Post-transition State Dynamics for Propene Ozonolysis: Intramolecular and Unimolecular Dynamics of Molozonide
The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. A direct chemical dynamics simulation, at the B3LYP/6-31G(d) level of theory, was used to study the post-transition state intramolecular and unimolecular dynamics for the O₃+propene reaction. Comparisons of B3LYP/6-31G(d) with CCSD(T)/cc-pVTZ and other levels of theory show that the former gives accurate structures and energies for the reaction’s stationary points. The direct dynamics simulations are initiated at the anti and syn O₃+propene transition states (TSs) and the TS symmetries are preserved in forming the molozonide intermediates. Anti↔syn molozonide isomerization has a very low barrier of 2–3 kcal/mol and its Rice-Ramsperger-Kassel-Marcus (RRKM) lifetime is 0.3 ps. However, the trajectory isomerization is slower and it is unclear whether this anti↔syn equilibration is complete when the trajectories are terminated at 1.6 ps. The syn (anti) molozonides dissociate to CH₃CHO+H₂COO and H₂CO+syn (anti) CH₃CHOO. The kinetics for the latter reactions are in overall good agreement with RRKM theory, but there is a symmetry preserving non-RRKM dynamical constraint for the former. Dissociation of anti molozonide to CH₃CHO+H₂COO is enhanced and suppressed, respectively, for the trajectory ensembles initiated at the anti and syn O₃+propene TSs. The dissociation of syn molozonide to CH₃CHO+H₂COO may also be enhanced for trajectories initiated at the syn O₃+propene TS. At the time the trajectories are terminated at 1.6 ps, the ratio of the trajectory and RRKM values of the CH₃CHO+H₂COO product yield is 1.6 if the symmetries of the initiation and dissociation TSs are the same and 0.6 if their symmetries are different. There are coherences in the intramolecular energy flow, which depend on molozonide’s symmetry (i.e., anti or syn). This symmetry related dynamics is not completely understood, but it is clearly related to the non-RRKM dynamics for anti↔syn isomerization and anti molozonide dissociation to CH₃CHO+H₂COO. Correlations are found between the stretching motions of molozonide, indicative of nonchaotic and non-RRKM dynamics. The non-RRKM dynamics of molozonide dissociation partitions vibration energy to H₂COO that is larger than statistical partitioning. Though the direct dynamics simulations are classical, better agreement is obtained using quantum instead of classical harmonic RRKM theory. This may result from the neglect of anharmonicity in the RRKM calculations, the non-RRKM dynamics of the classical trajectories, or a combination of these two effects. The trajectories suggest that the equilibrium syn/anti molozonide ratio is approximately 1.1–1.2 times larger than that predicted by the harmonic densities of state, indicating an anharmonic correction.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 921851
- Journal Information:
- Journal of Chemical Physics, 125:014317 1-16, Vol. 125, Issue 1; ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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