Trajectory surface hopping study of propane photodissociation dynamics at 157 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 H₂ elimination from the internal carbon atom (2,2-H₂ elimination) and terminal carbon atom (1,1-H₂ elimination) is the major process and follows a three centred synchronous concerted mechanism. 1,2-H₂ and 1,3-H₂ 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 CH₂ 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 CH₃ + C₂H₄ + 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, CH₃ + C₂H₄ + H completed in the ground singlet state following a nonadiabatic path (hopping from the first excited singlet S₁ to the ground state S₀) via the C-C and C-H dissociation coordinate conical intersection S₁/S₀. The products CH₃(1 ²A₂^″) + C₂H₄(¹A_g) + H, obtained are ground state methyl radicals and ground state ethylene. The trajectories those ended in a triple dissociation channel CH₃ + C₂H₄ + H adiabatically in the S₁ state lead to CH₃(1 ²A₂^″) + C₂H₄ (1 ³B₁) + H, where singlet methyl radicals and triplet ethylene are formed in their corresponding lowest electronic state via a spin conserving route. Two channels, CH₄ + CH₃CH and C₂H₆ + CH_2, are found to have minor contributions. In the case of methane elimination, the trajectories that follow an adiabatic path lead to CH₃CH(1 ¹A″) + CH_4,(1 ¹A₁), where ethylidene is in the excited state and methane is in the ground state. Methane elimination via nonadiabatic path leads to CH₃CH(1¹A^') + CH₄(1 ¹A₁), where both ethylidene and methane are in the ground electronic state. Ethane eliminations follow the adiabatic path leading to C₂H₆(1 ¹A_1g) + CH₂(1 ¹B₁) where ethane is in the ground state and methylene is in the first excited state.