Quantum Mechanical Study of Spin-Forbidden Reactions in Organometallic Catalysis and Astrochemistry
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Authors
Mirzanejad, Amir
Issue Date
2022
Type
Dissertation
Language
Keywords
Astrochemistry , Morse potential , Organometallic Catalysis , Spin-Forbidden Reactions
Alternative Title
Abstract
This dissertation presents a quantum mechanical investigation of spin-forbidden reactions in organometallic C-H bond activation and interstellar peptide-bond formation, as well as the derivation of the Morse empirical potential from an electrostatic model. In the first part, the role of triplet electronic state in spin-accelerated iron-catalyzed C(sp2)-H bond activation in a-benzoquinoline molecule is studied. The C-H activation facilitated by the electron donation from the sigma-bond to the iron dz2 orbital is promoted by the spin-orbit coupling of the triplet state to the quintet and singlet states. The calculated spin-orbit coupling values at the minimum energy crossing point geometries between the quintet-triplet and triplet-singlet potential energy surfaces indicate a strong coupling between these electronic states. This is in contrast to a very small spin-orbit coupling between the quintet and singlet states, responsible for the previously proposed two-state quintet-singlet reaction mechanism. Therefore, we propose a quintet-triplet-singlet multi-state mechanism that is thermodynamically more favorable than the two-state mechanism and the spin-allowed quintet pathway. The transition state (TS) in the quintet pathway is associated with a singly occupied dz2 orbital that accepts the C-H bond electron density, while a more stabilized TS in the multi-state mechanism is accessible via unoccupied dz2 orbital in the singlet state. In the second part, the spin-forbidden formation mechanisms of the acetamide and N-methyl-formamide (NMF) molecules in the interstellar medium are studied. Due to low temperature and particle densities in the interstellar medium, only barrierless bimolecular collisions lead to the formation of new molecular species. Two radiative association pathways are proposed for acetamide formation. The reaction mechanism proposed for the NMF formation is more complex and characterized by the formation of an intermediate species followed by an intramolecular hydrogen transfer. Moreover, we proposed a new formation mechanism for acetimidic acid, a molecule that is challenging to observe in the interstellar medium because of its small dipole moment. In the last part, we show that the Morse potential can be derived from a simple screened charge model in which an exponential function accounts for the electron screening of the positive nuclear charges. This derivation bridges the gap between the classical and quantum mechanical description of a chemical bond by connecting the electrostatic and covalent energies to the Morse potential.
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Creative Commons Attribution 4.0 United States