ethynylmethylallene Fulvene DMCB C 5 H 4 (isomers) + CH 3 Ethynylallene methyldiacetylene C 4 H 2 + C 2 H 5 Diacetylene A significant qualitative disagreement is found for the C 6 H 6 isomeric product distribution. Soorkia et al. detected cyclic DMCB and fulvene as the most significant C 6 H 6 products and, among the acyclic isomers, the order of the measured branching ratios was 3,4-hexadiene-1-yne > 1,3-hexadiyne > 2-ethynyl-1,3-butadiene. In our calculations, none of the cyclic isomers are formed and 2-ethynyl-1,3-butadiene is the major acyclic product, with minor contributions from 1,3-hexadiene-5-yne, 3,4-hexadiene-1-yne, and 1,3-hexadiyne.

19 How to explain the discrepancy between theory and experiment? 1) Can inaccuracies in our calculations be a source of the disagreement with experiment? Relative energies of various transition states are expected to have error bars of +2 kcal/mol. We decreased the critical barrier on the pathway leading to fulvene by 2 kcal/mol, while increasing the highest barriers on the pathways leading to the acyclic C 6 H 6 isomers, ethynylallene + CH 3, and diacetylene + C 2 H 5 also by 2 kcal/mol and repeated the RRKM calculations of the rate constants and first-order kinetics calculations of the branching ratios. Still virtually no fulvene could be produced. In general, the +2 kcal/mol variations in the energies of the critical transition states may significantly affectthe ratio of the 2-ethynyl-1,3-butadiene + H, ethynylallene + CH 3, and diacetylene + C 2 H 5 products. For the C 6 H 6 isomers, the relative yields can be affected for the minor products, 3,4-hexadiene- 1-yne, 1,3-hexadiyne, and 1,3-hexadiene-5-yne. However, the conclusion that 2-ethynyl-1,3- butadiene is the major C 6 H 6 isomer to be produced is not expected to change because the critical barrier for its formation is at least 10 kcal/mol lower than those for the other C 6 H 6 species. Deviations from the statistical behavior may enhance direct dissociation of the initial adducts, especially the cleavage of the bonds nearest to the carbon atom attacked by C 2 H. In this case, an increase of branching ratios may be expected for 1,3-hexadiyne + H and diacetylene + C 2 H 5.

20 How to explain the discrepancy between theory and experiment? 2) The experimental conditions of the room temperature (295 K) and 4 Torr (5.33 mbar) pressure are not exactly compatible with the single-collision (E col = 0-7 kcal/mol) conditions assumed in our kinetic calculations. However, the collision energies used in the calculations cover the kinetic energy range typical for 295 K and the zero yield of fulvene and DMCB is independent of T. The collisional stabilization of the C 6 H 7 species is not expected to enhance the yield of the primary cyclic fulvene and DMCB products because the pathways leading to them involve numerous isomerization steps, which would become even less probable if the C 6 H 7 intermediates are collisionally stabilized. On the other hand, it might be possible that secondary reactions involving stabilized C 6 H 7 may lead to fulvene, i.e., C 6 H 7 (INT 5) + H fulvene + H 2? C 6 H 7 (INT 7 or INT 8) + H fulvene + H + H?

21 How to explain the discrepancy between theory and experiment? 3) Contributions from secondary reactions 4) Difficulties with the accurate assignment of the experimental photoionization efficiency curve 2-ethynyl-1,3-butadiene formed in the C 2 H + 1-butyne reaction is exothermic by ~1.7 ev. Some fraction of this released energy goes to the kinetic energy of the C 6 H 6 + H products. It is plausible that a part of the energy remains in the form of the internal vibrational energy of 2-ethynyl-1,3-butadiene resulting in a reduction of its ionization energy. If 2-ethynyl-1,3-butadiene does not have enough time to completely thermalize before it is ionized under the experimental conditions, it may have an internal vibrational energy distribution different from that typical for the room temperature, further complicating the fit of the measured PIE curve. However, at the experimental conditions thermalization most likely does take place. Maybe, ionization energies of DMCB and 2-ethynyl-1,3-butadiene are even closer than 0.2 ev?

22 C 2 H + C 4 H 6 (2-butyne) products Methyldiacetylene + CH 3 are predicted to be the dominant reaction products

23 Conclusions The most typical mechanism of C 2 H reactions with unsaturated hydrocarbons is a C 2 H-for-H exchange where the C 2 H addition to a double or triple bond is followed by a C-H bond cleavage involving the attacked carbon atom: C 2 H + C 2 H 2 C 4 H 2 + H, C 2 H + C 4 H 2 C 6 H 2 + H, C 2 H + C 2 H 4 C 4 H 4 + H C 2 H + CH 3 CCH CH 3 CCCH (methyldiacetylene) + H, C 2 H + CH 2 CCH 2 ethynylallene + H / 1,4-pentadiyne + H C 2 H + 1-butyne 2-ethynyl-1,3-butadiene + H The reactions may also proceed by C 2 H-for-CH 3 or, more generally, C 2 H-for-C n H m exchange mechanisms where the C 2 H addition is followed by a cleavage of the weakest C-C bond of the carbon atom attacked by the ethynyl radical: C 2 H + CH 3 CCH C 4 H 2 + CH 3, C 2 H + 1-butyne ethynylallene + CH 3 / C 4 H 2 + C 2 H 5, C 2 H + 2-butyne methyldiacetylene + CH 3 C 2 H addition may result in a ring closure leading to the formation of an aromatic ring. The cyclization would occur only if the ring closure is faster than the competing C-H or C-C bond rupture processes. If the cyclization process is favorable, C 2 H additions may lead to the formation of a first aromatic ring or to the growth of PAH even at very low T: C 2 H + (substituted) 1,3-butadiene (substituted) benzene + H, EAM for PAH growth: consecutive C 2 H additions to phenylacetylene or styrene