Alessandro Bencini

Electron paramagnetic resonance of exchange coupled systems

Contents: Exchange and Superexchange.- Spin Hamiltonians.- Spectra of Pairs.- Spectra of Clusters.- Relaxation in Oligonuclear Species.- Spectra in Extended Lattices.- Selected Examples of Spectra of Pairs.- Coupled Transition-Metal Ions-Organic Radicals.- Biological Systems.- Low Dimensional Materials.- Excitons.- Appendix A: Second Quantization.- Appendix B: Properties of Angular Momentum Operators and Elements of Irreducible Tensor Algebra.

A Few Comments on the Application of Density Functional Theory to the Calculation of the Magnetic Structure of Oligo-Nuclear Transition Metal Clusters

First principle calculations of the magnetic structure of high nuclearity clusters appears challenging in order to validate fits of magnetic experiments. Density Functional Theory (DFT)-Broken Symmetry approach pair became, in the past few years, the most widely applied computational tool to investigating the chemical-physical properties of complex systems, in particular magnetic molecular compounds. However, the application of the Broken Symmetry formalism requires the knowledge of the energies of 2(N)/2 single Slater determinants, and this task can easily become difficult for large N. Three main approximations are therefore usually done in order to limit the computational efforts: the model dimer approach (MDA), the doped cluster approach (DCA), and the minimum cluster approach (MCA). The whole cluster approach (WCA) will be also applied as reference and in order to check the importance of spin Hamiltonian high order terms. A systematic comparison between these different approaches has been, therefore, performed. Since this study is aimed for being of help in choosing the best method of calculation, we check here the validity of the above approaches by computing the magnetic structure of some test systems: the tetrahedral system (HeH)4 and linear [Cu(II)]3 and [Mn(II)]4 complexes.

Exchange coupling in dinuclear copper(II) complexes with oxalato, oxamidato and oxamato ligands

We have synthesized a series of dinuclear copper-(II) complexes in which the metal ions are bridged by oxamidato, oxamato and oxalato ligands, with the purpose of studying the influence of the ligands on the extent of the antiferromagnetic coupling between the two metal ions. Magnetic susceptibility measurements between 77 and 300 K showed that the singlet-triplet splitting ranges from 330 cm−1 to 550 cm−1; J is found to increase in the order oxalato s oxamato s oxamidato.

Angular overlap calculations of the spin Hamiltonian parameters of transition metal ions in low symmetry environments. High spin iron(II), iron(III) and manganese(III)

A non-perturbative method is presented for the calculation of spin Hamiltonian parameters of d″ ions, 1 ≤ n ≤ 9. In its present form, it allows the calculation of the linear Zeeman and the second- and fourth-order zero-field spin Hamiltonian parameters. Symmetry restraints can easily be applied but application to non-symmetric molecules with determination of all 20 spin Hamiltonian parameters does not present particular difficulties. The model is based on a direct comparison of the spin Hamiltonian energies with the ligand field energies including spin-orbit coupling and Zeeman interaction of all-order perturbation. The model is applied to compute the spectromagnetic parameters of high spin iron(III), high spin iron(II) and high spin manganese(III) model chromophores.

Tuning the physical properties of a metal complex by molecular techniques: the design and the synthesis of the simplest cobalt-o-dioxolene complex undergoing valence tautomerism

Abstract A family of complexes of general formula [Co(CTH)(Phendiox)]Y·solv (CTH=tetraazamacrocycle; Phendiox=catecholato or semiquinonato form of 9,10-dioxophenanthrene; Y=PF 6 , I, BPh 4 ; solv=H 2 O, CH 2 Cl 2 , C 6 H 5 CH 3 ) were found to undergo valence tautomeric equilibria in the solid state. The interconversion involves an intramolecular electron transfer between a six-coordinated diamagnetic cobalt(III) metal ion and the coordinated polyoxolene ligand yielding a cobalt(II)–semiquinonato complex, the metal ion being in high spin configuration. All the synthesised complexes are diamagnetic at low temperature and high pressure and paramagnetic at high temperature and low pressure. These complexes were designed following electrochemical data. The critical interconversion temperature was found to depend on the nature of the counterion Y and on the nature of the solvent trapped in the lattice. No hysteresis was detected in the interconversion processes. However significant hysteresis effects were detected by substituting solvent from the lattice with its deuterated analogue and by using a partially deuterated ancillary macrocyclic ligand. The observed results are totally unpredictable and show how the emerging properties of a given set of molecules can be deeply influenced by very subtle and not controlled factors. Studies on closely related complexes were carried out for elucidating the electronic properties of these systems. In particular it was found that the [Co II (Me 4 cyclam)(SQ)]Y complexes (Me 4 cyclam=tetraazamacrocycle; SQ=semiquinonato radical ligand) are characterised by a weak anti-ferromagnetic coupling between the high-spin cobalt(II) ion and the paramagnetic ligand. In addition it was shown that the valence tautomeric interconversion could be photoinduced, the process involving two separate steps. DFT calculations show that in addition to the cobalt(III)–catecholato and hs -cobalt(II)–semiquinonato species which are involved in the thermal equilibrium, the third valence tautomer ls -cobalt(II)–semiquinonato species could be important from a photomagnetic point of view.

