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Theoretical Investigations on the Mechanisms for Several Hydrogen Abstraction Reactions

Author: LiNa
Tutor: LiJiLai；HuangXuRi
School: Jilin University
Course: Physical and chemical
Keywords: rate constants electron transfer reaction mechanism de-alkylation hydrogen transfer
CLC: O643.12
Type: Master's thesis
Year: 2011
Downloads: 23
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Abstract

Hydrogen transfer reaction is a fundamental issue in chemistry, which plays important roles in synthetic chemistry, gas phase molecular reactions, enzymatic reactions, proton and energy transfer processes, and so on. The Microscopic mechanism exploration is one of the hot topics currently. The utilization of modern theoretical chemistry calculation methods, in-depth study of hydrogen transfer reaction mechanism for further experimental studies, can provide valuable theoretical clues and supports and is of great significance for the exploration of life processes. This thesis covers the following:1. The reaction of C2 (A3Πu) with C2H6 has been investigated using coupled-cluster theory, density functional theory and ab initio dynamics methods. The rate constants are calculated over a wide temperature range of 50–3000 K, using the TST, CVT, CVT/SCT methods at the BMC–CCSD//BB1K/6–311+G(2d,2p) level of theory. The results demonstrate that the variational effect is small in high temperature range and is negligible in low temperature region. However, quantum effect plays an important role in rate constant calculations of the H-abstraction: the nonclassical reflection is predominant when T > 200K, while the tunneling effect is predominant in low temperature region T < 100K. The CVT/SCT rate constants are in good agreement with available experimental data and the normal Arrhenius expression is k(T) = 2.6996×10-11exp(–1034.17/T) cm3 molecule-1 s-1 between 298K and 673K. Our theoretical study is expected to gain further insight into the reaction dynamics behavior over a wide temperature range, in particular, the range where no experimental data are available so far.2. A thorough theoretical insight on AlkB-meciated oxidative demethylation in DNA Repair of the alkylation adenine, 1?meA were explored thoroughly by three different density functional theory, B3LYP, TPSSh, and PBE0. The overall mechanism is characterized by two steps: 1) hydrogen?atom abstraction from the substrate R–H via transition state TSHabs that leads to an iron(III)–hydroxyl species that is weakly bound to an alkyl radical RC? (intermediate I), and 2) hydroxyl back-transfer to the radical RC? via transition state TSReb to yield an iron(II) center and the hydroxylated product, R–OH. Theζ?πpathways of HS (SFe = 2), theπ?ζpathways of HS and theπ?ζpathways of IS (SFe = 1) pathways are found in the current research. We also found the hydrogen bond (HB) would be formed as the substrates 1?meA approach the Fe=O centre. To clarify the HB effects in the C?H bond activation, we also calculated the iron?oxo enzyme with the modified substrate 1?meA, in which the ?NH2 group was replaced by–H. Compared with the substrates without HB formed with iron(IV)–oxo, the newly formed H?bond with entirely exposed terminal oxo in the open core iron(IV)–oxo systems may exert novel influences on the C?H bonds activation. Our theoretical study is expected to provide helpful information for further research.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Chemical kinetics,catalysis > Chemical kinetics > The mechanism and kinetics of chemical reactions
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