Alessandro Soncini

PHI: a powerful new program for the analysis of anisotropic monomeric and exchange-coupled polynuclear d- and f-block complexes.

A new program, PHI, with the ability to calculate the magnetic properties of large spin systems and complex orbitally degenerate systems, such as clusters of d‐block and f‐block ions, is presented. The program can intuitively fit experimental data from multiple sources, such as magnetic and spectroscopic data, simultaneously. PHI is extensively parallelized and can operate under the symmetric multiprocessing, single process multiple data, or GPU paradigms using a threaded, MPI or GPU model, respectively. For a given problem PHI is been shown to be almost 12 times faster than the well‐known program MAGPACK, limited only by available hardware.

An electrostatic model for the determination of magnetic anisotropy in dysprosium complexes

Understanding the anisotropic electronic structure of lanthanide complexes is important in areas as diverse as magnetic resonance imaging, luminescent cell labelling and quantum computing. Here we present an intuitive strategy based on a simple electrostatic method, capable of predicting the magnetic anisotropy of dysprosium(III) complexes, even in low symmetry. The strategy relies only on knowing the X-ray structure of the complex and the well-established observation that, in the absence of high symmetry, the ground state of dysprosium(III) is a doublet quantized along the anisotropy axis with an angular momentum quantum number mJ=±(15)/2. The magnetic anisotropy axis of 14 low-symmetry monometallic dysprosium(III) complexes computed via high-level ab initio calculations are very well reproduced by our electrostatic model. Furthermore, we show that the magnetic anisotropy is equally well predicted in a selection of low-symmetry polymetallic complexes.

Single molecule magnetism in a family of mononuclear β-diketonate lanthanide(III) complexes: rationalization of magnetic anisotropy in complexes of low symmetry

The use of an amino-pyridyl substituted β-diketone, N-(2-pyridyl)-ketoacetamide (paaH), has allowed for the isolation of two new families of isostructural mononuclear lanthanide complexes with general formulae: [Ln(paaH*)2(H2O)4][Cl]3·2H2O (Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er (5) and Y (6)) and [Ln(paaH*)2(NO3)2(MeOH)][NO3] (Ln = Tb (7), Dy (8), Ho (9) and Er (10)). The dysprosium members of each family (3 and 8) show interesting slow magnetic relaxation features. Compound 3 displays Single Molecule Magnet (SMM) behaviour in zero DC field with an energy barrier to thermal relaxation of Ea = 177(4) K (123(2) cm−1) with τ0 = 2.5(8) × 10−7 s, while compound 8 shows slow relaxation of the magnetization under an optimum DC field of 0.2 T with an energy barrier to thermal relaxation of Ea = 64 K (44 cm−1) with τ0 = 6.2 × 10−7 s. Ab initio multiconfigurational calculations of the Complete Active Space type have been employed to elucidate the electronic and magnetic structure of the low-lying energy levels of compounds 2–5 and 8. The orientation of the anisotropic magnetic moments for compounds 2–5 are rationalized using a clear and succinct, chemically intuitive method based on the electrostatic repulsion of the aspherical electron density distributions of the lanthanides.

Why Downfield Proton Chemical Shifts Are Not Reliable Aromaticity Indicators

Traces of magnetizability, traces of magnetic shielding at the hydrogen nuclei, and nucleus-independent chemical shift are not reliable aromaticity quantifiers for planar conjugated hydrocarbons. A measure of aromaticity is provided by the out-of-plane tensor components, whose magnitude is influenced by the pi-ring currents. The failure of nucleus-independent chemical shift in this regard was proved for the molecule shown in the abstract graphic, sustaining a diatropic pi-current. The validity of the ring-current model is reaffirmed. [structure: see text]

Nuclear spin–spin coupling density functions and the Fermi hole

Nuclear spin–spin coupling density functions yield a three-dimensional picture of the interaction between two nuclear dipole moments mediated by electron spin density. A physical interpretation of the Fermi contact coupling density maps can be readily arrived at on account of the Fermi correlation between same-spin electrons as the mechanism whereby the spin polarization induced about one nucleus is transmitted to another nucleus coupled to it. It is shown that the Fermi hole density function, evaluated by an opportune choice of the reference electron, is characterized by morphological aspects very similar to those appearing in the plots of one- and two-bonds Fermi contact density functions. A comparison has been made for hydrogen fluoride, water, ammonia, and methane molecules at the Hartree–Fock level of theory. The results confirm the role of the Fermi correlation as the fundamental vehicle propagating nuclear-spin/electron-spin contact interaction, i.e., the process mainly responsible for nuclear spin...

