Z/E-photoisomerizations of olefins with 4npi- or (4n + 2)pi-electron substituents: zigzag variations in olefin properties along the T(1) state energy surfaces

[Figure: see text]. A quantum chemical study has been performed to assess changes in aromaticity along the T1 state Z/E-isomerization pathways of annulenyl-substituted olefins. It is argued that the point on the T1 energy surface with highest substituent aromaticity corresponds to the minimum. According to Baird (J. Am. Chem. Soc. 1972, 94, 4941), aromaticity and antiaromaticity are interchanged when going from S0 to T1. Thus, olefins with S0 aromatic substituents (set A olefins) will be partially antiaromatic in T1 and vice versa for olefins with S0 antiaromatic substituents (set B olefins). Twist of the C=C bond to a structure with a perpendicular orientation of the 2p(C) orbitals (3p*) in T1 should lead to regaining substituent aromaticity in set A and loss of aromaticity in set B olefins. This hypothesis is verified through quantum chemical calculations of T1 energies, geometries (bond lengths and harmonic oscillator measure of aromaticity), spin densities, and nucleus independent chemical shifts whose differences along the T1 PES display zigzag dependencies on the number of -electrons in the annulenyl substituent of the olefin. Aromaticity changes are reflected in the profiles of the T1 potential energy surfaces (T1 PESs) for Z/E-isomerizations because olefins in set A have minima at 3p* whereas those in set B have maxima at such structures. The proper combination (fusion) of the substituents of set A and B olefins could allow for design of novel optical switch compounds that isomerize adiabatically with high isomerization quantum yields.     

Electronic structure and bonding in Si70

We report a detailed AM1 investigation of the geometrical and electronic structure of Si₇₀. For this purpose, bond lengths, bond orders, charges and molecular energy levels are widely analyzed, and compared with previous theoretical and experimental data on Si₆₀ and homologous carbon clusters C₇₀ and C₆₀. The predicted D_5h structure of Si₇₀ is less delocalized than that of C₇₀. Furthermore, Si₇₀ presents the lowest ionization potential (7.63eV), the highest electron affinity (3.61 eV) and the smallest HOMO-LUMO gap (4.02eV) of the four studied clusters.     

Latex Fractionation by Sedimentation and Colloidal Crystallization: The Case of Poly(styrene-co-acrylamide)

Poly(styrene-co-acrylamide) (PS-AAM) latex was prepared, fractionated by sedimentation under gravity, and characterized by PCS, infrared spectra, secondary and backscattered electron imaging in the scanning electron microscope, and electron spectroscopy imaging in an analytical transmission electron microscope. Three latex fractions were obtained. The lower fraction was opalescent and its particles were the more uniform, concerning size, chemical composition, and topochemical features. This lower fraction was still further fractionated by zonal centrifugation in a density gradient, yielding two fractions with similar macrocrystal-forming abilities but different sizes and chemical compositions. These results confirm those previously obtained for the PS-HEMA latex. Copyright 2000 Academic Press.     

Cathodoluminescence from deformed doped MgO crystals

The cathodoluminescence (CL) from indented Fe doped MgO single crystals has been studied with the scanning electron microscope and the results have been compared with the CL from undoped crystals. As in the undoped crystals, the iron doped crystals show in the deformed regions an enhanced luminescence emission but it has been found that iron acts as a quencher of luminescence. The results indicate that in the blue emission of the deformed crystals there is not a noticeable contribution of iron. From the evolution of CL with the irradiation time in the electron microscope it is concluded that the total Fe content influences the CL emission more than the Fe³⁺ content.     

Halo structure of (14)Be

The two-neutron halo nucleus (14)Be has been investigated in a kinematically complete measurement of the fragments ((12)Be and neutrons) produced in dissociation at 35 MeV/nucleon on C and Pb targets. Two-neutron removal cross sections, neutron angular distributions, and invariant mass spectra were measured, and the contributions from electromagnetic dissociation (EMD) were deduced. Comparison with three-body model calculations suggests that the halo wave function contains a large nu(2s(1/2))(2) admixture. The EMD invariant mass spectrum exhibited enhanced strength near threshold consistent with a nonresonant soft-dipole excitation.     

Self-assembled tin-doped ZnO nanowire and nanoplate structures grown by thermal treatment of ZnS powder

Sintering of a ZnS–SnO₂ mixture under argon flow leads to the growth of columnar nanoplate arrays as well as arrays of nanowires, nanorods and nanoplates with six-fold symmetry. The six-fold nanoplate structures correspond to a more advanced stage of growth than the nanowire structures. Cathodoluminescence (CL) in the scanning electron microscope (SEM) shows that the structures contain Sn, but the amount of this element is normally under the detection limit of X-ray energy-dispersive spectroscopy (EDS). The formation of branches in the hierarchical structures depends on the presence of Sn and on defects in the mixture powder.