Dynamics of the C(³P) + Ethylene Reaction: A Trajectory Surface Hopping Study

Abstract

The direct dynamics trajectory surface hopping (DDTSH) method has been employed to study the reaction of C(P) with ethylene (CH). Our trajectory simulations show that at a reagent collision energy of 7.36 kcal/mol, there are two possible product channels: propargyl (HCCCH) + H and carbene (CH) + acetylene (HCCH). Estimated branching ratios based on trajectory propagations indicate that propargyl radical formation is the dominant channel contributing (94.1 ± 5.2) % of the overall products formation with (5.9 ± 1.7)% contribution from the minor CH + HCCH channel. These findings are consistent with earlier experimental observations and theoretical predictions that propargyl (HCCCH) formation is the dominant channel for the C(P) + CH collision reaction. Our trajectory simulations, however, unravel five distinctly different dynamical pathways, unlike earlier experimental and theoretical predictions of only two pathways proposed for the formation of propargyl radical, and three different dynamics are followed for the CH + HCCH channel (this channel was not detected experimentally). The computed translational energy distribution for the propargyl + H channel is narrower and showed peak maximum at a lower energy compared to the experimental one. While the center of mass product angular distribution based on our trajectory propagation is nearly isotropic in nature indicating formation of long-lived intermediate complexes, the experimental one was reported to be backward-forward distributed with more intensity in the forward direction indicating the formation of an osculating complex. Our trajectory surface hopping calculations confirm that the effect of intersystem crossing (ISC) is not important for the title reaction presumably because of weak spin-orbit coupling values (<10 cm) for the (C + CH) system. No trace of cyclic products formation was obtained from our trajectory simulations, which however was predicted to be a minor (2%) product channel, experimentally.

Discovery of the propargyl radical (CH2CCH) in TMC-1: One of the most abundant radicals ever found and a key species for cyclization to benzene in cold dark clouds

“…which has been measured to be rapid down to 15 K (Chastaing et al 1999) and yields propargyl radical as main product, as indicated by extensive experimental and theoretical evidence (Kaiser Bergeat & Loison 2001;Geppert et al 2003;Chin et al 2012;Mandal et al 2018). A second formation pathway, which is also the main route to the two C 3 H 4 isomers, is provided by the dissociative recombination of C 3 H n + ions with electrons…”

Mechanistic Insight and Intersystem Crossing Dynamics of the C(3P) + H2CO/D2CO Reaction

“…In recent past, Li et al investigated the C­( 3 P) + C 3 H 6 reaction and observed that the electrophilic addition of C­( 3 P) to C = C π bond to produce a cyclic ring, followed by ring opening to form trans -CH 3 CHCCH 2 isomer. In our earlier work in studying dynamics of the reaction of C­( 3 P) with C 2 H 4 , we observed three different type of initial approaches without any entrance channel barrier. These three initial approaches are (1) the C­( 3 P) attacks one of the carbon atoms of the ethylene forming very short-lived biradical (ĊCH 2 ĊH 2 ), (2) the C­( 3 P) adds to the C = C π-bond of ethylene yielding c-propylidene, and (3) the C­( 3 P) gets inserted into one of the C – H bonds of the ethylene forming venylmethylene (H 2 C = CHCH).…”

“…The reactions of the ground state atomic carbon with hydrocarbons play an important role in combustion, − hydrocarbon synthetic chemistry − and interstellar chemistry. , In particular, reactions of the ground state atomic carbon are ubiquitous in the dense interstellar clouds, where the temperature ranges 10 to 50 K. Atomic carbon (in the 3 P, 1 D and 1 S electronic states) generally involves in the abstraction of hydrogen, addition to π bond and insertion into a C – H bond reactions with unsaturated hydrocarbons and hetero-organics compounds. Addition of the triplet carbon atom C­( 3 P) to saturated hydrocarbons are less reactive and take place only at high collision energy, but it shows a high reactivity with unsaturated hydrocarbons through a barrier-less addition. − The triplet atomic carbon atom primarily attacks the organic molecules containing π bond through electrophilic addition. In recent past, Li et al investigated the C­( 3 P) + C 3 H 6 reaction and observed that the electrophilic addition of C­( 3 P) to C = C π bond to produce a cyclic ring, followed by ring opening to form trans -CH 3 CHCCH 2 isomer.…”

A Combined Experimental and Computational Study on the Reaction Dynamics of the 1-Propynyl (CH3CC)–Acetylene (HCCH) System and the Formation of Methyldiacetylene (CH3CCCCH)

“…The propynyl radical holds a C 3 v symmetric point group with an X 2 A 1 electronic ground state and lies 168 kJ mol –1 (0 K) above the propargyl radical. , Literature on the availability and reactivity of 1-propynyl during combustion is scarce. Geppert et al suggested that higher-energy C 3 H 3 isomers like 1-propynyl might represent up to 14% of the total C 3 H 3 yield in crossed molecular beams studies on the reaction of atomic carbon (C­( 3 P)) with ethylene (CH 2 CH 2 ) at a collision energy of 30.8 kJ mol –1 . , However, recent studies on this system were unable to confirm their hypothesis. , Methylacetylene (propyne; CH 3 CCH) could be a source of 1-propynyl radicals, but considering the bond energies of the acetylenic hydrogen bond to those of the acetylene-methyl (C–CH 3 ) and methyl hydrogen bonds (H–C 2 H), the yield would be extremely small . Indeed, hydrogen atom reactions with methylacetylene at combustion-relevant temperatures and pressures produce mainly acetylene by methyl (CH 3 ) displacement, allene (H 2 CCCH 2 ) via hydrogen-assisted isomerization, and the propargyl radical by direct hydrogen abstraction …”

“…33,34 However, recent studies on this system were unable to confirm their hypothesis. 35,36 Methylacetylene (propyne; CH 3 CCH) could be a source of 1-propynyl radicals, but considering the bond energies of the acetylenic hydrogen bond to those of the acetylene-methyl (C−CH 3 ) and methyl hydrogen bonds (H−C 2 H), the yield would be extremely small. 37 Indeed, hydrogen atom reactions with methylacetylene at combustion-relevant temperatures and pressures produce mainly acetylene by methyl (CH 3 ) displacement, allene (H 2 CCCH 2 ) via hydrogen-assisted isomerization, and the propargyl radical by direct hydrogen abstraction.…”