Project A - Disentangling catalysis at real surfaces
The project involves a new approach to the measurement of chemical reaction rates at surfaces, a topic of great relevance to heterogeneous catalysis. We have recently succeeded in measuring the specific rates of reactions at specific active sites on a catalytic surface - see Nature, (2018) DOI : 10.1038/s41586-018-0188-x, where we report step and terrace specific reaction rate constants for CO oxidation on Pt. We intend to systematically obtain kinetic data for important catalytic reactions involving multiple reaction sites and in cooperation with theoretical collaborators construct accurate atomic scale dynamical descriptions and detailed maps of the elementary steps of the reaction mechanisms.
Project B – Barriers of protonation reactions of organometallics
Typical organometallics, such as Grignard reagents RMgX, are notorious for their complexity in solution: they easily switch between different aggregation and coordination states (so-called Schlenk equilibria). This dynamic behavior severely complicates any kinetic analysis in solution. For ionic species, the latter problem can be circumvented by transferring them into the gas phase via electrospray ionization and isolating them by tandem mass spectrometry. The project will use this approach for examining the microscopic reactivity of a wide range of organometallic ions toward proton donors as model electrophiles. The experimentally obtained rate constants shall then serve as benchmarks for theory. In combination, experiment and theory, thus, promise to unravel the mechanism of prototypical organometallic reactions and afford fundamental insight relevant to synthesis and catalysis.
Project C - Thermochemistry of proton-coupled electron transfer reactions involving 3d metal ions
Redox reactions are often, if not most, coupled to proton transfer events. Such proton-coupled electron transfer (PCET) reactions are important in chemical energy conversion reactions as the water oxidation reaction or in fuel cells, which are usually mediated by metal compounds. The underlying elementary steps are also commonly subject to quantum chemical benchmarking, from ionization to electron attachment, or even hydrogen atom transfer. The latter reference data are, however, far from the systems and the conditions commonly featured in catalysis, focusing on gas phase reactivity with light main-group elements. In the BENCh project we will explore PCET reactions involving 3d metal/ligand reactivity in solution. The solution thermochemistry of the elementary steps of formally spin forbidden and spin allowed PCET reactions will be investigated in different, aprotic solvents as well as the solution BDFE of the HAT reaction.
Project D - Hydrogen atom transfer reactivity of 5d oxo- and nitrene species
Heavy (5d) transition metals (TM) are important for many catalytic transformations, such as oxo/nitrene transfer. Owing to the transient character of the TM oxo/nitrene key intermediates, mechanistic evaluation of such reactions heavily relies on computational analysis. We want to benchmark the thermochemistry of 5d TM complexes relevant to catalysis, specifically with respect to spin-orbit relativistic contributions, which can be as large as 50 kcal/mol for bare transition metal ions in the gas phase. We will systematically evaluate the thermochemistry of hydrogen atom transfer to late 5d TM oxo/imido complexes as a key reaction to oxo/nitrene transfer catalysis via a radical rebound mechanism. This will allow for assessing the relevance of relativistic spin-orbit effects in the condensed phase and benchmarking of the computational methods.
Project H - Chirality recognition as an experimental benchmarking tool
Chiral molecules recognize the relative chirality of partner molecules through subtle intermolecular interactions. This molecular handshake has spectroscopic, thermodynamic and kinetic implications for the molecular pairs that are best explored at low temperature in the gas phase. One can even smell the chirality of molecules in favorable cases. By studying these molecular recognition effects using unique vibrational spectroscopy setups in the Suhm group, one can test the performance of electronic structure methods quite rigorously, down to the sub-kJ/mol level. It has even been speculated that there may be new kinds of chirality-dependent forces based on spin polarization. In any case, there are intriguing competitions between attractive and repulsive forces to be discovered and concepts like chirality discrimination, chirality induction and chirality synchronization to be dissected using terpenes, lactates, and other chiral model systems.
Project I – Understanding environmental effects on molecules – step by step
Quantum chemistry is best at calculating isolated molecules at 0 K. Chemists like to study them with some thermal excitation, embedded in a solvent. We aim to provide bridges between these two worlds. Supersonic jet spectroscopy offers rigorous answers for well-defined partial problems such as stepwise solvation and mode-selective thermal excitation. In this project, interesting model systems are put under spectroscopic scrutiny by adding weak solvent perturbations and by exciting selected vibrational modes. Weak hydrogen bonding, vibrational mode coupling and shock heating will be explored to tame the complexity of environmentally influenced molecules and to benchmark both electronic structure and nuclear dynamics theory.
Project T1 - New approaches to electronic structure diagnostics
How can one work towards a more predictive, robust use of computational procedures for chemical simulations? Part of the solution is understanding the caveats of current methods, starting with the electronic structure. In this project, we explore a series of local orbital descriptions to analyze and diagnose wave function and density functional theory approaches in the computation of a wide range of properties and systems, ranging from non-covalent interactions in the gas phase to chemical reactivity in solution. The successful candidates will come in contact with state-of-the-art electronic structure methods and embedding techniques for the description of molecules in condensed phases.
Project T2 - Molecular Dynamics with Machine Learning Potentials
The simulation of dynamical processes in chemistry is still a substantial challenge because of the high complexity of realistic structural models and because of the need to reach statistically converged results. In spite of the availability of high performance supercomputers the use of ab initio molecular dynamics simulations is still prohibitively expensive for many interesting problems. In recent years, machine learning potentials have become promising new tools to transfer the accuracy of first principles methods to larger time and length scales. In this project we seek candidates to develop and apply high-dimensional neural network potentials to solve chemical problems in different fields like heterogeneous catalysis, vibrational spectroscopy and chemistry in solution.