Abstract
The role of valence and semicore correlation in differentially stabilizing the intermediate spin state of Fe(II)-porphyrins is analyzed. For CASSCF treatments of valence correlation, a (32,34) active space containing metal 3d, d′ orbitals and the entire π system of the porphyrin is necessary to stabilize the intermediate spin state. Semicore correlation provides a minor (−1.6 kcal/mol) but quantitatively significant correction. Accounting for valence, semicore, and correlation beyond the active space enlarges the (3Eg–5A1g) spin gap to −5.7 kcal/mol.
Highlights
The role of valence and semicore correlation in differentially stabilizing the intermediate spin state of Fe(II)porphyrins is analyzed
We have performed a series of Stochastic-CASSCF and coupled cluster calculations to analyze the origins of the stabilization of the intermediate spin state in a model system for Fe(II)-porphyrin
We analyzed the charge redistribution due to electron correlation via CASSCF[32,34] calculations, which reveal that the triplet state is stabilized by a correlation enhanced σ-donation/π-back-donation interaction with the macrocycle
Summary
All calculations were performed on the same model system as our earlier work, where the carbon atoms in β-positions of the porphyrin ring have been replaced with H atoms This model preserves the chemical properties of the larger metal-porphyrin complexes and, crucially, includes a π-conjugated macrocycle with the four Gouterman orbitals involved in determining the spin energetics. The calculations were all performed at a single geometry, chosen to match the ground state (triplet) geometry of the Fe(II)-porphyrin, which has a small Fe−N bond length At this geometry ROHF, CASSCF, and CASPT2 (with small active space wave functions) all predict the ordering of the spin states incorrectly. The reference determinant was obtained by first performing a ROHF calculation with 32 or 40 electrons in the 34 or 38 active orbitals, respectively. An ad hoc interface was created to translate orbital coefficients from the Molcas to the MOLPRO format.
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