Highly-branched symmetric

Thus the hydrogen-bonding is apparently dependent, amongst other factors, upon the nature of the alkyl substituents. Perhaps the more highly branched secondary alkyl substituents increase the hydrophobic environment about the hydrogen bond, thereby favoring a symmetrical interaction. In any event, it is apparent that this interaction is best described by a very broad, shallow potential function. 

As indicated above (Ohme and Schmitz [38a, p. 339]) primary amines converted into the alkylamides of sulfuric acid can be oxidized with sodium hypochlorite to azo compounds. The reaction appears to proceed by way of an intermediate hydrazine, which is ultimately oxidized [38a], The reaction is suitable for the formation of symmetrically substituted azoalkanes. Highly branched primary aliphatic amines have been oxidized with sodium hypochlorite in an aqueous dioxane medium [77],... 

Fromont, C., Bradley, M. High-Loading Resin Beads for Solid Phase Synthesis Using Triple Branching Symmetrical Dendrimers. Chem. Commun. 2000, 283-284. 

For satisfactory diemo- and stereoselectivity, most catalytic, direct cross-aldol methods are limited to the use of non enolizable (aromatic, a-tert-alkyl) or kineti-cally non enolizable (highly branched, ,/funsaturated) aldehydes as acceptor carbonyls. With aromatic aldehydes, however, enantioselectivity is sometimes moderate, and the dehydration side-product may be important. With regard to the donor counterpart, the best suited pronucleophile substrates for these reactions are symmetric ketones (acetone) and ketones with only one site amenable for enolization (acetophenones). With symmetric cyclic or acyclic ketones superior to acetone, syn/anti mixtures of variable composition are obtained [8b, 11, 19a]. Of particularly broad scope is the reaction of N-propionylthiazolidinethiones with aldehydes, which regularly gives high enantioselectivity of the syn aldol adduct of aromatic, a,fi-unsaturated, branched, and unbranched aldehydes [13]. 

Since 1985, Newkome and co-workers [92-94] have reported the synthesis of related symmetrically branched macromolecules which they refer to as arborols . These macromolecules are highly branched dendrimers which are not amenable to size control as a function of generation in that reiterative chemistry for advancing concentric growth has not been reported. These prototypes have been used successfully for controlling molecular shape and, in some cases, surface moieties. To date, examples of uni-, di- and tri-dendron arborols have been reported [92-94] (see Sect. 5). 

Miller et al. reported21,22,23 that highly symmetrical, mostly single-element, highly branched polyphenylenes were prepared by the stepwise... 

The name dendrimer comes from the ancient Greek words dendron , which means tree, and meros , which means part. Dendrimers are synthetic highly branched globular molecules.They consist of a central core molecule with three attachment points dendrons which bind to these attachment points look like tree arms and sometimes have surface modification (see Figure 11.5). They form highly symmetric, generally mono-disperse and pure nanocarriers which have well-defined architecture, size and shape. 

Growing out of the catenation work was the development with C. Smith concerned with the preparation and examination of the completely symmetrical and highly branched types. One representative of this group is tetrakis(trimethylsilyl)silane. 

Another unique highly branched and symmetrical organopolysilane, developed with Harrell, has become available recently. This is hexakis(tri-methylsilyl)disilane. 

At the nanoscale dimensional level it was shown by Hirsch et al. [276] that [60]-fullerenes could be used as a core tecton to construct a core-shell molecule with 7b-symmetrical Ceo core and an extraordinarily high branching multiplicity of 12. 

Dendrimers are nanometer-sized, highly branched and monodisperse macromolecules with symmetrical architecture. They consist of a central core, branching units and terminal functional groups. The core together with the internal units, determine the environment of the nanocavities and consequently their soiuhiiizing properties, whereas the external groups the solubility and chemical behaviour of these poiymers [53]. 

To consider that deagglomeration and agglomeration are two symmetrical processes is surely a strong hypothesis and likely the reason for the deficiencies of the Kraus model. In order to somewhat circumvent it, recent theoretical developments were made with an explicit reference to the fractal description of CB aggregafes, and by considering that, above a percolation threshold, highly branched aggregates can flocculate and form a secondary... 

Hyperbranched pol)miers are prepared by the direct, one-step polycondensation of AB monomers with two different types of ftmctional groups (A and B) that can react with each other to form a covalent bond, and the total number of reactive sites is x + 1 (x > 2) [48]. Hyperbranched polymers are highly branched like dendrimers, but their structure is not regular or highly symmetrical because linear segments can be formed, as shown in Scheme 10. It is important to note that in 1952 Flory [49] recognized the unique polymer structural type that... 

The effect of the AK = 1 selection rule, compared with AK = 0 for an transition, is to spread out the sets of P, Q, and R branches with different values of K. Each Q branch consists, as usual, of closely spaced lines, so as to appear almost line-like, and the separation between adjacent Q branches is approximately 2 A — B ). Figure 6.29 shows such an example, E — A band of the prolate symmetric rotor silyl fluoride (SiH3F) where Vg is the e rocking vibration of the SiH3 group. The Q branches dominate this fairly low resolution specttum, those with AK = - -1 and —1 being on the high and low wavenumber sides, respectively. 

The selection rules are the same for oblate symmetric rotors, and parallel bands appear similar to those of a prolate symmetric rotor. However, perpendicular bands of an oblate symmetric rotor show Q branches with AK = - -1 and — 1 on the low and high wavenumber sides, respectively, since the spacing, 2 C — B ), is negative. 

---