A Theoretical Study of the Reaction Mechanism and Product Branching Ratios of C₂H + C₂H₄ and Related Reactions on the C₄H₅ Potential Energy Surface

Abstract

Ab initio and density functional RCCSD(T)/cc-pVQZ//B3LYP/6-311G** calculations of various stationary points on the C(4)H(5) global potential energy surface have been performed to resolve the C(2)H + C(2)H(4) and C(2)H(3) + C(2)H(2) reaction mechanisms under single-collision conditions. The results show vinylacetylene + H as the nearly exclusive products for both reactions, with exothermicities of 26.5 and 4.3 kcal/mol, respectively. For C(2)H + C(2)H(4), the most important mechanisms include a barrierless formation of the CH(2)CH(2)CCH adduct c6 (56.9 kcal/mol below the reactants) in the entrance channel followed either by H loss from the vicinal CH(2) group via a barrier of 35.7 kcal/mol or by 1,2-H migration to form CH(3)CHCCH c3 (69.8 kcal/mol lower in energy than C(2)H + C(2)H(4)) via a 33.8 kcal/mol barrier and H elimination from the terminal CH(3) group occurring with a barrier of 49.4 kcal/mol. RRKM calculations of energy-dependent rate constants for individual reaction steps and branching ratios for various channels indicate that 77-78% of vinylacetylene is formed from the initial adduct, whereas 22-21% is produced via the two-step mechanism involving the 1,2-H shift c6-c3, with alternative channels contributing less than 1%. The theoretical results support the experimental crossed molecular beams observations of vinylacetylene being the major product of the C(2)H + C(2)H(4) reaction and the fact that CH(2)CHCCH is formed via a tight transition state with an exit barrier of 5-6 kcal/mol and also confirm that vinylacetylene can be produced from C(2)H + C(2)H(4) under low temperature conditions of Titan's atmosphere. The prevailing mechanism for the C(2)H(3) + C(2)H(2) reaction starts from the initial formation of different n-C(4)H(5) conformers occurring with significant entrance barriers of approximately 6 kcal/mol. The n-C(4)H(5) isomers reside 35-38 kcal/mol lower in energy than C(2)H(3) + C(2)H(2) and can rapidly rearrange to one another overcoming relatively low barriers of 3-5 kcal/mol. H loss from the n-C(4)H(5) species then gives the vinylacetylene product via exit barriers of approximately 6 kcal/mol with the corresponding transition states lying 1.2-1.6 kcal/mol above the C(2)H(3) + C(2)H(2) reactants. Since the C(2)H(3) + C(2)H(2) reaction is hindered by relatively high entrance barriers, it is not expected to be important in Titan's atmospheric environments but can produce n-C(4)H(5) or vinylacetylene under high temperature and pressure combustion conditions.

Theoretical study on the reaction of butadiynyl radical (C4H) with ethylene (C2H4) to form C6H4 and H

“…These results are different from those found in the C 2 H þ C 2 H 4 reaction where the direct interaction two C atoms of C 2 H and both C atoms of C 2 H 4 makes an initial adduct with a four-membered ring (C 4 H 5 , see q2 structure in Figure 1 in Ref. [4]). Rather long chain of C 4 H may distort the PES and cause the different reaction step.…”

“…The IM1 is directly connected to the P1 through the transition state, TS1-P1, via H elimination. A similar result is found in the C 2 H þ C 2 H 4 reaction [4]. Alternatively, one H atom of the ethylene moiety of IM1 can migrate to C1 atom of the C 4 H moiety, thereby forming IM2.…”

“…We will return to this in a later section (section 4). It should be noted that all these processes in this reaction pathway are quite similar to those of the reaction of C 2 H with C 2 H 4 [4].…”

“…We found only one theoretical study for the reaction of C 4 H with C 2 H 2 [5]. On the other hand, experimental investigations for C 4 H with various saturated and unsaturated hydrocarbons using the pulsed laser photolysis have been recently reported [9,10]. The rate coefficients for these reactions have been provided, but their detailed reaction mechanisms are still unknown.…”

“…Experimental and theoretical investigations for this kind of unusual hydrocarbon (or hydrogen-deficient) compounds are important not only for combustion chemistry (because of long chain of C 4 H, it can be crucial role for the formation of soot) but also for better understanding the chemistry of the interstellar medium and planetary atmospheres. Despite numerous theoretical investigations of the reactions involving ethynyl (C 2 H) radical [3][4][5][6][7][8], which is also considered as a precursor to polycyclic aromatic hydrocarbons such as C 4 H, a theoretical study for the reaction involving C 4 H is still lacking. We found only one theoretical study for the reaction of C 4 H with C 2 H 2 [5].…”