Photodissociation Dynamics of Propyne and Allene:  A View from ab Initio Calculations of the C3Hn (n = 1−4) Species and the Isomerization Mechanism for C3H2

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 → 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.