Date on Master's Thesis/Doctoral Dissertation

12-2023

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Kozlowski, Pawl

Committee Member

Buchanan, Robert

Committee Member

Thompson, Lee

Committee Member

Sathitsuksanoh, Noppadon

Author's Keywords

intermediate omega; cobaloxime; Co-C bond dissociation; CASSCF/NEVPT2; dimethylamination of aromatic compounds; Fe-C bond dissociation

Abstract

The efficient application of DFT and TD-DFT has been harnessed to study bond-breaking processes in some molecules which play a prominent role in enzymatic reactions. The first application includes Radical S-adenosyl methionine (SAM) enzymes which are fundamentally important sources of organic radicals to initiate diverse radical reactions. Recently a bio-organometallic intermediate (Ω) that contains an Fe‒C bond has been characterized and shown to be a common feature of radical SAM enzymes. The strength of Fe‒C bond in Ω has been computed using broken-symmetry density functional theory (BS‒DFT). Additionally, Fe‒C bond dissociation energy (BDE) in Ω has been compared to that with S‒C bond in SAM and Co‒C bond in adenosylcobalamin (AdoCbl). The second application includes Ethyl(aqua)cobaloxime (Cbx) which is commonly used as a model compound to study vitamin B12 derivatives such as AdoCbl and methylcobalamin (MeCbl). The mechanism of Co‒C bond photodissociation in Cbx in presence of a benzophenone (acting as photocatalyst, PC) has been investigated by means of time-dependent density functional theory (TD-DFT). The calculation of low-lying singlet and triplet excited states of the substrate and PC reveal that the photodissociation process is mediated by a repulsive triplet state via Dexter energy transfer from PC to Cbx. Lastly, DFT and TD‒DFT have been applied to investigate photo-assisted cleavage of N, N-dimethylformamide (DMF). Potential energy surfaces (PES) have been constructed using DFT to investigate the cleavage of C‒F bond of perfluoronaphthalene (PFN) and simultaneous attachment of DMF N-atom to C-atom. Excited state PESs, which were calculated using TD‒DFT, reveals that the energy barrier for electron transfer from DMF to PFN is significantly reduced in presence of light. This enables a redistribution of the electronic charge between the DMF and PFN leading to accelerated reductive cleavage of the C‒F bond and subsequent quick reaction between radical ions to give amination products.

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