The epr spectra of the inhibitor derivatives of cobalt carbonic anhydrase.

The epr spectra at 4.2 K of inhibitor derivatives of cobalt carbonic anhydrase have been recorded. The spectra can be grouped into two classes according to whether the low-field signal is broad or sharp and with g ranges of 6.1-6.8, 2.3-2.9, 1.6-1.8, and 5.8-6.2, 2.2-2.8, 1.5-1.8, respectively, The two kinds of spectra have been empirically related to the features of the room-temperature solution electronic spectra. A third kind of epr spectrum with a single broad signal is obtained when the inhibitor is in large excess. The possibility of using the epr spectra for deducing the geometry of cobalt enzymes is discussed.

Magnetic Properties and Vapochromic Reversible Guest-Induced Transformation in a Bispyrazolato Copper(II) Polymer: an Experimental and Dispersion-Corrected Density Functional Theory Study

Dispersion-corrected density functional theory (DFT-D) calculations, Electron Spin Resonance spectroscopy (EPR), and variable temperature magnetic moment measurements were used to investigate the structure and the electronic/magnetic properties of bispyrazolato-copper(II) coordination polymer and of its hydration product. The Cu(II) ions are antiferromagnetically coupled through the sigma system of the pyrazolate rings in both compounds. Theoretical electron density maps reveal that water molecules interact simultaneously and to a comparable extent with two Cu(II) centers (through the electronegative O end) and two pyrazolate rings (through the partly positively charged H atoms), which is compatible with the observed internuclear distances. DFT-D calculations indicate that low kinetic barriers are involved in the rearrangement of the host structure.

Electronic Structure and Nature of the Ground State of the Mixed‐Valence Binuclear Tetra(μ‐1,8‐naphthyridine‐N,N′)‐bis(halogenonickel) Tetraphenylborate Complexes: Experimental and DFT Characterization

The ground state electronic structure of the mixed-valence systems [Ni(2)(napy)(4)X(2)](BPh(4)) (napy=1,8-naphthyridine; X=Cl, Br, I) was studied with combined experimental (X-ray diffraction, temperature dependence of the magnetic susceptibility, and high-field EPR spectroscopy) and theoretical (DFT) methods. The zero-field splitting (zfs) ground S=3/2 spin state is axial with /D/ approximately 3 cm(-1). The iodide derivative was found to be isostructural with the previously reported bromide complex, but not isomorphous. The compound crystallizes in the monoclinic system, space group P2(1)/n, with a=17.240(5), b=26.200(5), c=11.340(5) A, beta=101.320(5) degrees. DFT calculations were performed on the S=3/2 state to characterize the ground state potential energy surface as a function of the nuclear displacements. The molecules can thus be classified as Class III mixed-valence compounds with a computed delocalization parameter, B=3716, 3583, and 3261 cm(-1) for the Cl, Br, and I derivatives, respectively.

Synthesis, x-ray crystal structure and bonding in [(PPh3)2PtSe]2

Abstract The compound [(PPh 3 ) 2 PtSe] 2 has been obtained by reacting a selenide solution with (PPh 3 ) 2 PtCl 2 and its structure has been determined by X-ray diffraction analysis. The molecular geometry of the complex consists of a four membered Pt 2 Se 2 ring with two triphenylphosphine ligands coordinated to each platinum, giving a planar (P 2 Pt) 2 Se 2 arrangement. MO calculations have shown that the planar geometry is indeed slightly favoured for the neutral compound but anticipate a bent geometry for a hypothetical cationic species, in agreement with what has been found for related tellurium derivatives.

DFT Description of the Magnetic Properties and Electron Localization in Dinuclear Di‐μ‐oxo‐Bridged Manganese Complexes

Density functional theory (DFT) was applied to describe the magnetic and electron-transfer properties of dinuclear systems containing the [MnO2Mn]n+ core, with n=0,1,2,3,4. The calculation of the potential energy surfaces (PESs) of the mixed-valence species (n=1,3) allowed the classification of these systems according to the extent of valence localization as Class II compounds, in the Robin-Day classification scheme. The fundamental frequencies corresponding to the asymmetric breathing vibration were also computed.