A very important open question related to the pygmy dipole resonance is about its quite elusive collective nature. In this paper, within a harmonic oscillator shell model, generalizing an approach introduced by Brink, we first identify the dipole normal modes in neutron rich nuclei and derive the energy weighted sum rule exhausted by the pygmy dipole resonance. Then solving numerically the self-consistent Landau-Vlasov kinetic equations for neutrons and protons with specific initial conditions, we explore the structure of the different dipole vibrations in the 132 Sn system and investigate their dependence on the symmetry energy. We evidence the existence of a distinctive collective isoscalar-like mode with an energy well below the Giant Dipole Resonance (GDR), very weakly dependent on the isovector part of the nuclear effective interaction. At variance the corresponding strength is rather sensitive to the behavior of the symmetry energy below saturation, which rules the number of excess neutrons in the nuclear surface. PACS numbers: 25.70.Pq, 25.70.Mn, 21.65.Ef, 24.10.Cn Keywords:One of the important tasks in many-body physics is to understand the emergence of the collective features as well as their structure in terms of the individual motion of the constituents. The steady progress of experimental methods of investigation opens now the possibility to study very neutron rich nuclei, beyond the limits of stability. The goal is to have a unified picture of the evolution of various nuclear properties with mass and isospin and to test the validity of our theoretical understanding over an extended domain of analysis.New exotic collective excitations show up when one moves away from the valley of stability [1]. Their experimental characterization and theoretical description is a challenge for modern nuclear physics. Recent experiments provided several evidences about their existence but the available information is still incomplete and their nature is a matter of debate.An interesting exotic mode is the Pygmy Dipole Resonance (PDR) which was observed as an unusually large concentration of the dipole response at energies clearly below the values associated with the GDR. The latter is one of the most prominent and robust collective motions, present in all nuclei, whose centroid position varies, for medium-heavy nuclei, as 80A −1/3 M eV . Adrich et al. [2] reported the observation of a resonant-like shape distribution with a pronounced peak around 10M eV in 130 Sn and 132 Sn isotopes. A concentration of dipole excitations near and below the particle emission threshold was also observed in stable Sn nuclei, a systematics of PDR in these systems being presented in [3]. It was concluded that the strongest transitions locate at energies between 5 and 8.5M eV and a sizable fraction of the Energy-Weighted Sum Rule (EWSR) is exhausted by these states. From a comparison of the available data for stable and unstable Sn isotopes a correlation between the fraction of pygmy strength and isospin asymmetry was noted [4]. In general t...

Encapsulation of a nitroxide spin label into a host cavity can prolong the lifetime of the spin label in biological tissues and other environments. Although such paramagnetic supramolecular complexes have been extensively studied experimentally, there is yet little understanding of the role of the encapsulation on the magnetic properties of the spin labels and their performance at the atomistic level. In this work, we approach this problem by modeling encapsulation induced changes of the magnetic properties of spin labels for a prototypical paramagnetic guest-host complex, 2,2,6,6-tetramethyl-4-methoxypiperidine-1-oxyl, enclosed in the hydrophobic cavity of cucurbit[8]uril, using state-of-the-art hybrid quantum mechanics/molecular mechanics methodology. The results allow a decomposition of the encapsulation shift of the electronic g-tensor and the nitrogen isotropic hyperfine coupling constant of nitroxide radical into a set of distinct contributions associated with the host cavity confinement and with changes of the local solvent environment of the spin label upon encapsulation. It is found that the hydrophobic cavity of cucurbit[8]uril only weakly influences the electronic g-tensor of the 2,2,6,6-tetramethyl-4-methoxypiperidine-1-oxyl but induces a significant encapsulation shift of the nitrogen hyperfine coupling constant. The latter is caused by the change of topology of the hydrogen bonding network and the nature of the hydrogen bonds around the spin label induced by the hydrophobic cavity of the inclusion host. This indirect effect is found to be more important than the direct influence of the cavity exerted on the radical. The ramification of this finding for the use of approximate methods for computing electron paramagnetic resonance spectra of spin labels and for designing optimal spin labels based on guest-host templates is discussed.

A series of computational experiments performed with various methods belonging to wave-function and density functional theories approaches the issue of bonding regime and exchange coupling in the title compounds. Gd 2 @C 80 is computed with a very weak exchange coupling, the sign depending on the method, while Gd 2 @C 79 N has resulted with a strong coupling and ferromagnetic ground state, irrespective of the computational approach. The multi-configuration calculation and broken symmetry estimation are yielding closely coincident coupling constants, of about J ∼ 400 cm −1 . No experimental estimation exists, but the ferromagnetic ground state of Gd 2 @C 79 N is confirmed from paramagnetic resonance data. The different behaviour is due to particularities of electron accommodation in the orbital scheme. The exchange effects localised on atom lead to preference for parallel alignment of the electrons placed in the 4f and 5d lanthanide shells, determining also a ferromagnetic inter-centre coupling. The structural insight is completed with a ligand field analysis of the density functional theory results in the context of frozen density embedding. The energy decomposition analysis of bonding effects is also discussed. Finally, with the help of home-made codes (named Xatom+Xsphere), a model for the atom encapsulated in a cage is designed, the exemplified numeric experiments showing relevance for the considered endohedral metallo-fullerene issues.

A density functional restricted-unrestricted approach, capable of evaluating hyperfine coupling constants with the inclusion of spin polarization effects in a spin-restricted Kohn-Sham method, has been extended to incorporate environmental effects. This is accomplished by means of a hybrid quantum mechanics/molecular mechanics formalism which allows for a granular representation of the polarization and electrostatic interactions with the classically described medium. By this technique, it is possible to trace the physical origin of hyperfine coupling constants in terms of spin polarization and spin density contributions and disentangle the dependence of these contributions on molecular geometry and solvent environment, something that increases the prospects for optimal design of spin labels for particular applications. A demonstration is given for the nitrogen isotropic hyperfine coupling constant in di-tert-butyl nitroxide solvated in water. The results indicate that the direct spin density contribution is about 5 times smaller than the spin polarization contribution to the nitrogen isotropic hyperfine coupling constant and that the latter contribution is solely responsible for the solvent shift of the constant. The developed approach is found capable of achieving satisfactory accuracy in prediction of the hyperfine coupling constants of solvated di-tert-butyl nitroxide and other similar nitroxides without the inclusion of solvent molecules in the quantum region provided polarizable force fields are used for the description of these molecules.

We present computational analyses, methodological advances, and heuristic conclusions applied on a series of polyaromatic systems condensed in the shape of regular triangles, carrying spin, because of topological reasons. A new clue about the classification of title systems in three equivalency classes is presented. A conjugated hydrocarbon having n-hexagonal rings at one edge, carrying n − 1 unpaired electrons, will be called n-triangulene in the generalization of the experimentally known structures with n = 2 (phenalenyl) and n = 3 (triangulene). To be distinguished from most of the previous computational approaches, done by density functional theory, we challenged the problem in the key of valence bond (VB) paradigm in both ab initio and phenomenological manners. The Heisenberg spin Hamiltonian was used to simulate the computed spectrum of VB states for the phenalenyl radical (n = 2), predicting with the fitted parameters the effective VB description of n = 3 triangulene and other related systems. The outcome has practical importance in the prospects of spin chemistry because the VB ab initio calculations are prohibitive beyond the n = 2 case. The results are made transparent to the chemical intuition using the language of resonance structures.