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Electronic Structure Assessment: Combined Density Functional Theory Calculations and Ru L2,3-Edge X‑ray Absorption Near-Edge Spectroscopy of Water Oxidation Catalyst
journal contribution
posted on 2016-02-18, 21:14 authored by Igor Alperovich, Dooshaye Moonshiram, Javier J. Concepcion, Yulia PushkarDensity
functional theory (DFT) is now widely used for analysis
of the electronic structure and reactivities of transition metal complexes.
However, large
variability in how well different combinations of exchange-correlation
potentials/basis sets reproduce real molecular geometries and electronic
configurations remains a problem. Experimental X-ray absorption near-edge
structure (XANES) spectra directly reflect the electronic structure
of transition metal complexes. Combined analysis of theoretical calculations
and experimental data is highly beneficial for DFT validation as well
as for understanding limitations of the DFT. Ruthenium-based molecular
water oxidation catalyst cis,cis-[(bpy)2(H2O)RuIIIORuIVO(OH)(bpy)2]4+ is a complex coordination compound with two Ru centers
in different oxidation states bound by μ-oxo bridge. Multiple
DFT calculations of this catalyst in different oxidation states have
been reported previously but it was never clear whether DFT is truly
capable of describing its geometry and electronic structure. We tested
a variety of DFT potentials with two relativistic models for electronic
structure calculations and simulated experimental Ru L2,3-edges XANES spectra. We found that the local density approximation
(LDA) exchange-correlation potential reproduces the experimental geometry
and XANES spectra, while more advanced potentials fail. We also report
combined DFT calculations/Ru L2,3-edges XANES spectra for
three protonation forms: cis,cis-[(bpy)2(H2O)RuIIIORuIVO(H2O)(bpy)2]5+, cis,cis-[(bpy)2(H2O)RuIIIORuIVO(OH)(bpy)2]4+, and cis,cis-[(bpy)2(OH)RuIIIORuIVO(OH)(bpy)2]3+. We
found that selected DFT technique captures well small modifications
of the electronic structure caused by changes in the protonation state
of the ligand. Small differences in the Ru L-edges XANES for various
protonation states are reflected in the corresponding theoretical
spectra. We also observed that overall DFT-based XANES modeling is
sensitive to various subtle changes in molecular geometries of Ru
complex.