PNNL

Abstract

Understanding the solvation structure of transition metal ions is important due to its broad impact on the kinetics, stability, and reactivity in a variety of applications in geochemistry, biochemistry, energy storage, and environmental chemistry. Using water as the common ligand, we study the X-ray absorption pre-edge and near-edge spec- tra at the K-edge of a near-complete series of hydrated first-row transition metal ions with d-orbital occupancy ranging from d2 to d10. Starting with optimized structures that were derived from an explicit solvation treatment at the density functional the- ory (DFT) level, we then compute the pre-edge X-ray absorption spectra at the metal ion K-edge at the time dependent density functional theory (TDDFT) and restricted active-space second-order perturbation theory (RASPT2) levels of theory. TDDFT cal- culations provide accurate results for spectra that are dominated by single excitations, while signicant improvements were obtained with RASPT2 calculations that correctly distinguish between singly and doubly excited states, where relevant, with quantita- tive accuracy compared with experiment. We analyze and assign the dierent pre-edge features for each metal ion in order to reveal the impact of the variations in the d or- bital occupancy on the rst-shell coordination environment. For completeness, we also report the lowest energy ligand-eld d-d transitions for all the transition metal aqua ions considered in this study using complete active space second order perturbation theory (CASPT2).