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Recent work has documented conjugate polycyclic hydrocarbons presenting unusual properties: accepting full on-bond electron pairing, they could be considered as closed-shell architectures, but their ground-state wave function is actually a pure diradical singlet, free of any ionic component, in contrast to diradicaloids. These so-called entangled molecules also differ from disjoint diradicals, which do not accept on-bond electron pairing, in that their singly occupied molecular orbitals (SOMOs) are spatially entangled rather than disjoint. The present work first extends the study to a broad series of architectures exhibiting the same properties, namely: they present two degenerate SOMOs in the topological Hückel Hamiltonian, and their pure diradical wave functions lead to symmetry-keeping geometries. These solutions are always of lower energy than the closed-shell solutions that break symmetry and destroy aromaticity of some six-membered rings. A topological criterion ensuring that a given conjugate hydrocarbon will behave as an entangled pure diradical is then formulated. Next, a second set of molecules is proposed, still exhibiting two degenerate Hückel SOMOs, but with smaller contrast between the energies of open-shell and closed-shell solutions. Conservation of six-membered rings aromaticity appears as the driving factor ruling the stability of diradical solutions.
A trinuclear Co(II)-containing complex was assembled using the non-innocent hexahydroxytriphenylene bridging ligand. Cyclovoltammetry and spectroelectrochemistry studies revealed that the central ligand sustained four reversible redox events, leading to different species with diverse optical behavior. Complementary analysis of the molecular structure confirmed by ab initio theoretical calculations were consistent with the bridge in the tris-semiquinone (sq) state for the trinuclear complex. The exchange coupling among the electrons of the bridge resulted in a spin doublet (s = ½) localized close to one of the three Co2+ ions, as suggested by the experimental magnetic data. The central doublet underwent one large antiferromagnetic exchange coupling with one Co(II) and almost no coupling with the two other metal ions.
In quantum chemistry, single-reference Coupled Cluster theory, and its refinements introduced by Bartlett, has become a “gold-standard” predictive method for taking into account electronic correlations in molecules. In this article, we introduce a new formalism based on a Coupled Cluster expansion of the wave function that is suited to describe model periodic systems and apply this methodology to the case of hole-doped antiferromagnetic two-dimensional (2D)-square spin-lattices as a proof of concept. More precisely, we focus our study on 1/5 and 1/7 doping ratios and discuss the possible ordering effect due to large hole–hole repulsion. Starting from one of the equivalent single determinants exhibiting a full spin alternation and the most remote location of the holes as a single reference, the method incorporates some corrections to the traditional Coupled Cluster formalism to take into account the nonadditivity of excitation energies to multiply excited determinants. The amplitudes of the excitations, which are possible on the excited determinants but impossible on the reference, are evaluated perturbatively, while their effect is treated as a dressing in the basic equations. The expansion does not show any sign of divergence of the wave operator. Finally, the probabilities of holes moving toward the first- and second-neighboring sites are reported, which confirms the importance of the hole–hole repulsion and offers a picture of how stripes expand around its central line in the “stripe phases” observed in cuprates.
The crystal field parameters are determined from first-principles calculations in the [An<sup>III</sup>(DPA)<sub>3</sub>]<sup>3-</sup> series, completing previous work on the [Ln<sup>III</sup>(DPA)<sub>3</sub>]<sup>3-</sup> and [An<sup>IV</sup>(DPA)<sub>3</sub>]<sup>2-</sup> series. The crystal field strength parameter follows the Ln(III) < An(III) < An(IV) trend. The parameters deduced at the orbital level decrease along the series, while J-mixing strongly impacts the many-electron parameters, especially for the Pu(III) complex. We further compile the available data for the three series. In some aspects, An(III) complexes are closer to Ln(III) than to An(IV) complexes with regard to the geometrical structure and bonding descriptors. At the beginning of the series, up to Pu(III), there is a quantitative departure from the free ion, especially for the Pa(III) complex. The magnetic properties of the actinides keep the trends of the lanthanides; in particular, the axial magnetic susceptibility follows Bleaney’s theory qualitatively.
Actinide +IV complexes with six nitrates [AnIV(NO3)6]2− (An = Th, U, Np, and Pu) have been studied by 15N and 17O NMR spectroscopy in solution and first-principles calculations. Magnetic susceptibilities were evaluated experimentally using the Evans method and are in good agreement with the ab initio values. The evolution in the series of the crystal field parameters deduced from ab initio calculations is discussed. The NMR paramagnetic shifts are analyzed based on ab initio calculations. Because the cubic symmetry of the complex quenches the dipolar contribution, they are only of Fermi contact origin. They are evaluated from first-principles based on a complete active space/density functional theory (DFT) strategy, in good accordance with the experimental one. The ligand hyperfine coupling constants are deduced from paramagnetic shifts and calculated using unrestricted DFT. The latter are decomposed in terms of the contribution of molecular orbitals. It highlights two pathways for the delocalization of the spin density from the metallic open-shell 5f orbitals to the NMR active nuclei, either through the valence 5f hybridized with 6d to the valence 2p molecular orbitals of the ligands, or by spin polarization of the metallic 6p orbitals which interact with the 2s-based molecular orbitals of the ligands.
Sujets
Model Hamiltonians
Wave functions
Iridate
Dynamical mean field theory
Lanthanide
Actinides
Imidazolium salt
Covalency
Iodine
Crystal field parameters
Correlated relativistic ab initio calculations
MOLCAS calculations
Luminescence
Electron spin
Calculs ab initio relativistes et corrélés
First-order spin–orbit coupling
Dynamical mean-field theory
Perturbation theory
Déplacements chimiques paramagnétiques
NMR
Actinide
Effets magnéto-résistifs
Configuration interactions
Binuclear compounds
Lanthanides
Magnetic Susceptibility
Décontamination de spin
MACROCYCLIC POLYARYLMETHYL POLYRADICALS
Bleaney's theory
Diagonalisations exactes
Magnetic anisotropy
Configuration interaction
Magnetic properties
Exchange and superexchange interactions
Crystal field theory
Divalent cobalt
Electronic correlation
Magnétisme moléculaire
Ab initio calculation
Spin-orbit coupling
Crystal-field theory and spin Hamiltonians
Iridates
AB-INITIO
Density functional theory
Heavy fermions
Calculs ab initio
FOS Physical sciences
Metal-insulator transition
Magnetism in organic systems
Bleaney's model
Calcul ab initio
Manganites
Hamiltonien modèle
Basis sets
Determinants
Electron paramagnetic resonance
Complexes de métaux de transition
Excitation energies
Dzyaloshinskii–Moriya interaction
Magnétisme dans les systèmes organiques
MECHANISM
Cooperative effect
Modèle de Bleaney
Hyperfine coupling
Spin-orbit interactions
HIGH-SPIN
Excited states
High pressure
Magnetism
Ionic liquid
Hyperfine structure
Modeling
Anderson mechanism
Bleaney
Model hamiltonian
Isotropic and anisotropic exchange
Free radicals
Finite nucleus effects
Ground states
Model Hamiltonian derivation
Exact diagonalization
CLUSTERS
Electron g-factor
MOLECULAR MAGNETIC-MATERIALS
Double exchange model
DOTA ligand
Anisotropy
Ligand-field theory
Effective Hamiltonian theory
Disordered Systems and Neural Networks cond-matdis-nn
Ab initio calculations
Magneto-resistive effects
Magnetic susceptibility
Anisotropie magnétique
Relativistic corrections
Dzyaloshinskii-Moriya interaction
Heptacoordination
Electron paramagnetism
Coupled cluster calculations
Electronic structure