Theoretical Investigations of Unimolecular and Bimolecular Reaction Dynamics in Gas Phase.

Abstract

For decades, classical trajectory simulations have been used to determine reaction mechanisms, energy flow pathways, product branching ratio, etc. Though atoms and molecules are quantum mechanical in nature, classical mechanics is used because of the inherent computational complexities associated with full quantum dynamics calculations. In a classical trajectory simulation, Newton’s or Hamilton’s equations of motion are time evolved using an appropriately selected set of initial coordinates and momenta. The time propagated coordinates and momenta are used to compute final properties of the system. A crucial aspect of trajectory integrations is selecting an appropriate potential energy surface. Conventionally, this is done with model potentials (Classical force fields) and such calculations are fast but limited by accuracy. With the advancement in parallel computing techniques and sophisticated algorithms, it is possible to compute the required potentials and gradients (for trajectory integration) from a suitable electronic structure theory. Such an on-the-fly approach known as direct dynamics- is quite popular today and has led to identification of new mechanisms and pathways. Combining this method with electronic structure calculations, few unimolecular and bimolecular reactions were modeled in the present work. Selected reactions are of interest in combustion and interstellar chemistry. The first reaction investigated was the bimolecular collision dynamics of H₃ CO in the gas phase. The bimolecular reaction of H₃ + CO is one of the cornerstone chemical processes in the interstellar media. The products of this reaction are either formyl (HCO+) or isoformyl (HOC+) cation along with H₂ molecule. These are barrier-less proton transfer and exoergic processes which results in the two isomers via ion-dipole complex formation. The reaction products are known to initiate the formation of important organic molecules in the interstellar media. Several experimental and theoretical investigations of the reaction probing structure and energetic, reaction mechanism, product branching ratios, and HCO+ = HOC+ isomerization have been reported. Ionic products of this reaction initiate different reactions networks in the interstellar media and their relative abundance in the space is a crucial quantity of interest. Direct dynamics simulations of h₃ + CO bimolecular reaction were performed using density functional PBE0/aug-cc-p VDZ level of theory to model a recently reported velocity imaging experimental studies of the same reaction. Reaction mechanisms, branching ratios, product energy and scattering angle distributions were computed from the trajectory data. Results are in qualitative agreement with experiments and detailed atomic level mechanisms are presented. The second reaction studied was the unimolecular dissociation of y- ketohydroperoxide (KHP). Y- ketohydroperoxide [(3-hydroperoxy)propanal] is an important reagent in synthetic chemistry. KHP is considered to be the primary source of radicals in low temperature combustion. Automated reaction discovery methods were utilized previously to study the unimolecular decomposition pathways of KHP. In the present work, direct chemical dynamics simulations at the B3LYP/6-31+G* level of theory were performed to model the unimolecular decomposition of KHP identifying important dissociation pathways. Simulations were carried out at three different total energies mimicking thermal reaction conditions. Three dissociation channels among the previously reported pathways were identified to be important. Korcek decomposition, which was proposed earlier as a source of carbonyl compounds from thermal decomposition of KHP, was not observed in the present high-temperature simulations. However, trajectories showed the formation of carbonyl compounds such as aldehydes via other pathways. Further, Rice-Ramsperger-Kassel-Marcus (RRKM) rate constants were computed and compared with the trajectory data. The third reaction studied was the unimolecular dissociation of Thiophene. Thiophene is an organo-sulfur aromatic molecule present in fossil fuels and alternate fuels such as shale oils and contributes to air pollution via fuel burning. Hence, it is essential to remove thiophene and its derivatives during the refining process. In this regard, experimental and electronic structure theory studies investigating the thermal decomposition of thiophene have been reported in the literature. In the present work, high temperature thermal decomposition of thiophene was investigated. The trajectory integrations were performed on-the-fly at the density functional B3LYP/6-31+G* level of electronic structure theory to investigate the atomic level decomposition mechanisms. Simulations results show that C-S cleavage accompanied by an intramolecular proton transfer to C is the dominant initial dissociation step. Acetylene was observed as the primary decomposition product and the results are in agreement with previous experimental studies.