Unbiased Angular Overlap Model parameters from ab initio calculations

• Main Author: Buchhorn, Moritz
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: 2024
• Subjects: Orca
• Online Access: http://tuprints.ulb.tu-darmstadt.de/26712/; https://tuprints.ulb.tu-darmstadt.de/26712/1/dissertation_buchhorn.pdf; https://doi.org/10.26083/tuprints-00026712

In this work, molecular transition metal complexes are investigated with a quantum chemicalmulticonfigurational method in conjunction with ligand field theory (LFT). Ligand field theory is an established tool to parameterize electronic and magnetic properties of transition metal complexes. The parameterization used is the angular overlap model (AOM), which parameterizes the ligand field locally, i.e. each parameter set refers to a specific metal-ligand pair. The advantage of the AOM compared with other ligand field schemes is thus the possibility to assess the effect of particular ligands in a complex instead of relying on global parameters that can only describe whole complexes. The downside of this approach is the high number of parameters, often leading to underdetermined problems that make an AOM fit difficult. Common approaches to solve this issue are artificial relationships and assumptions that introduce biased parameters. In this work, complete active space (CAS) calculations and the ab initio ligand field theory routine implemented in the ORCA quantum chemistry software are used to calculate electronic state energies that are necessary to fit AOM parameters. The calculated data is equivalent to the information obtained by spectroscopic methods that yield the energy difference of excited states, e.g. UV-Vis spectroscopy. The use of computational methods comes with an array of advantages compared to spectroscopic measurements, most importantly the unambiguous assignment of states and the possibility to investigate structural snapshots instead of thermally averaged structures. These properties can be used to create structural samples of low symmetry, which have fewer degenerate electronic states and therefore permit AOM parameterizations that are impossible when only considering experimental data. By this, it is not necessary anymore to avoid AOM underdetermination by fixing parameters or introducing relational rules. It is thus an approach to remove bias and obtain more objective parameter sets. The ...