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Autori: J. Paulovic, F. Cimpoesu, M. Ferbinteanu, K. Hirao
Editorial: JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 126, p.3321-3331, 2004.
This paper offers the first series of state-of-the-art quantum chemical calculations (CASSCF, CASPT2, MS-CASPT2) and analytical models for the well-known problem of quasi-general ferromagnetic coupling in copper-gadolinium complexes. A system chosen from the chemical report of Costes et al. was taken as prototype. At the CASSCF level, calculated results for the experimental structure reproduced the magnetic coupling constant well ( Jcalcd = +7.67 cm-1 vs Jexp =+7.0 cm-1). For more insight, the study molecule was further idealized by geometry optimization to C2 v symmetry. Systematic ab initio computation experiments were designed and performed. Owing to specific problems related to the non- aufbauground configuration of the [CuL-Gd] complexes, the calculations were conducted in a nonstandard manner. We found that the qualitative mechanism of Kahn, assigned to the electron jump from 3 d of Cu(II) to 5 d shell of Gd(III), can be presented effectively as the cause of the phenomenon, if CASPT2 MOs are taken as magnetic orbitals. We showed that the ferromagnetic coupling is also matched and magnified by spin polarization effects over the ligand, in line with the early assumption of Gatteschi. To be distinguished from the initial hypothesis of Gatteschi, which assumed the role of 6 sAO of Gd(III), we found that one 5 d-type AO is actually involved in the polarization scheme. In fact, the Gatteschi and Kahn mechanisms are not mutually contradictory, but are even interconvertible with appropriate changes of the magnetic orbitals.
Within C2 v symmetry of complexes, the ferromagnetic coupling can be qualitatively regarded as the
preponderant influence of interaction channels exhibiting orbital orthogonality (four 3 d-4 f contacts) over the non orthogonal ones (two 3 d-4 f contacts). The effective preponderance from ferromagnetic pathways is supported by CASPT2 results. One may explain the generality of Cu(II)-Gd(III) ferromagnetic coupling as being correlated with the large occurrence of approximate pseudo- C2 v geometry of complexes. The observed orbital regularity is lost in lower symmetries. Thus, the antiferromagnetic exceptions occur when
the molecular asymmetry is advanced (e.g., owing to strong chemical nonequivalence of the donor atoms).
Cuvinte cheie: Ferromagnetic Coupling; d-f system; Ab-initio calculation