Eliseo Ruiz

Broken symmetry approach to calculation of exchange coupling constants for homobinuclear and heterobinuclear transition metal complexes

The application of broken symmetry density functional calculations to homobinuclear and heterobinuclear transition metal complexes produces good estimates of the exchange coupling constants as compared to experimental data. The accuracy of different hybrid density functional theory methods was tested. A discussion is presented of the different methodological approaches that apply when a broken symmetry wave function is used with either Hartree–Fock or density functional calculations. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1391–1400, 1999

About the calculation of exchange coupling constants in polynuclear transition metal complexes

The application of theoretical methods based on the density functional theory with hybrid functionals provides good estimates of the exchange coupling constants for polynuclear transition metal complexes. The accuracy is similar to that previously obtained for dinuclear compounds. We present test calculations on simple model systems based on H · · · He and CH2 · · · He units to compare with Hartree–Fock and multiconfigurational results. Calculations for complete, nonmodeled polynuclear transition metal complexes yield coupling constants in very good agreement with available experimental data.

About the calculation of exchange coupling constants using density-functional theory: the role of the self-interaction error.

The effect of the correction of the self-interaction error on the calculation of exchange coupling constants with methods based on density-functional theory has been tested in simple model systems. The inclusion of the self-interaction correction cancels the nondynamical correlation energy contributions simulated by the commonly used functionals. Hence, such correction should be important in the accurate determination of exchange coupling constants. We have also tested several recent functionals to calculate exchange coupling constants in transition-metal complexes, such as meta-GGA functionals or new formulations of hybrid functionals. The influence of the basis set and of the use of pseudopotentials on the calculated J values has also been evaluated for a Fe(III) dinuclear complex in which the paramagnetic centers bear several unpaired electrons.

Exchange Coupling in Carboxylato-Bridged Dinuclear Copper(II) Compounds: A Density Functional Study

A computational study of the exchange coupling is presented for a selected sample of carboxylato-bridged dinuclear copper(II) compounds. Model calculations have been used to examine the influence of several factors on the coupling constants: a) the electron-withdrawing power of the bridging ligands; b) the nature of the axial ligands; c) the number of bridging carboxylato groups; d) some structural distortions frequently found in this family of compounds; and e) the coordination mode of the carboxylato bridge. Coupling constants calculated for some complete structures, as determined by X-ray diffraction, are in excellent agreement with experimental data, confirming the ability of the computational strategy used in this work to predict the coupling constant for compounds for which experimental data are not yet available.

Mononuclear single-molecule magnets: tailoring the magnetic anisotropy of first-row transition-metal complexes.

Magnetic anisotropy is the property that confers to the spin a preferred direction that could be not aligned with an external magnetic field. Molecules that exhibit a high degree of magnetic anisotropy can behave as individual nanomagnets in the absence of a magnetic field, due to their predisposition to maintain their inherent spin direction. Until now, it has proved very hard to predict magnetic anisotropy, and as a consequence, most synthetic work has been based on serendipitous processes in the search for large magnetic anisotropy systems. The present work shows how the property can be predicted based on the coordination numbers and electronic structures of paramagnetic centers. Using these indicators, two Co(II) complexes known from literature have been magnetically characterized and confirm the predicted single-molecule magnet behavior.

Origin of slow magnetic relaxation in Kramers ions with non-uniaxial anisotropy

Transition metal ions with long-lived spin states represent minimum size magnetic bits. Magnetic memory has often been associated with the combination of high spin and strong uniaxial magnetic anisotropy. Yet, slow magnetic relaxation has also been observed in some Kramers ions with dominant easy-plane magnetic anisotropy, albeit only under an external magnetic field. Here we study the spin dynamics of cobalt(II) ions in a model molecular complex. We show, by means of quantitative first-principles calculations, that the slow relaxation in this and other similar systems is a general consequence of time-reversal symmetry that hinders direct spin-phonon processes regardless of the sign of the magnetic anisotropy. Its magnetic field dependence is a subtle manifestation of electronuclear spin entanglement, which opens relaxation channels that would otherwise be forbidden but, at the same time, masks the relaxation phenomenon at zero field. These results provide a promising strategy to synthesize atom-size magnetic memories.

Spin Density Distribution in Transition Metal Complexes: Some Thoughts and Hints

Abstract The spin density distribution in transition metal complexes is discussed in qualitative terms, taking into account the coexistence of spin delocalization and spin polarization mechanisms, with the help of numerical results for several complexes obtained from density functional calculations. The covalent character of the metal-ligand bonds as well as the σ- or π-characteristics of the partially filled d orbitals must be taken into account to qualitatively predict the sign of the spin density at a particular atom within a ligand. The same patterns can be applied to binuclear complexes and can be helpful in determining the ferro- or antiferromagnetic character of the exchange coupling between two paramagnetic ions when the energy gap between the partially occupied molecular orbitals is small. An attempt is made to establish a link between the qualitative-Hay-Thibeault-Hoffmann model of exchange coupling and the of spin polarization model.

Can large magnetic anisotropy and high spin really coexist

This theoretical study discusses the interplay of the magnetic anisotropy and magnetic exchange interaction of two Mn6 complexes and suggests that large magnetic anisotropy is not favoured by a high spin state of the ground state.

Exchange Coupling in Oxalato-Bridged Copper(II) Binuclear Compounds: A Density Functional Study

A recently developed computational strategy is applied to examine the influence of several factors on the exchange coupling constants of oxalato-bridged copper(II) binuclear complexes (shown schematically here); molecular topology, the nature of terminal ligands and selected structural parameters are discussed.

Shedding Light on the Single-Molecule Magnet Behavior of Mononuclear DyIII Complexes

General requirements for obtaining Dy(III) single-molecule magnets (SMM) were studied by CASSCF+RASSI calculations on both real and model systems. A set of 20 Dy(III) complexes was considered using their X-ray crystal structure for our calculations. Theoretical results were compared with their experimental slow relaxation data, and general conclusions about the calculated key parameters related with SMM behavior are presented. The effect of the coordination geometry and nature of ligands is discussed based on calculations on real and model systems. We found two different patterns to exhibit SMM behavior: the first one leads to the largest axial anisotropy in complexes showing heterolepticity of the ligand environment (more important than symmetric requirements), while the second one corresponds to sandwich-shaped complexes with a smaller anisotropy. Thus, most existing mononuclear zero-field SMMs adopting a heteroleptic coordination mode mixing neutral and anionic ligands present the same pattern in the electrostatic potential induced by their ligands, with a lower potential island related to the presence of neutral ligands inside a high potential background related with anionic groups. The existence of different electrostatic regions caused by the ligands induces a preferential orientation to reduce the electron repulsion for the electron density of the Dy(III) cations, resulting in the magnetic anisotropy.