Tomohisa Soda

Theoretical studies on effective spin interactions, spin alignments and macroscopic spin tunneling in polynuclear manganese and related complexes and their mesoscopic clusters

Abstract Theoretical efforts to investigate molecular magnetic materials are reviewed mainly from the viewpoint of our interest. Ab initio calculations of effective exchange interactions between spins are performed for HH, HHeH and simplified models of binuclear manganese and related complexes by using the spin unrestricted Hartree–Fock (UHF) and spin-polarized density functional (DFT), and UHF plus DFT hybrid methods. The scope and limitation of these broken-symmetry approaches are briefly discussed in relation to several computational schemes of effective exchange integrals ( J ab ). The calculated J ab values for the three systems are summarized for comparison of the computational methods. The natural orbitals (UNO or DNO) of the UHF and DFT solutions for magnetic clusters are determined by diagonalizing their first-order density matrices. They are used for MO-theoretical interpretation of superexchange interactions. The effective spin Hamiltonians such as the Heisenberg model are constructed for polynuclear complexes assuming the calculated and experimental effective exchange integrals. The macroscopic quantum tunneling (MQT) and coherence (MQC) of spins in the manganese oxide clusters are analyzed using the Heisenberg model, and the tunneling rate of spins is calculated by the coherent state path integral method. The topological rules for MQT and MQC are derived from this analysis. The path integral formulations are extended to tunneling probabilities for clusters of clusters and spin lattices with mesoscopic size. The resulting ideas are also applied to the molecular design of mesoscopic clusters of clusters in intermediate and strong correlation regimes. The active control of spins are finally discussed from the viewpoint of functionalities in molecular and biological materials, and technological applications of mesoscopic molecular magnets to quantum computing.

Density functional and post-Hartree-Fock studies on effective exchange interaction ofd-?-d conjugated systems involvingm-phenylene-type bridge

Density functional and post–Hartree–Fock calculations have been carried out for the metal–π conjugated complexes involving pyrimidine as an m-phenylene-type unit. Total energies for the lowest and highest spin states are used for investigation of d–π–d magnetic interactions via pyrimidine of Mn(II)2–pyrimidine, Cu(II)2–pyrimidine, and Mn(II)Cu(II)–pyrimidine. The natural orbital analysis of the unrestricted Hartree–Fock (UHF) and density functional theory (DFT) solutions of these complexes has been performed to elucidate relative contributions of spin polarization and spin delocalization (superexchange) interactions for determination of the sign of effective exchange integrals. Implications of the calculated results have been discussed in relation to the molecular magnetism of Mn(II) and Cu(II) complexes from the viewpoint of charge and spin density distributions, shapes of the natural orbitals, and their occupation numbers.

Theoretical studies of the effective exchange interactions and photoinduced magnetism in manganese and copper di(4-pyridyl)carbene complexes

Abstract UHF, UMP n ( n =2,4), UCC and DFT (UBLYP and UB3LYP) calculations using several basis sets were performed in order to elucidate the sign and magnitudes of the effective exchange interactions ( J ab ) of models of 1:1 complex of Mn(hfac) 2 or Cu(hfac) 2 with (4-pryidyl)methylene. The J ab value between Mn(II) and divalent carbon (:C) through 4-pyridyl group by the UMP and UCCSD(T)/triple-zeta basis set was antiferromagnetic, while it was ferromagnetic between Cu(II) and :C. This is wholly consistent with the experiments. The signs of J ab values are explained by mechanisms of spin–exchange couplings through the configuration interaction pictures.

Theoretical study of the antiferromagnetic model clusters for K2MX4 type solids

Abstract Magnetic properties for the K2MX4 type solids (M=Cu2+, Ni2+ X=F−, O2−. Cl−) were investigated by the ab initio calculations using UHF and the hybrid Density Functional methods (B2VWN, S2VWN and UB3LYP). It was found that these type solids had the antiferromagnetic interactions.

Theoretical Studies on Magnetic Interactions of Dichromium Tetraacetate by Using Hybrid Density Functional Method

Abstract The hybrid parameters of the density functional theory (DFT) for the open shell system in terms of the effective exchange integrals (Jab ). By using the parameters, Jab value for Cr2(O2CCH3)4(H2O)2 are calculated and Cr-Cr direct exchange interaction are estimated by using DFT natural orbital (DNO) CAS CI method.

Theoretical study on necessary conditions for reversible photoinduced magnetization : Cobalt-Iron cyanide system

Abstract We study metastability of the Cobalt-Iron Prussian blue compound. DFT calculations are performed for model clusters (a) (NC)5-Fe-CN-Co-(NC)5 and (b) (NC)5-Fe-CN-Co-(NC)3 (H2O)2 and the energy difference between the low spin state (S = 0) and the high spin state (S = 1) is estimated. Correction due to impurity effect is evaluated and screening effect in a solid state is also discussed.

Theoretical Studies on Magnetic Interaction of Di-μ-oxo Bridged Manganese Dimers

Abstract We performed the density functional theory (DFT) calculations of several manganese model complexes in order to investigate the effective exchange interaction of di-μ-oxo bridged manganese(IV) dimers. We calculated the effective exchange integrals (Jab ) of the model molecules and discussed geometry and ligand effects on them.

Theoretical study on magnetic interactions of Mn-π conjugated system

Abstract Magnetic interaction of Mn-π conjugated system, especially Mn(II)2-py-rimidine was investigated by the ab initio UHF, post-UHF, and DFT approximations. The calculated effective exchange integrals (Jab ) are negative, even though the m-phenylene type bridge is employed. This indicates that the superexchange effect plays a predominant role to determine the sign of Jab in Mn-π conjugated systems.