Variation of Spacer

Entry/No. R Z X Y Solid state (X-ray diffraction analysis) Solution (NMR spectroscopy, molecular weight determination) Ref. 

In solution, they each show one isomer only, which is most likely the cis one. All these compounds are chiral, but they crystallize as racemic mixtures and no efforts have yet been made to separate the enantiomers. 

Formal replacement of the acetylene bridge in the compounds mentioned above by an oxygen atom provides the disiloxanyl-bridged tetraorganodistannoxanes shown in entries 35-37. Like their acetylene-bridged analogs, they adopt B-type stmctures, but exclusively crystallize as frani -isomers. The 

The distance between the two Sn303 layers amounts to approximately 12.2 A. Most interestingly, as with some of the tetraorganodistannoxanes (see discussion below) the molecule shows a twist (37°) between the upper and lower SnsOs rings making it chiral. 

As with the identity of the organic substituents R and the spacers Z, the electronegative substituents X and Y also have a profound influence on the structure of spacer-bridged tetraorganodistannoxanes. Like the parent compounds, enhanced kinetic stability in solution is observed for those cases in which the substituents are different (X Y). This is especially pronounced for the phenyl-substituted derivatives 39-43 (entries 1-5) and also valid for the acetylene-bridged compounds (entries 32-34). 

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A. Patel and T. B. Lindhorst, Multivalent glycomimetics Synthesis of nonava-lent mannoside clusters with variation of spacer properties, Carbohydr. Res., 341 (2006) 1657-1668. 

The variation of the transition temperatures of these polybibenzoates with the number of methylene units in the spacer is shown in the lower part of Fig. 5. Melting temperatures, Tm, (crystal-isotropic melt transition) are obtained [9] for m > 7 and m = 3 (monotropic behavior), while for the other members, Tm really represents the... 

N-heterocyclic compounds containing six-membered rings (pyridine and analogues) behave as excellent -acceptors and in turn they provide a rather soft site for metal ion coordination. The 7r-excessive five-membered pyrazole is a poorer 7r-acceptor and a better 7r-donor and it acts as relatively hard donor site. Inclusion of six- and five-membered N-heterocycles like pyridine and pyrazole in one ligand system leads to very attractive coordination chemistry with variations of the electronic properties.555 The insertion of a spacer (e.g., methylene groups) between two heterocycles, which breaks any electronic communication, makes the coordination properties even more manifold. 

Fichert T, Regelin A, Massing U. Synthesis and transfection properties of novel non-toxic monocationic lipids. Variation of lipid anchor, spacer and head group structure. Bioorg Med Chem Lett 2000 10(8) 787-791. 

Fig. 9.18 The polymer spacer concept for the construction of a biomimetic cell membrane on solids. Mesogenic units, coupling groups and the flexible polymer can be combined either in form of a statistical terpolymer (above). Variation of the ratio of the three monomers allows an easy tuning of the system. In an alternative system, an end-functio-nalized linear hydrophilic polymer chain bearing a coupling group at the proximal and the mesogen at the distal end was employed.

The variation of the temperature interval of structure formation is also accounted for by the effect of side chain mobility on mesophase nucleation. The solutions of cholesterol-containing polymers with longer spacer groups have to be cooled further for mesophase nuclei to be formed. For instance, in a series of PChMO-n polymers the interval of structure formation is for PChMO-14 — 308-313 K, for PChMO-lO — 323-333 K and for PChMO-5 the internal structure is formed at even higher temperatures 187). 

Many variations of K2K dendrons have also evolved. A symmetrical version with a p-alanine spacer 39 at the a-amine of Lys provides a core unit that has symmetrical branching. To limit the flexibility of the branched lysyl template, ornithine, the lower homologue of Lys, as well as more constrained diamino acids 23-25 have been used. 39 62,64 Nonbranched polylysine dendrons 26 that are linear, and helical 71 and cyclic peptides 27 have also been developed to impart different forms of dendrimeric architecture (Scheme 5). 

Variation of the spacer length between CMPO groups attached to the 1,3-and 2,4-positions of the calixarene platform did not lead to appreciable improvements, in terms of extraction or selectivity. 

Phosphoryl-containing podands are acyclic molecules with polyether spacer(s) linking two (in monopodands), three (in dipodands and tripodands), or four (in tripo-dands) terminal phosphine oxide groups (Figure 5.6c). Variations of the lengths of the spacers, of the molecular topology, and of the substituents at the phosphorus... 

The structural mutation of our 29 by variation of the macrocycle (via its heteroatoms and carbon bridges), the lumophore, and the spacer is obviously a rich vein to mine for luminescent PET sensors. Kubo s laboratory (formerly at Kyushu University, now at Kanagawa University, Japan) has systematically explored this line [81-84], 33 has a straight swap of 1-pyrenyl for 9-anthryl [81]. 34 to 36 [81-83] use two nitrogens in the crown ether instead of the one in 29,... 

Table 4.4 Variation of pK, with spacer length in macrocycles of type Hm[4n]aneN4m+ (n = 2-4 m = 3 and 4).

In this family of polymers, with saturated cyclic side chain the effect of the side chain structure and also the effect of the spacer group on the viscoelastic behaviour is clearly illustrated in Fig. 2.11a and b in which it is possible to observe the variation of tan 8 for five members of the series with temperature for 8 and y relaxations. 

Fig. 2.14 (a) Molecular dynamic simulation results of the variation of the free energy change (A G/R) of the chair-to-chair conformational change against of the number of carbon atoms of the spacer group, (b) Probability distribution of the torsional angle obtained at 1000 K for PCHMA... 

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