Analysis of the stability of transition states in chemical reactions using tools from the Quantum Theory of Atoms in Molecules

Author(s)

Gabriel Kossaka Macedo

• Document type: Doctoral Thesis
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Instituto de Química de São Carlos (IQSC/BT)
• Defense date: 2023-08-10

Examining board members

Roberto Luiz Andrade Haiduke; Ataualpa Albert Carmo Braga; Sergio Emanuel Galembeck; Teodorico de Castro Ramalho; Aguinaldo Robinson de Souza

• Advisor: Roberto Luiz Andrade Haiduke

Abstract

In this work, the stability of transition states in chemical reactions was investigated, focusing on the bonds broken and formed during such processes, evaluating the topology of the electronic density at the bond critical points and their covalent/ionic character. Furthermore, the intra- and interatomic energy contributions, obtained according to the precepts of the Quantum Theory of Atoms in Molecules, were studied in detail. In a first step, the performances of different methods based on the Density Functional Theory (DFT) were investigated, in order to determine which are the best exchange-correlation functionals for describing the electron density and energy partition of transition states. In this case, BB1K showed results in better agreement with the theoretical reference values obtained from CCSD calculations. The investigation of classical reactions was then conducted with such tools, via the use of the BB1K functional. Thus, the results for the second-order elimination reactions (E2) of chloroethane were determined with chloride as the base and with different substituents on the carbon atom bonded to the eliminated hydrogen (H, F and CH3). Data for the reactions of E2 with other bases (NH2-, PH2- and AsH2-) and two leaving groups (Cl- and Br-) were also discussed. Therefore, it was possible to understand more deeply the origin of the variations of reaction barriers in the different cases analyzed, and the results obtained from the atomic energy partition, the topological data and the QTAIM charges complement each other in this process. The most relevant results discussed here were that the atoms that contribute most to composing the electronic barrier of the reaction are the hydrogen that is eliminated, the carbon attached to it, the leaving group, and the base, where almost always the first three terms are destabilizing while the latter is stabilizing. It was also observed that substituents such as methyl lower the reaction barrier and substituents such as fluorine tend to increase the reaction barrier (via a destabilization of the substituent itself), indicating that more electronegative substituents tend to lower the reaction rate and more electropositive substituents tend to increase the reaction rate. It was noted that larger (or weaker) bases tend to increase the reaction barrier and therefore decrease the reaction rate (effect rationalized mainly via destabilization of the eliminated hydrogen and the base). The reason why bromine provides a better leaving group than chlorine, with a reduction in the reaction barrier, was explained from the greater electronic charge gain of the former compared to the latter in the process of transforming reagents into the transition state. (AU)

• FAPESP's process: 18/12675-6 - Stability analysis of transition states in chemical reactions by means of tools from the quantum theory of atoms in molecules
• Grantee: Gabriel Kossaka Macedo
• Support Opportunities: Scholarships in Brazil - Doctorate (Direct)