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

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Abstract

Potential energy surfaces of various primary and secondary products from the photodissociation of propyne and allene, including the C3Hn (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 H2 elimination from C3Hn (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 C3H2 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 1E excited state. After internal conversion into the vibrationally excited ground electronic state, propyne can either dissociate to produce HCCCH + H2 or isomerize to allene which, in turn, undergoes the H2 elimination giving H2CCC. The HCCCH produced from propyne can have sufficiently high internal energy to rearrange to H2CCC. In both mechanisms, the formation of C3 + H2 from propyne and allene goes via the same intermediate, which explains the identical rotational distribution of the C3 products in experiment. The H2 elimination is a minor channel of propyne photodissociation and the major channel is elimination of the acetylenic hydrogen atom. The rearrangement mechanism of C3H2 in the ground electronic state also has been studied. Automerization of H2CCC 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-C3H2 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 → H2CCC 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-C3H2 via TS7 (barrier 37.4 kcal/mol) and to H2CCC 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 C3H2 isomers and to assign their UV absorption spectra.

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