Iron-sulfur bond covalency from electronic structure calculations for classical iron-sulfur clusters

Area of Science:

• Inorganic Chemistry
• Computational Chemistry
• Biochemistry

Background:

• Iron-sulfur (Fe-S) clusters are vital in biological systems, influencing electronic, magnetic, and reactive properties.
• X-ray absorption spectroscopy (XAS) provides experimental bond covalency data, serving as a benchmark for theoretical calculations.
• Theoretical bond covalency is sensitive to population analysis methods, a factor often overlooked.

Purpose of the Study:

• To evaluate the impact of different population analysis methods on the theoretical bond covalency of iron-sulfur clusters.
• To identify the most reliable computational approach for determining the electronic structure of Fe-S clusters.
• To compare theoretical findings with experimental XAS data for iron tetrathiolates and classical [2Fe-2S] and [4Fe-4S] clusters.

Main Methods:

• Utilized ab initio wave function-based (QCISD) calculations and experimental S K-edge XAS.
• Investigated various population analyses: Mulliken, Minimum Basis Set Mulliken, Natural, Coefficients-Squared, Hirshfeld, and "Atoms in Molecules" (AIM) theory.
• Analyzed iron tetrathiolates and classical [2Fe-2S] and [4Fe-4S] clusters with thiolate ligands.

Main Results:

• Orbital compositions varied significantly based on the population analysis method used (frontier orbitals, spin densities, AIM theory).
• The AIM theory, a triple-ζ basis set, and the B(5%HF)P86 hybrid functional provided the most consistent electronic structure.
• Discrepancies were observed between different methods in deriving theoretical bond covalency.

Conclusions:

• The choice of population analysis method critically affects the calculated electronic structure of Fe-S clusters.
• "Atoms in Molecules" (AIM) theory offers a robust approach for electronic structure determination in these systems.
• The optimized computational strategy aids in accurately predicting the properties and reactivity of iron-sulfur clusters.