Elucidating the Chemical Dynamics of the Elementary Reactions of the 1-Propynyl Radical (CH₃CC; X²A₁) with Methylacetylene (H₃CCCH; X¹A₁) and Allene (H₂CCCH₂; X¹A₁)

Abstract

The reactions of the 1-propynyl radical (CH3CC; X2A1) with two C3H4 isomers, methylacetylene (H3CCCH; X1A1) and allene (H2CCCH2; X1A1), along with their (partially) deuterated counterparts were explored at collision energies of 37 kJ mol–1, exploiting crossed molecular beams to unravel the chemical reaction dynamics to synthesize distinct C6H6 isomers under single collision conditions. The forward convolution fitting of the laboratory data along with ab initio and statistical calculations revealed that both reactions have no entrance barrier, proceed via indirect (complex-forming) reaction dynamics involving C6H7 intermediates with life times longer than their rotation period(s), and are initiated by the addition of the 1-propynyl radical with its radical center to the π-electron density of the unsaturated hydrocarbon at the terminal carbon atoms of methylacetylene (C1) and allene (C1/C3). In the methylacetylene system, the initial collision complexes undergo unimolecular decomposition via tight exit transition states by atomic hydrogen loss, forming dimethyldiacetylene (CH3CCCCCH3) and 1-propynylallene (H3CCCHCCCH2) in overall exoergic reactions (123 and 98 kJ mol–1) with a branching ratio of 9.4 ± 0.1; the methyl group of the 1-propynyl reactant acts solely as a spectator. On the other hand, in the allene system, our experimental data exhibit the formation of the fulvene (c-C5H4CH2) isomer via a six-step reaction sequence with two higher energy isomershexa-1,2-dien-4-yne (H2CCCHCCCH3) and hexa-1,4-diyne (HCCCH2CCCH3)also predicted to be formed based on our statistical calculations. The pathway to fulvene advocates that, in the allene–1-propynyl system, the methyl group of the 1-propynyl reactant is actively engaged in the reaction mechanism to form fulvene. Because both reactions are barrierless and exoergic and all transition states are located below the energy of the separated reactants, the hydrogen-deficient C6H6 isomers identified in our investigation are predicted to be synthesized in low-temperature environments, such as in hydrocarbon-rich atmospheres of planets and their moons such as Titan along with cold molecular clouds such as Taurus Molecular Cloud-1.

Directed Gas‐Phase Synthesis of Triafulvene under Single‐Collision Conditions

“…RRKM rate constants were then used to evaluate product branching ratios by solving first-orderk inetic equations within steady-state approximation. [25,40] This ab initio/RRKMapproach has been shown to closely reproduce experimental product branching ratios when the system exhibits statistical behavior. [32,[41][42][43]…”

A chemical dynamics study on the gas-phase formation of triplet and singlet C 5 H 2 carbenes

“…These data were computed either at our regular CCSD(T)-F12/cc-pVTZ-f12//B2PLYPD3/cc-pVTZ level or at the CASPT2(13, 10)/cc-pVTZ//B3LYP/cc-pVTZ level for the i2-p1 and i3-p2 paths where several structures showed a significant multireference character. More detail on our approach to conventional RRKM and variational transition state theory energy-dependent calculations are provided in an earlier publication (62), but here we employed our recently developed inhouse Unimol software code (63) to generate all rate constants and product branching ratios depending on the collision energy.…”

Directed Gas Phase Formation of the Elusive Silylgermylidyne Radical (H3SiGe, X2A′′)

“…Rice‐Ramsperger‐Kassel‐Marcus (RRKM) theory, [35] was used to compute energy‐dependent rate constants of all unimolecular reaction steps on the SiGeH 5 PES following the initial formation of the i0 complex in the entrance channel. Internal energy dependent rate constants were computed within the harmonic approximation using B2PLYPD3/cc‐pVTZ frequencies using our in‐house code, [36] which automatically processes GAUSSIAN 09 log files to assess numbers of states for transition states and densities of states for local minima employing the direct count method. The internal energy was assumed to be equal to the sum of the collision energy and the chemical activation energy, that is, negative of the relative energy of a species with respect to the reactants.…”

“…Only one energy level was considered throughout as at a zero‐pressure limit corresponding to crossed molecular beams conditions. The RRKM‐computed rate constants were utilized to obtain product branching ratios by solving first‐order kinetic equations within steady‐state approximation [36–37] …”