Modeling of Transition Metals and Lanthanides Containing Materials by Ab Initio Methods

During this Phd thesis the analysis at the post-Hartree Fock level was focalized on transition metal ion and lanthanide containing systems characterized by a pronounced magnetic anisotropy. The work was progressively focused on molecular systems with higher level of complexity: mononuclear complexes, 2p-4f polinuclear systems, 2p-4f-3d-heterospin complexes and, finally, 2p-4d one dimensional magnetic systems. The computational protocol employed is the Complete Active Space Self-Consistent Field (CASSCF), followed by Complete Active Space State Interaction (CASSI) for the calculation of the Spin-Orbit interaction. CASSCF is a post-HF multiconfigurational computational protocol. This method is necessary in order to take into account the electronic correlation in the calculation of the excited states of the system: the electronic states indeed have to be computed at an high level of theory in order to map the magnetic anisotropy. The first part of this thesis was dedicated to the computational investigation of lanthanide based single ion magnets (SIMs). Ab initio calculations of the ground and excited states of the Dy(LH)3 complex showed to reproduce experimental magnetic data (susceptometry, magnetization and cantilever torque magnetometry). Moreover, the most probable relaxation path between the two opposite sides of the anisotropy barrier was evaluated allowing the interpretation of the AC data. The analysis of the DyDOTA complex confirmed the influence of the water molecule directly bonded to the lanthanide atom on the orientation of the magnetic anisotropy axis. Calculations were also performed where the water molecule was substitued by its multipolar expansion. These results undermined the common idea that the magnetic properties are only determined by the purely electrostatic interaction between the 4f-orbitals and the electric multipolar moments of the ligands. The protocol employed was then applied on the isostructural series of Ln(DOTA) (Ln = Ce, Pr, Nd, Eu, Gd, Er). Regarding heterospin sytems, the exchange couplings between radicals, the lanthanide ions and transition metals were computed by CASSCF methods. The magnetic anisotropy of the isostructural series of binuclear complexes, Ln2dppnTEMPO, containing two lanthanide ions bridged by an organic radical, was computed. The electronic structure, g-tensor and crystal field parameters were simulated. Two heterotrispin systems based on coupling a lanthanide ion with an organic radical and/or a transition metal ion were compared and a qualitative analysis of the relaxation path of the magnetization was attempted, giving an insight on the different observed magnetic behaviour. Calculations on cobalt containing Single Chain Magnets were performed. The study was focused on the reproduction of the structural, magnetic and electronic data of this system in the bulk phase. A reliable computational protocol is, indeed, necessary in order to unambiguously characterize a molecular system on surface, where the common techniques of structural investigation are not employable. The single repetitive unit of the coordination polymer was studied by post-Hartree Fock methods, then optimization of the geometry, with and without the co-crystallized solvent, and mapping of the magnetic exchange constants at periodic DFT level was performed. The optimized geometry showed only minimal modifications from with the X-Ray structure. However, in order to reproduce magnetic properties, i.e. the exchange coupling constants, the addition of a localizing potential, the Hubbard’s U parameter, to the revPBE functional was crucial.