Dumb-bell structures

Regular dumb-bell structures have been detected in face-centred cubic elementary crystals [113] After the relaxation of the structiue, which involves the interstitial atom at the centre of the unit cell (oct dled al interstice) and a face-centring atom (comer of the octahedron), such a symmetrical situation is created. However, none of these structural details influences the phenomenological description of dilute defects. 

G.-W. Wang, K. Komatsu, Y. Murata, M. Shiro, Synthesis and X-ray Structure of Dumb-Bell-Shaped C120 , Nature 1997, 387 583-586. 

The simplest case of comb polymer is the H-shaped structure in which two side arms of equal length are grafted onto each end of a linear cross-bar [6]. In this case the backbones may reptate, but the reptation time is proportional to the square of Mj, rather than the cube, because the drag is dominated by the dumb-bell-like frictional branch points at the chain ends [45,46]. In this case the dependence on is not a signature of Rouse motion - the relaxation spectrum itself exhibits a characteristic reptation form. The dynamic structure factor would also point to entangled rather than free motion. 

The term proteasome is used to describe two kinds of multisubunit proteolytic complexes, the 26S and 20S, based on their sedimentation coefficient. The 26S proteasome degrades ubiquitinated protein substrates. The 26S complex contains the 20S as a core and regulatory caps on either end like a dumb bell. Each cap of the 26S proteasome is known as the 19S regulatory complex (19S RC). The 20S core is a cylindrical structure consisting of the catalytic part of the proteasome. ... 

FIGURE 20 The structures of CesPd Pb and Y NigPb. Cerium (yttrium), palladium (nickel), and lead atoms are drawn as medium gray, black filled, and open circles, respectively. The lead filled Pd6 (white) and the empty Ce2Pd4 (gray) octehedra of the Ce8Pd24Pb structure are emphasized. In the Y NigPb structure, the main structural motifs are lead centered icosahedra and Ni2 dumb-bells. 

In the oxygen atom which is next, there are two unpaired electrons in the lowest energy state of this atom, each in a different p level. Here the electrons available for bonding, therefore, form two dumb bells along mutually perpendicular axes. A rectangular structure... 

For the production of three bonds the carbon atom will have to be brought into the excited configuration already mentioned. In the formation of three equivalent bonds with the electrons of the hydrogen atoms one electron naturally remains over unpaired. The three electron pairs will occupy the functions with the lowest energy 2s + 2 X 2p, for example 2s + 2px + sp, thus sp2 hybridization and bond angles of 120° the lone electron is then a pz electron. The radical will then have a plane structure with the dumb-bell-shaped wave function of... 

For the case of a warped Jahn-Teller surface the probability functions, as shown in Fig. 8b, are localised at the positions of the Jahn-Teller minima and now the Ad(M-L) values show a 2/3 and 1/3 probabilities at displacements of — l/3x and +2/3x respectively. This structure reflects the underlying tetragonal elongation at the minima of the potential surface. Again, for these dynamic Jahn-Teller effects all six ligands would show the same probability distribution centred at equal bond lengths, but now with asymmetric dumb-bell shaped thermal ellipsoids. 

In 2-17 compounds, the atoms located in the dumb-bell sites make a strong contribution to the anisotropy. These sites are preferentially occupied by the substituents when Co is partially replaced by other TM atoms, and this can even change the sign of 7C1, inducing EA behavior (Deportes et al. 1976). This effect has also been described in terms of a single-ion, local crystal-field model but certain results seem to require that band-structure changes also be considered (Perkins and Strassler 1977) i.e., that 3d-electrons be treated as collectivized. 

The deviations from the Debye law are, undoubtedly, connected with the structure of the electron shells of atoms and their shape. By the shape of the atom or ion, one may understand the shape of the surface of equal electron density of the outer-shell electrons of the atom. From this point of view, the ions whose outer electrons are the s electrons have spherical symmetry, whereas the p electrons and d electrons form dumb-bells, the shapes of which are shown in Fig. 

GaSe, shown in Figure 3, has a iayer structure with a very pronounced cieavage between the layers. The Ga atoms form dumb-bells. The non-bonding orbitals on the Se atoms are directed towards the space between the layers. 

Fig. 5 Postulated stacking arrangements for dumb-bell-shaped arborols. Top orthogonal stacking of the alkane arborols. Bottom non-orthogonal relationships between adjacent molecules of alkyne arborols leads to A—helical structures, B—ribbon structures. Reproduced from reference [38b] with the kind permission of Wiley-VCH...

In a similar general approach, Namazi and Adeli reported thermore-versible hydrogels formed from ABA-type copolymers. The A blocks were constituted from dendritic citric acid and the B block was a polyethylene glycol unit (Fig. 10) [47]. This dendrimer has a dumb-bell-type structure, somewhat reminiscent of Newkome s bola-amphiphiles, only with a polymeric spacer chain. First- and second-generation dendritic systems were reported and it was found that the gels could also be formed in the presence... 

The development of methods for the synthesis of topologkal compounds with elements of directionality began as early as studies by Schill Thus, fee synthesis by this method involves an attempt to bind temporarily the chains with the macrocycle, which results in compound 1 with a prerotaxane structure (see the diagram below) and its isomer 2, which are in conformational equilibrium After connection of bulky end groups and breaking of temporary chemical bonds, a mixture is form consisting of a rotaxane, a dumb-bell , and a macrocycle ... 

---