Core polarization effects and giant quadropole resonances in the A ≈ 90 region

The existence, in A ≈ 90 nuclei, of large E2 core polarization effects evident from experimental and shell model fits to decay rates is examined in two independent model calculations: a linearized Hartree-Fock calculation studying the nuclear response function and a macroscopic model involving excitations of giant quadrupole resonances. Both investigations confirm the need for large renormalization of proton and neutron E2 effective charges consistent with the recently discovered isoscalar and isovector giant quadrupole resonances.     

Effects beyond first-order perturbation theory in the polarization charges forE4 transitions

Recent spectroscopy and electron scattering measurements indicate that severalE4 transitions in thef-p shell are severely retarded compared to shell model estimates. We examine whether core polarization mechanisms could be invoked to explain these anomalousE4 transitions and conclude that open shell effects wellbeyond core polarization are necessary to explain the data.     

Can spin-flip quadrupole resonances be observed?

Shell model calculations ofJ=2⁺,S=0 andS=1 states indicate that the spin flip quadrupole strength is much more fragmented than itsS=0 analog, at least in light nuclei. A sum rule technique is then used to study the coexistence betweenS=0 andS=1 states; it illustrates why theS=0 strength is fairly immune to changes in the noncentral part of the nuclear interaction. In¹⁶O, someE2 spin flip strength is predicted to lie in the 40 MeV region where it could be located by inelastic electron and pion scattering experiments.     

Sur le théorème en moyenne d'Akcoglu-Sucheston

Sans résumé     

Structural characterization of artificial self-assembling porphyrins that mimic the natural chlorosomal bacteriochlorophylls c, d, and e

We report two crystal structures of a synthetic porphyrin molecule which was programmed for self-assembly. The same groups which ensure that bacteriochlorophylls c, d, and e can self-assemble into the chlorosomal nanorods, the photosynthetic antenna system of some green bacteria, have been engineered into desired positions of the tetrapyrrolic macrocycle. In the case of the 5,15-meso-substituted anchoring groups, depending upon the concentration, by using the same crystallization solvents, either a tetragonal or a layered structure of porphyrin stacks were encountered. Surprisingly, pi-pi interactions combined with extensive dispersive interactions, which also encompass cyclohexane, one of the crystallization solvents, win over putative hydrogen bonding. We are aware that our compounds differ considerably from the natural bacteriochlorophylls, but based upon our findings, we now question the hydrogen-bonding network, previously proposed to organize stacks of bacteriochlorophylls. Transmission electron microscopy (TEM), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS) on various isomeric compounds support our challenge of current models for the chlorosomal antenna as these show structures, astonishingly similar to those of chlorosomes.     

Glycol-cleavage reagents also act on stannylene derivatives.

The well-known cleavage reactions of vicinal diols by such reagents as Pb(OAc)₄, NaIO₄, PhI(OAc)₂ and Ph₃Bi^V-compounds have now been observed for the first time on covalent derivatives of these vicinal diols, their dibutylstannylenes.