Molecular Spin Density Research Articles

Chiral Jahn-Teller Distortion in Quasi-Planar Boron Clusters.

In this work, we have observed that some chiral boron clusters (B16-, B20-, B24-, and B28-) can simultaneously have helical molecular orbitals and helical spin densities; these seem to be the first compounds discovered to have this intriguing property. We show that chiral Jahn-Teller distortion of quasi-planar boron clusters drives the formation of the helical molecular spin densities in these clusters and show that elongation/enhancement in helical molecular orbitals can be achieved by simply adding more building blocks via a linker. Aromaticity of these boron clusters is discussed. Chiral boron clusters may find potential applications in spintronics, such as molecular magnets.

Electron spin resonance of single iron phthalocyanine molecules and role of their non-localized spins in magnetic interactions.

Electron spin resonance (ESR) spectroscopy is a crucial tool, through spin labelling, in investigations of the chemical structure of materials and of the electronic structure of materials associated with unpaired spins. ESR spectra measured in molecular systems, however, are established on large ensembles of spins and usually require a complicated structural analysis. Recently, the combination of scanning tunnelling microscopy with ESR has proved to be a powerful tool to image and coherently control individual atomic spins on surfaces. Here we extend this technique to single coordination complexes-iron phthalocyanines (FePc)-and investigate the magnetic interactions between their molecular spin with either another molecular spin (in FePc-FePc dimers) or an atomic spin (in FePc-Ti pairs). We show that the molecular spin density of FePc is both localized at the central Fe atom and also distributed to the ligands (Pc), which yields a strongly molecular-geometry-dependent exchange coupling.

Partition density functional theory and its extension to the spin-polarized case

Partition density functional theory (PDFT) [P. Elliott, K. Burke, M.H. Cohen, and A. Wasserman, Phys. Rev. A 82 (2), 024501 (2010)] is a formally exact method for obtaining molecular properties from Kohn–Sham calculations on isolated fragments. Here, we express the partition energy of PDFT as an implicit functional of the molecular spin-densities for a given choice of fragmentation, and use the principle of von Barth and Hedin to formulate the spin-decomposed version of PDFT. We introduce a partition energy functional of the spin-up and spin-down electronic densities and derive the associated polarized partition potentials, which are found to be global quantities that influence every fragment in the molecule. Along with the formal theory, we propose a simplified approach to computing the spin-partition potentials, and illustrate its utility and accuracy with two simple examples. Finally, we propose a viable approach to including external electric and magnetic fields in the framework of spin-PDFT.

Intercalation of stable organic radicals into layered inorganic host matrices: Preparation and structural characterization of Cd 1−xPS 3( metaMPYNN) 2x

The radical cation 2-(3- N-methylpyridinium)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3- N-oxide (abbreviated as m-MPYNN) is successfully intercalated into the layered host structure CdPS 3. The reaction proceeds via an ion exchange reaction from methanol solutions containing the radical iodide salt, leading to materials described by the compositional formula Cd 1− x PS 3( metaMPYNN) 2 x . Detailed characterization of the intercalates by chemical analysis, X-ray powder diffraction, EPR, and NMR spectroscopy indicate that the maximum uptake corresponds to x=0.20; attempts to produce higher intercalated materials result in the formation of a side product with inferred composition CdI 4( metaMPYNN) 2. Magnetic susceptibility measurements indicate Curie-like behavior for x=0.13, while at higher intercalation levels antiferromagnetic coupling is observed. The magnetic properties may be linked to the orientation of the radical ions relative to the host layers, which is also found to depend on x. While for the low-intercalated regime ( x=0.13) both a transversal intercalation with the N–O director pointing towards the layers, and a longitudinal orientation, with the long axis of the molecule pointing towards the layers are found, at larger x-values only the longitudinal orientation is present. 1H NMR chemical shifts indicate that the orientation greatly influences the molecular spin density distributions. Modern single and double resonance solid state NMR techniques have been introduced successfully to study the structural modifications of the host lattice and the details of the intermolecular guest/host interactions. Specifically the internuclear distance correlations extracted from selectively measured 1H– 31P magnetic dipole–dipole couplings via 31P/ 1H rotational echo double resonance (REDOR) experiments allow important conclusions regarding the orientation of the guest species relative to the inorganic layers.

