Photodissociation Dynamics of Propyne and Allene:  A View from ab Initio Calculations of the C₃H_n (n = 1−4) Species and the Isomerization Mechanism for C₃H₂

Abstract

Potential energy surfaces of various primary and secondary products from the photodissociation of propyne and allene, including the C 3 H n (n ) 1-3) species, have been investigated at the CCSD(T)/6-311+G-(3df,2p)//B3LYP/6-311G(d,p) level of theory. The calculated heats of the reactions and the activation barriers for H 2 elimination from C 3 H n (n ) 2-4) have been employed to analyze the experimental translational energy distribution for different photodissociation channels. The electronic spectra of propyne and various isomers of C 3 H 2 have been calculated by using the CASSCF, MRCI, and EOM-CCSD methods with the ANO(2+) basis set. The calculations suggest that the photodissociation of propyne at 193 nm involves a Franck-Condon transition to the 1 E excited state. After internal conversion into the vibrationally excited ground electronic state, propyne can either dissociate to produce HCCCH + H 2 or isomerize to allene which, in turn, undergoes the H 2 elimination giving H 2 CCC. The HCCCH produced from propyne can have sufficiently high internal energy to rearrange to H 2 CCC. In both mechanisms, the formation of C 3 + H 2 from propyne and allene goes via the same intermediate, which explains the identical rotational distribution of the C 3 products in experiment. The H 2 elimination is a minor channel of propyne photodissociation and the major channel is elimination of the acetylenic hydrogen atom. The rearrangement mechanism of C 3 H 2 in the ground electronic state also has been studied. Automerization of H 2 CCC can take place either via a cyclopropyne transition state (the barrier is 37.5 kcal/mol, ref 18) or through isomerization to cyclopropenylidene and backward via TS6 (the barrier is 41.7 kcal/mol). Isomerization of triplet propargylene to cyclo-C 3 H 2 occurs by the ring closure via the triplet-singlet seam of crossing MSX1, and the activation energy is predicted to be about 41 kcal/mol. Cyclopropenylidene can undergo automerization by the 1,2-H shift via TS10 with the barrier of 32.4 kcal/mol. The direct triplet HCCCH f H 2 CCC isomerization proceeds by the 1,3-hydrogen shift via MSX2 and TS8 or TS9 with a high activation energy of 78-81 kcal/mol. The singlet propargylene can also rearrange to cyclo-C 3 H 2 via TS7 (barrier 37.4 kcal/mol) and to H 2 CCC via TS8 or TS9. The calculated PES for the ground and excited states have allowed us to explain the experimentally observed automerizations and isomerizations of C 3 H 2 isomers and to assign their UV absorption spectra.

Photochemistry of C3Hphydrocarbons in Titan’s stratosphere revisited

“…Therefore the photodissociation will be dominated by these absorptions, which mainly lead to C-H bond rupture. The photodissociations of c−C 3 H and l−C 3 H are dominated by the visible and near-UV regions and should yield only C 3 + H. The products of c−C 3 H 2 and l−C 3 H 2 photodissociation were derived from Mebel et al (1998), Park et al (2006) and Großet al (2008) and were set to l−C 3 H + H (30%), c−C 3 H + H (30%), and C 3 + H 2 (40%). The C 3 + H 2 exit channel shows a notable exit barrier Park et al 2006).…”

“…The C 3 + H 2 exit channel shows a notable exit barrier Park et al 2006). The products of t−C 3 H 2 photodissociation were also derived from Mebel et al (1998) and Park et al (2006) and were set to l−C 3 H + H (50%) and c−C 3 H + H (50%), although the spin-allowed 3 C + C 2 H 2 exit channel may not be negligible. The absorption cross sections of C 3 H and C 3 H 2 are given in Figs.…”

Methylacetylene (CH3CCH) and propene (C3H6) formation in cold dense clouds: A case of dust grain chemistry

“…As a result, C 3 (and also l,c-C 3 H and l,c-C 3 H 2 but at lower levels) is efficiently formed in the gas phase in our model, and can stick on the surface, undergo successive hydrogenations to form closed shell s-C 3 H 4 molecules (s-C 3 H 4 represents C 3 H 4 residing on the grain surface). As, by comparison with the gas phase, these hydrogenation reactions are considered to be barrierless (or with very low barriers) up to s-C 3 H 4 (Mebel et al 1998, Nguyen et al 2001, Mebel & Kaiser 2002, Miller & Klippenstein 2003, Hébrard et al 2013, this is an efficient route for s-C 3 H 4 synthesis (both methylacetylene and allene, but only the methylacetylene isomer is considered in this work). Also by comparison with the gas phase, the subsequent hydrogenation step to s-C 3 H 5 shows a small barrier (Whytock et al 1976, Faravelli et al 2000, Miller et al 2008, Vuitton et al 2012) which can be overcome through tunneling.…”

The chemistry of C2and C3in the coma of Comet C/1995 O1 (Hale-Bopp) at heliocentric distancesrh≥2.9 AU

“…Therefore we consider in this work only the two linear isomers as possible grandparents of C 3 . For the photodissociation of propyne at 1930 Å, Mebel et al (1998) predicted that it occurs via a fast pathway including internal excitation and a slow pathway including internal isomerization into allene. In the fast pathway, propyne is excited and quickly eliminates the acetylenic hydrogen.…”