Trajectory surface hopping study of propane photodissociation dynamics at 157 nm

Abstract

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.

Ultraviolet photochemistry of ethane: implications for the atmospheric chemistry of the gas giants

“…Many of the present interpretations align with the results of recent quasi-classical trajectory surface hopping calculations for the next larger alkane, propane (C 3 H 8 ), following excitation at l ¼ 157 nm, wherein it was concluded that most dissociations occur aer internal conversion to the S 0 PES, that the energy disposal in the resulting fragments is governed by dynamical rather than statistical factors, and that the three-body fragmentation processes occur sequentially. 46 (e) Implications for modelling the atmospheres of the gas giants This work provides detailed new insights into the VUV photochemistry of ethane. The results discussed in detail in the above subsections are summarised below in terms of their implications for modelling the atmospheres of the gas giants.…”

Photodissociation Dynamics of Methyl Hydroperoxide at 193 nm: A Trajectory Surface-Hopping Study

“…In this study, we have used trajectory surface-hopping (TSH) methodology as implemented in the NEWTON-X package . This methodology is similar to one used previously by our group to study the photodissociation of propyne and propane. , In brief, the potential energies, energy gradients, and nonadiabatic couplings are calculated using the COLUMBUS electronic structure program interfaced with NEWTON-X for integrating trajectories. Initially, the molecule is excited to the first singlet (S 1 ) state.…”