Photodissociation dynamics of propyne at 193 nm: a trajectory surface hopping study

Abstract

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.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…The results of several early experimental studies of methylacetylene (propyne, H3CCCH) photolysis at 193.3 nm were interpreted as showing acetylenic C-H bond fission as the dominant decay process, [34][35][36] as has also been suggested in one recent trajectory surface-hopping theoretical study. 37 However, H Rydberg atom photofragment translational spectroscopy (HRA-PTS) studies of propyne and its isomer allene (propadiene), at several wavelengths in the range 193.3    213.3 nm, returned essentially identical TKER spectra with a form that matched well with that obtained by assuming an approximate statistical model predicated on population of all possible vibrational states of the propargyl (H2CCCH) product. Such behaviour was rationalised by assuming efficient coupling to high vibrational levels of the S0 state and isomerisation (including H atom migration) prior to fragmentation.…”

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.…”