Molecular spintronics using noncollinear magnetic molecules

We investigate the spin transport through strongly anisotropic noncollinear magnetic molecules and find that the noncollinear magnetization acts as a spin-switching device for the current. Moreover, spin currents are shown to offer a viable route to selectively prepare the molecular device in one of two degenerate noncollinear magnetic states. Spin-currents can be also used to create a non-zero density of toroidal magnetization in a recently characterized Dy_3 noncollinear magnet.

Nuclear spin-spin coupling density in molecules

It is shown that nuclear spin-spin coupling in a molecule can be rationalized in terms of property density functions which depend on the position in three-dimensional space. The spin-spin coupling density surface, calculated as a table of values for a grid of coordinates on a plane through the molecular domain, yields a direct physical picture and offers a physical interpretation of the phenomenology, by showing the path whereby coupling takes place. The different role and the relative importance of the Fermi contact, spin-dipolar, and diamagnetic and paramagnetic spin-orbit mechanisms is readily assessed. The display of the spin-spin density reveals that the major contribution comes from the electrons close to the coupled nuclei. The economy of thinking achieved by the use of functions of three coordinates in real space, instead of n-electron wave functions depending on 3n coordinates in Hilbert space, is evident in the present case. The utility of spin-spin coupling densities has been discussed for the ...

Nonperturbative ab initio calculations in strong magnetic fields using London orbitals

A self-consistent field (SCF) London-orbital computational scheme to perform gauge-origin independent nonperturbative calculations for molecules in strong magnetic fields is presented. The crucial difference in the proposed approach with respect to common-origin finite-field SCF implementations consists in the evaluation of molecular integrals over the field-dependent molecular basis functions, which is tantamount to computing molecular integrals in a hybrid Gaussian and plane-wave basis set. The implementation of a McMurchie-Davidson scheme for the calculation of the molecular integrals over London orbitals is discussed, and preliminary applications of the newly developed code to the calculation of fourth-rank hypermagnetizabilities for a set of small molecules, benzene, and cyclobutadiene are presented. The nonperturbative approach is particularly useful for studying the highly nonlinear response of paramagnetic closed-shell systems such as boron monohydride, or the pi-electron response of cyclobutadiene.

Molecular spintronics in mixed-valence magnetic dimers: the double-exchange blockade mechanism.

We theoretically investigate the charge and spin transport through a binuclear Fe(III)Fe(III) iron complex connected to two metallic electrodes. During the transport process, the Fe(III)Fe(III) dimer undergoes a one-electron reduction (Coulomb blockade transport regime), leading to the reduced mixed-valence Fe(II) Fe(III) species. For such a system, the additional electron may be fully delocalized leading to the stabilization of the highest spin ground state S = 9/2 by the double exchange mechanism, while the original Fe(III)Fe(III) has usually an S = 0 spin ground state due to the antiferromagnetic exchange coupling between the two Fe(III) ions. Intuitively, the spin delocalization within the mixed-valence complex may be thought to favor charge and spin transport through the molecule between the two metallic electrodes. Contrary to such an intuitive concept, we find that the increased delocalization leads in fact to a blocking of the transport, if the exchange coupling interaction within the Fe(III)Fe(III) dimer is antiferromagnetic. This is due to the violation of the spin angular momentum conservation, where a change of half a unit of spin (DeltaS = 1/2) is allowed between two different redox states of the molecule. The result is explained in terms of a double-exchange blockade mechanism, triggered by the interplay between spin delocalization and antiferromagnetic coupling between the magnetic cores. Consequently, ferromagnetically coupled dimers are shown not to be affected by the double-exchange blockade mechanism. The situation is evocative of the onset and removal of giant magnetoresistance in the conductance of diamagnetic layers, as a function of the relative alignment of the magnetization of two weakly antiferromagnetically coupled ferromagnetic contacts. Numerical simulations accounting for the effect of vibronic coupling show that the spin current increases as a function of spin delocalization in Class I and Class II mixed-valence dimers. The signature of vibronic coupling on sequential spin-tunneling processes through Class I and Class II mixed-valence systems is identified and discussed.

Ring currents in the porphyrins: π shielding, delocalisation pathways and the central cation

It is shown that the ipsocentric orbital-based model explains how the charge of the central cation drives the delocalisation pathway in metalloporphyrins. A positive charge +Ze at the centre of the porphin ring gives rise to a two-way radial transfer of charge within the pi structure of the porphin macrocycle. This manifests itself in a change of pathway of the global pi current, as Z increases from Z = 0, from an inner- through a bifurcated- to an outer-pathway. Changes of pathway can be interpreted in terms of a specific pi shielding effect whereby electrons in high-lying pi orbitals are screened from the central charge by the electrons in lower-lying orbitals of the same symmetry. These changes in pi structure are essentially independent of accompanying changes in the sigma structure.