Near-Quantitative Predictions of the First-Shell Coordination Structure of Hydrated First-Row Transition Metal Ions Using K-Edge X-ray Absorption Near-Edge Spectroscopy

The solvation structure of transition metal ions is important for applications in geochemistry, biochemistry, energy storage, and environmental chemistry. We study the X-ray absorption pre-edge and near-edge spectra at the K-edge of a nearly complete series of hydrated first-row transition metal ions with d orbital occupancy from d(2) to d(10). We optimize all of the structures at the density functional theory (DFT) level with explicit solvation and then compute the pre-edge X-ray absorption spectra with time-dependent DFT (TDDFT) and restricted active space second-order perturbation theory (RASPT2). TDDFT provides accurate results for spectra that are dominated by single excitations, while RASPT2 correctly distinguishes between singly and doubly excited states with quantitative accuracy compared with experiment. We analyze the pre-edge features for each metal ion to reveal the impact of the variations in d orbital occupancy on the first-shell coordination environment. We also report the lowest-energy ligand field d-d transitions using complete active space second-order perturbation theory.