Wavefunction methods for X-ray processes
Abstract
X-ray spectroscopy is an element-specific probe that can be used as a local probe of electronic structure. With new experimental capabilities through free-electron x-ray lasers and high brilliance synchrotrons, it is possible to collect high-resolution time-resolved spectral data of many transition-metal enzymes and solution catalysts. Theoretical simulations play important roles in designing new experiments, rationalizing results, and assigning electronic structure. We have expanded the use of a method, based on multiconfigurational wavefunction theory, to simulate X-ray spectra of both closed and open-shell systems. With recent implementations of transition intensities beyond the electric dipole approximation, we can now model a large variety of absorption, emission and scattering techniques used, thus opening up new possibilities to understand chemical processes. The coming year we will make fundamental studies of how charge and spin density are expressed in x-ray spectra. We will also target applications with relevance to photon harvesting and water oxidation in solar fuel catalysis. More specifically, we want to continue our studies of electronic and structural dynamics in valence excited states by extending our current X-ray emission spectroscopy approach to new complexes. We will also benchmark new approaches, including those based on density-functional theory to facilitate calculations on a large number of complexes and to enable a quicker prediction process required to help in the design of new experiments. For metal-oxo systems we will look at synthetic manganese(V)-oxo complexes and their precursors to cover the full range of manganese oxidation states and both one-electron and hydrogen-abstraction reactions. We will also use mononuclear manganese models with similar ligand environments as in natural photosynthesis, where structures can be obtained both from time-resolved x-ray diffraction as well as published mechanistic studies. We will model a range of x-ray spectroscopic processes, including L-edge XAS and Kβ XES, to see how they express these electronic structure differences. The right combination of experiments should then make it possible to identify reactive intermediates, even in an environment with multiple metal centers.
