Vera Krewald

Mössbauer spectroscopy is a key method for characterizing the electronic structures of iron compounds. The resulting Mössbauer parameters show a well-established temperature dependence. For quantum chemical calculations that are often used to aid in the interpretation of Mössbauer spectra, this poses a major challenge: they commonly employ a single, rigid equilibrium structure that is assumed to deliver good results for low temperatures up to ca. 80 K. In this work, we reveal this approach to be a rather severe approximation. We use a structural sampling approach that captures the effect of temperature dependent molecular vibrations on the quadrupole splitting. Even for low temperatures, this approach delivers a significantly improved agreement with experimental data. In addition, the T-dependence of the quadrupole splitting can be qualitatively reproduced for four iron(II) high spin complexes. The results are analyzed in terms of the individual EFG tensor contributions, which shows the origin of the temperature dependence being rooted in different electronic structures with varying contributions at different temperatures. Limitations in the sampling approach and electronic structure method are discussed. The approach presented herein may provide better agreement with experiment in future in situ and operando studies at low, ambient and elevated temperatures.

Combined experimental and theoretical studies shed light on the proton-coupled electron transfer mechanism of a polyoxometalate-based alcohol oxidation in water, highlighting the importance of substrate pre-coordination to the reactive site.

Late transition metal hydroxides are important intermediates in many catalytic processes. In [Ni,Fe] carbon monoxide dehydrogenase enzymes (CODHs), which catalyze the reversible CO/CO2 interconversion, CO insertion in the Ni–(μ–OH) bond of a Ni/Fe-bridging hydroxide is the proposed C–O bond-forming step during CO oxidation. Here, we report that CO reacts with the dinickel(II) complex LNi2(μ–OH) (L3– is a pyrazolato-centered ligand with two {N3} compartments), finally leading to HLNiII(CO); this is coupled to the formation of CO2 and Ni(CO)4, which is the driving force of the overall reaction. Mechanistic details have been investigated by 13CO labeling experiments and DFT calculations, and the product derivative HLNiII(CNBn) has been crystallographically characterized. The findings demonstrate that a nickel(II)-bound μ–OH can serve as a nucleophile toward CO and promote its oxidation to CO2, which is suggested to proceed via a metal-bridging O-protonated carbonite intermediate, emulating a key step of [Ni,Fe]-CODH reactivity. For CO2 release, in the present system, one of the nickel ions serves as a terminal electron sink and the corresponding ligand {N3} site of L3– as a proton acceptor.

Both electron transfer (ET) and proton transfer (PT) are common steps in energy conversion across chemistry and biology. Coupling of these two transfer events reduces the energy demand relative to either individual process, but introduces a fundamentally new process—proton‐coupled electron transfer (PCET)—with its own demands for a suitable theoretical description. Conceptualization of PCET usually involves a square scheme representing ET, PT, and PCET steps, each generating states with different energies. While intuitive, these square schemes do not offer structural insights such as the identities and contributions of nuclear motions to PCET. Herein, we present a computational approach that maps these square schemes onto real space coordinates (Å), from which ground and excited‐state potential energy surfaces can be generated. This mapping involves the identification of PT and ET coordinates and reconstruction of the potential energy surfaces in orthogonalized coordinates. We find qualitative differences in key features of the surfaces for two distinct processes within PCET, namely concerted proton–electron transfer and hydrogen atom transfer, which may help to distinguish different PCET scenarios.

Both electron transfer (ET) and proton transfer (PT) are common steps in energy conversion across chemistry and biology. Coupling of these two transfer events reduces the energy demand relative to either individual process, but introduces a fundamentally new process—proton‐coupled electron transfer (PCET)—with its own demands for a suitable theoretical description. Conceptualization of PCET usually involves a square scheme representing ET, PT, and PCET steps, each generating states with different energies. While intuitive, these square schemes do not offer structural insights such as the identities and contributions of nuclear motions to PCET. Herein, we present a computational approach that maps these square schemes onto real space coordinates (Å), from which ground and excited‐state potential energy surfaces can be generated. This mapping involves the identification of PT and ET coordinates and reconstruction of the potential energy surfaces in orthogonalized coordinates. We find qualitative differences in key features of the surfaces for two distinct processes within PCET, namely concerted proton–electron transfer and hydrogen atom transfer, which may help to distinguish different PCET scenarios.

