Photodissociation dynamics of propyne at 193 nm are studied using the fewest switches nonadiabatic trajectory surface hopping method on its first excited singlet electronic state (1(1)A''). The trajectories are propagated based on potential energies, gradients and nonadiabatic couplings calculated at the MRCIS(6,7) level with the 6-31++G(d,p) basis set. Our trajectory calculations have revealed that H + H3CCC is the major dissociation channel, which has also been predicted experimentally. For the primary photodissociation channel H + H3CCC we demonstrate that nonadiabatic dynamics do not play a significant role. This observation is however contradictory to most of the previously reported experimental predictions. The calculated product translation energy distribution for the acetylenic H atom elimination peaked at ∼ 18 kcal mol(-1), indicating that the dissociation occurs adiabatically on a moderately repulsive excited surface that correlates with the ground state products (CH3C ≡ C + H). The H atom elimination process from the methyl fragment involving a transition state, which has to compete with the acetylenic H atom dissociation channel with no barrier in the excited singlet surface, was found to be too less probable to make a contribution to product branching. We observed that a fewer but significant number of trajectories led to CH3 + CCH product formation which has not been observed experimentally when propyne is excited at 193 nm.
The photodissociation dynamics of propane molecules has been studied using the quasiclassical trajectory surface hopping (TSH) method in conjunction with Tully's fewest switches algorithm. The trajectories are propagated on potential energy surfaces computed on-the-fly using the multiconfiguration and multireference ab initio method starting in the lowest excited singlet state (HOMO → 3s Rydberg state) of propane at 157 nm with the emphasis on the site specificity of atomic hydrogen elimination, molecular hydrogen elimination, and their product branching ratios. Our dynamics simulation revealed that there are three primary dissociation channels: the atomic hydrogen elimination, the molecular hydrogen elimination, and the C-C bond scission. The trajectories indicate that the H elimination from the internal carbon atom (2,2-H elimination) and terminal carbon atom (1,1-H elimination) is the major process and follows a three centred synchronous concerted mechanism. 1,2-H and 1,3-H eliminations on the other hand are minor processes and exclusively follow the roaming mediated nonadiabatic dynamics. The probability of elimination of the hydrogen atom from two terminal groups (terminal hydrogen elimination) is greater than that from the internal CH group (internal hydrogen elimination). Almost 83% of atomic hydrogen elimination occurs through the asynchronous concerted mechanism from the terminal carbon atom via triple dissociation leading to CH + CH + H products. This finding is in good agreement with a recent experimental observation. The present TSH study indicates that approximately one-third of the trajectories those resulted in a triple dissociation channel, CH + CH + H completed in the ground singlet state following a nonadiabatic path (hopping from the first excited singlet S to the ground state S) via the C-C and C-H dissociation coordinate conical intersection S/S. The products CH(1 A) + CH(A) + H, obtained are ground state methyl radicals and ground state ethylene. The trajectories those ended in a triple dissociation channel CH + CH + H adiabatically in the S state lead to CH(1 A) + CH (1 B) + H, where singlet methyl radicals and triplet ethylene are formed in their corresponding lowest electronic state via a spin conserving route. Two channels, CH + CHCH and CH + CH are found to have minor contributions. In the case of methane elimination, the trajectories that follow an adiabatic path lead to CHCH(1 A″) + CH(1 A), where ethylidene is in the excited state and methane is in the ground state. Methane elimination via nonadiabatic path leads to CHCH(1A) + CH(1 A), where both ethylidene and methane are in the ground electronic state. Ethane eliminations follow the adiabatic path leading to CH(1 A) + CH(1 B) where ethane is in the ground state and methylene is in the first excited state.
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