The Hydrogen Bonding Strategy. A New Approach Towards Purely Organic/Molecular Ferromagnets

Abstract The variation of the number and position of OH substituents at the phenyl ring of α-phenyl nitronyl nitroxide radicals yields different H-bonded molecular self-assemblies in the solid state but do not change essentially their molecular spin density distributions. In accordance with the distinct structural dimensionalities, the obtained radicals show a large variety of magnetic behaviors, being the mono ortho substituted isomer the most remarkable one since it shows a 3-D network of weak H-bonds exhibiting a bulk ferromagnetic transition below 0.45K.

On the molecular spin density and the electrostatic potential as determinants of the relaxivity of metalloporphyrins

The origin of the unexpectedly large relaxivity of a manganese metalloporphyrin is explored through computer simulations that include a description of the molecular spin density and the molecular electrostatic potential. These molecular properties describe not only the distribution of unpaired electrons in the coordination complex, but also a major driving force responsible for the dynamic behavior of the water molecules. By comparing the computed properties for the manganese complex with those of its congeneric iron complex, we gain insight into the origin of the unusually large relaxivity of the manganese metalloporphyrin. In the process, we learn how to use the computed molecular properties to formulate rules on how to modify the chemical structure of the metalloporphyrins to improve their relaxivity. Specifically, we show how to spatially direct the molecular spin density by the splitting of d orbitals and by the delocalization of electronic spin across unsaturated rings. We also learn how to attract water protons to the areas of high spin density by designing electrostatic focusing fields.

Spin-dependent unitary group approach. I. General formalism

A new spin-dependent unitary group approach to the many-electron correlation problem is investigated. It is demonstrated that the matrix elements of the U(2n) generators, in the U(n)×U(2) adapted electronic Gel’fand basis, are determined by the matrix elements of a single U(n) adjoint tensor operator, herein denoted by Δij(1≤i, j≤n), where Δ is a polynomial of degree two in the U(n) matrix E=[Eij]. The method is then applied, in the second paper of the series, to derive a simple segment level formula for the matrix elements of all U(2n) generators. The advantages of this new procedure for computer implementation are discussed and the central role played by the matrix Δ for the determination of molecular spin densities is highlighted.

Wavefunctions in molecular crystals

Abstract The diffraction experiment provides explicit data which relate directly to the electron density (X-ray scattering) or the magnetization density (polarized neutron scattering). A summary is given of the determination of spin densities in a number of paramagnetic molecular ions with orbitally non-degenerate ground states. The observed densities are compared with theoretical models up to the level of unrestricted Hartree-Fock. Electron correlation is amongst the most important features in the molecular spin densities; observed spin transfer from the metals to the ligands can be matched only with a theory based on a quite extensive basis set.

Solid state spin densities in triplet-exciton TCNQ salts

The combination of molecular spin densities and of structural data is shown to provide a sensitive test for the fine structure splittings and principal axes of triplet spin excitons in organic ion-radical crystals, in support of weakly-perturbed molecular sites in these solids. The exchange pathway in Rb(TCNQ) and the occurrence of dimer radicals, with fractional charges, in several tetrameric (TCNQ) 4 2− stacks illustrate the comparisons afforded by computations of fine structure parameters for triplets states in the solid.

Intermolecular contribution to relaxation of the molecular spin density matrix

The equation of motion for the mean density matrix in spin space of one molecule is derived for fluids where intermolecular spin-lattice interactions are important. The contribution of the intermolecular dipolar interaction to the relaxation coefficients is evaluated.