Ferrocene-based radicals and diradicals with ligand-centered spin density are highly promising building blocks for extended magnetic materials. In addition, the unpaired electrons may dress such systems with intriguing ligand-centered chemical reactivity that can be utilized for structural diversification of the functional material. Just like their inorganic analogues, metallonitridyls and metallonitrenes, nitrogen-centered organic radicals and diradicals, like iminyls and nitrenes, are particularly appealing spin centers because they offer an elegant entry into a wide spectrum of chemical reactivities ranging from remote CC-functionalization and radical relay to CH-amination and amidation. Here, we report on the photochemical generation of an exceptional metal-ligand ferrocene diradical from a photolabile diamagnetic azidoferrocene precursor. The diradical product has a triplet electronic ground state and contains the organic cyclopentadiene-iminyl neutral radical ligand (S = 1/2) that is ferromagnetically coupled to a low-spin (S = 1/2) iron(I) center. This unique species appears within 13 ps after optical excitation of the precursor and is formed with a primary quantum yield of 50%. Within less than 1 ps after impulsive electronic excitation in its metal-to-ligand charge-transfer region, the azidoferrocene finds itself in an excited quintet azide-ππ* state, which is electronically preconfigured to adiabatically release dinitrogen and to form a quintet iminyl intermediate. The latter then undergoes a rapid intersystem crossing to the triplet iminyl final product. The presence of the terminal iminyl moiety was verified unambiguously by conducting the optical excitation in the presence of an isonitrile quencher, thereby trapping the photochemical diradical product as a ferrocenyl carbodiimide.

In mechanistic studies of Ru-catalyzed olefin metathesis using UV/Vis spectroscopy, the origin of multiple bands has been elusive. This study uncovers the origin of those bands and assigns them to previously unknown Ru species that arise during oxygen-induced decomposition of the active catalyst. Pathways for such decomposition have been proposed, but spectroscopic evidence of the respective Ru species has not yet been reported. Therefore, in a multispectroscopic and theoretical approach to mechanistic investigation, using operando nuclear magnetic resonance (NMR), operando UV/Vis and operando combined NMR-UV/Vis spectroscopy, high-resolution mass spectrometry (HR-MS), and time-dependent density functional theory, we connect two open strands in metathesis literature: (i) explaining the origin of the previously unassigned UV/Vis bands and (ii) experimentally substantiating and refining the proposed pathway for oxygen-induced decomposition. With that, we propose the occurrence of the 505 and 640 nm UV/Vis bands as a convenient spectroscopic indicator for oxygen-induced decomposition occurring in Ru-catalyzed olefin metathesis.

Electron transfer between strongly coupled redox centers has posed several challenges for decades, including the relative time scales of spectroscopic measurements and the electron transfer event itself and the interplay of a mixed-valent solute and its solvation shell regarding the degree of electronic (de-)localisation. Using multi-scale atomistic simulations of the most prominent example of a strongly coupled mixed-valent transition metal complex, the Creutz–Taube ion, (𝜇–pyrazine) [Ru(NH ₃ ) ₅ ] ₂ ⁵⁺ , we provide an independent perspective on the roles that intramolecular vibrations and solvent motion play on its electronic structure and electron transfer events. To analyze the explicitly solvated system in atomistic detail, we developed a new methodology that yields a spectrum of electron transfer rates as a Fourier transform of the cross-correlation function of the spin populations at the redox centers. Surprisingly, the analysis reveals that multiple electron transfer mechanisms are operative, and that solute vibrations and solvent reorganization act independently to affect electron transfer. The methodology presented herein is general and transferrable to other electron transfer systems.

Opaque computational simulations pose a distinctive epistemic challenge for learners: they must evaluate outputs generated by processes they cannot directly inspect. How students navigate this condition in situated discourse remains insufficiently understood. This study examined this challenge in the context of molecular dynamics (MD) simulations, where well-established physical principles are implemented through computational procedures that remain inaccessible to students. Eight undergraduate chemistry students completed a five-hour Jupyter Notebook-based laboratory session on MD simulations and subsequently participated in three semi-structured focus group interviews. The interviews were analyzed through structured qualitative content analysis, guided by the Epistemic Practices in Opaque Computational Learning Environments – Chemistry and Modeling (EPOCLE-CM) analytical framework, yielding 189 coded segments across five epistemic dimensions and a supplementary Domain Knowledge category. Three configurations of epistemic practice emerged: Reflective Model Evaluation, Boundary-Based Model Critique, and Calibrated Trust. Trust did not function as a stable attitude but was produced through sustained reasoning about the simulation–experiment relationship: students calibrated their reliance on outputs by reflecting on representational scope, model boundaries, and conditions under which simulation results could count as evidence (φ = .509 between trust calibration and simulation–experiment reflection). For instruction, boundary testing and deliberate use of simulation breakdowns emerged as productive entry points for fostering epistemic engagement with opaque computational tools. As computational systems become less transparent, understanding how learners navigate opacity is relevant beyond chemistry education.