Crossed-beam reaction of carbon atoms with hydrocarbon molecules. II. Chemical dynamics of n-C4H3 formation from reaction of C(3P j) with methylacetylene, CH3CCH (X1A1)

Abstract

The reaction between ground-state carbon atoms, C(3Pj), and methylacetylene, CH3CCH (X1A1), was studied at average collision energies of 20.4 and 33.2 kJ mol−1 using the crossed molecular beams technique. Product angular distributions and time-of-flight spectra of C4H3 at m/e=51 were recorded. Forward-convolution fitting of the data yields weakly polarized center-of-mass angular flux distributions isotropic at lower, but forward scattered with respect to the carbon beam at a higher collision energy. The translational energy flux distributions peak at 30–60 kJ mol−1 and show an average fractional translational energy release of 22%–30%. The maximum energy release as well as the angular distributions are consistent with the formation of the n-C4H3 radical in its electronic ground state. Reaction dynamics inferred from these distributions indicate that the carbon atom attacks the π-orbitals of the methylacetylene molecule via a loose, reactant like transition state located at the centrifugal barrier. The initially formed triplet 1-methylpropendiylidene complex rotates in a plane almost perpendicular to the total angular momentum vector around the B\C-axes and undergoes [2,3]-hydrogen migration to triplet 1-methylpropargylene. Within 1–2 ps, the complex decomposes via C–H bond cleavage to n-C4H3 and atomic hydrogen. The exit transition state is found to be tight and located at least 30–60 kJ mol−1 above the products. The explicit identification of the n-C4H3 radical under single collision conditions represents a further example of a carbon–hydrogen exchange in reactions of ground state carbon atoms with unsaturated hydrocarbons. This channel opens a versatile pathway to synthesize extremely reactive hydrocarbon radicals relevant to combustion processes as well as interstellar chemistry.

On the ionization energies of C4H3 isomers

“…The different product spectra could be the result of a combination of distinct production methods of the radicals (supersonic be-ams versus combustion flames) and different pressure conditions in the flame and within the supersonic expansion. Further, the present experiments support the prediction from previous crossed molecular beam studies of ground state carbon atom with methylacetylene at lower collision energies that a higher energy C 4 H 3 isomer, possibly of cyclic structure, likely contributes to reactive scattering signal [11]. Theoretically, the reaction mechanism was suggested based on electronic structure calculations of the triplet C 4 H 4 PES [25].…”

“…The n-C 4 H 3 isomer (HCCCHCH; X 2 A') was identified as a reactive intermediate formed in the initial addition of the ethynyl radical (CCH) to acetylene (HCCH). However, a detailed analysis of the center-of-mass functions as extracted from the crossed beam reactions of ground state carbon atoms with methylacetylene [11] suggested that an elusive high-energy isomer, might have also been synthesized. The molecular structure of this isomer is still unknown.…”

“…Chemical dynamics experiments exploiting the crossed molecular beam method showed that C 4 H 3 radicals can be formed under single collision conditions as a product of bimolecular reactions. These involve reactions of ground state carbon atoms (C( 3 P j )) with methylacetylene (CH 3 CCH) [11] and allene (H 2 CCCH 2 ) [12] as well as bimolecular collisions of dicarbon in its ground and first electronically excited states, i.e. C 2 (X 1 Σ g + ) and C 2 (a 3 Π u ), with ethylene (H 2 CCH 2 ); these processes lead to the synthesis of the C s symmetric i-C 4 H 3 isomer: HCCCCH 2 (X 2 A').…”

Crossed Molecular Beam Study of the Reaction Br + O3

“…This is in contrast to the reactions of ground state carbon atoms, C( 3 P j ), with allene and methylacetylene, where under single collision conditions only the C s symmetric, resonantly stabilized i-C 4 H 3 radical was formed. 38,41 Because in the allene system, the hydrogen atoms are equivalent, it is not feasible to utilize partially deuteration of the reactants as done in the methylacetylene-d 3 reaction to elucidate the role of thermodynamically less stable isomers p2-p4 on the reaction dynamics. A comprehensive study of the global BC 3 H 4 PES combined with RRKM calculations is currently underway.…”

Crossed Molecular Beam Experiments of Radical-Neutral Reactions Relevant to the Formation of Hydrogen Deficient Molecules in Extraterrestrial Environments

“…Kaiser et ale (1996b); b. Kaiser et al (1996a); c. Kaiser et al (1996c); d. Kaiser et al (1997a); e. Kaiser et ale (1999a)j f. Kaiser et aI. (1997b); g. Kaiser et al (1999b) The explicit identification of the carbon-hydrogen exchange under single collision conditions opens a versatile, one step pathway to free hydrocarbon radicals in extraterrestrial environments and even in our solar system.…”