Transition metal compounds: outstanding challenges for multiconfigurational methods

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Relative energy of the high-„ 5T2g… and low-„ 1A1g… spin states of the ferrouscomplexes [Fe(L)(NHS4)]: CASPT2 versus density functional theory

High-level ab initio calculations using multiconfigurational perturbation theory complete active space with second-order perturbation theory CASPT2 were performed on the transition energy between the lowest high-spin corresponding to 5T2g in Oh and low-spin corresponding to 1A1g in Oh states in the series of six-coordinated Fe II molecules [Fe(L)(NHS4)] , where NHS4 is 2,2'-bis(2-mercaptophenylthio)diethylamine dianion and L = NH3, N2H4, PMe3, CO, and NO+. The results are compared to previous and presently obtained results from density functional theory DFT calculations with four functionals, which were already shown previously by Casida and co-workers [Fouqueau et al., J. Chem. Phys. 120, 9473 2004 ; Ganzenmüller et al., ibid. 122, 234321 2005 ; Fouqueau et al., ibid. 122, 044110 2005 ; Lawson Daku et al., ChemPhysChem 6, 1393 2005] to perform well for the spin-pairing problem in these and other Fe II complexes, i.e., OLYP, PBE0, B3LYP, and B3LYP*. Very extended basis sets were used both for the DFT and CASPT2 calculations and were shown to be necessary to obtain quantitative results with both types of method. This work presents a sequel to a previous DFT/CASPT2 study of the same property in the complexes [Fe(H2O)6]2+, [Fe(NH3)6]2+, and [Fe(bpy)3]2+ [Pierloot et al., J. Chem. Phys. 125, 124303 2006]. The latter work was extended with new results obtained with larger basis sets and including the OLYP functional. For all considered complexes, the CASPT2 method predicts the correct ground state spin multiplicity. Since experimental data for the actual quintet-singlet free energy differences are not available, the performance of the different DFT functionals was judged based on the comparison between the DFT and CASPT2 results. From this, it was concluded that the generalized gradient OLYP functional performs remarkably well for the present series of ferrous compounds, whereas the success of the three hybrid functionals varies from case to case.status: publishe

The dioxygen adducts of iron and manganese porphyrins: electronic structure and binding energy

In this paper, we present a thorough study of the electronic structures and binding energies of O2 to iron and manganese porphyrins (FeP and MnP), employing a state-of-the-art computational technique known as second-order perturbation theory based on density matrix renormalization group (DMRG-CASPT2). By investigating an extensive list of different binding modes and spin states, we provide a clear and conclusive description of the ground state of MnP-O2, confirming available experimental evidences. Our results show that MnP-O2 favours a side-on quartet structure, with strong charge transfer between MnP and O2. We also calculated the standard binding enthalpies of O2 to different metal porphyrins and showed that an agreement between calculated results and experimental data to within 2 kcal mol-1 can be achieved. Our calculations confirm the experimental observation that the binding of O2 to manganese porphyrin is stronger by around 4-6 kcal mol-1 than to the corresponding ferrous porphyrin.status: publishe

Electronic Structure of N-Bridged High-Valent Diiron-Oxo

Density functional theory (DFT) and an advanced ab initio technique based on density matrix renormalization group (DMRG-CASPT2) were employed to investigate a reactive N-bridged high-valent diiron-oxo species involved in H-abstraction reactions. We studied in detail two important doublet states, the ground state with two iron(IV) centers and a mixed valence FeV -FeIV excited state. We found that the latter state is low-lying. Furthermore, its electronic structure and spin density imply that it has significantly higher H-abstraction reactivity than the ground state. This low-lying excited state might be the reason behind the high oxidation reactivity of this diiron-oxo species towards methane.status: publishe