Computational Electrochemistry of 3d Transition Metal Complexes

Abstract

The topic of this thesis is the computational quantum chemical (QC) description of homogeneous first-row (3d) transition metal (TM) electrocatalysis. This branch of chemistry holds great potential for employing Earth-abundant 3d TMs in renewable energy concepts. Therefore, routine predictions for the reactivity of 3d TM electrocatalysts are desirable, but due to numerous challenges, they are only possible to a limited extent. The thesis describes the development, assessment, and application of QC methods with the aim of such predictions. <br /> In the first Chapter, an introduction to the QC treatment of 3d TM electrocatalysis is given, followed by a brief overview of the different QC methods in the second Chapter. <br /> The computationally most affordable methods are the semiempirical quantum mechanical (SQM) methods, which are the topic of the third Chapter, where the inclusion of spin-polarization in the extended tight-binding Hamiltonian (xTB) is elaborated. The next higher QC level is density functional theory (DFT), which is the topic of Chapter four. Here, the extension of the non-empirical r²SCAN density functional approximation (DFA) to the hybrid level, resulting in the r²SCANh, r²SCAN0, and r²SCAN50 DFAs, is described. At the highest DFT level are the double-hybrids (DHs), which are the subject of Chapter five. Their applicability is extended with the domain-based local pair natural orbital (DLPNO) approximation for second-order Møller–Plesset perturbation theory (MP2). The highest level belongs to the wave function theory (WFT) methods. Their application can face severe difficulties in 3d TM electrocatalysis due to multireference (MR) character, which is the subject of the sixth Chapter. Here, the recognition of MR systems and the calculation of their ionization potentials (IPs) is studied at the highest feasible WFT level. For this purpose, a new benchmark set of electrocatalysts, termed 3dTMV, is compiled, and coupled cluster calculations (CCSD(T)) as well as quantum Monte Carlo (ph-AFQMC) calculations were conducted. Chapter seven deals with the application of SQM and DFT methods for the elucidation of electrocatalytic cycles with three-legged piano-stool iron complexes. An efficient workflow is presented for the calculation of Gibbs free energies yielding a free energy map that is used to propose an initial catalytic cycle. The extension of the free energy map to also include kinetics by transition state theory is shown in Chapter eight. Finally, in the ninth Chapter, the findings of this work are summarized, and their impact on the theoretical description of 3d TM electrocatalysis and 3d TM chemistry in general, are evaluated. Novel QC workflows can benefit from the methods and findings presented in this work and accelerate the discovery of efficient (electro-)catalysts employing Earth-abundant 3d transition metals.