⁵⁷Fe Mössbauer parameters from domain based local pair-natural orbital coupled-cluster theory

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy
• Solid-State Chemistry

Background:

• Accurate calculation of spectroscopic parameters like isomer shifts and quadrupole splittings is crucial for understanding electronic structure in transition metal complexes.
• Iron-57 (⁵⁷Fe) spectroscopy is a key technique for probing the local environment of iron atoms in various chemical systems.
• Existing methods, including density functional theory (DFT), face challenges in accurately predicting these parameters for systems with complex ligands and varying iron states.

Purpose of the Study:

• To apply and evaluate the domain-based local pair-natural orbital coupled-cluster singles and doubles (DLPNO-CCSD) method for calculating ⁵⁷Fe isomer shifts and quadrupole splittings.
• To compare the performance of DLPNO-CCSD with established DFT functionals (RPBE, TPSS, B3LYP, B2PLYP) and second-order Møller-Plesset perturbation theory (MP2).
• To investigate the impact of scalar relativistic effects on the accuracy of electron density calculations for ⁵⁷Fe.

Main Methods:

• Utilized the DLPNO-CCSD method with an analytic derivative scheme to compute electron densities and electric field gradients.
• Incorporated scalar relativistic effects using the first-order Douglas-Kroll-Hess Hamiltonian and a Gaussian charge distribution model for the nucleus.
• Performed calculations on a training set of iron complexes with diverse iron charge, spin, and oxidation states, and large molecular ligands.

Main Results:

• DLPNO-CCSD demonstrated excellent correlation between calculated electron densities and experimental ⁵⁷Fe isomer shifts, with a calibration constant (α) matching experimental values.
• The method showed high accuracy for quadrupole splittings as well, yielding a nuclear quadrupole moment for ⁵⁷Fe of 0.15 barn, consistent with prior studies.
• DLPNO-CCSD results were robust against intrinsic method approximations, unlike DFT results, which showed dependency on the chosen functional, although overall accuracies were statistically comparable.

Conclusions:

• DLPNO-CCSD is a highly accurate and reliable method for predicting ⁵⁷Fe isomer shifts and quadrupole splittings in complex iron systems.
• The method provides a robust theoretical framework, less sensitive to approximations than DFT, for spectroscopic studies of iron compounds.
• Scalar relativistic effects can be effectively included in DLPNO-CCSD calculations to improve the accuracy of electron density and related properties.