Trimer links

A more complex variation of the structure is observed in cytosine monohydrate [CYTOSH], which was studied as the deuterated compound by neutron diffraction. It contains cyclic trimers linking -ND2, D20 and C=0 groups (Fig. 15.11b). 

As a structural model of the zinc(II)-OH species of carbonic anhydrase. the first mononuclear OH -bound zinc(TI) complex 6 was prepared by mixing zinc(II) ion. the ligand, and KOH. and was characterized by x-ray crystal analysis.The zinc(II)-OH complex lb is a cyclic trimer linked by three hydrogen bonds between each zinc(II)-bound hydroxide group, as shown by x-ray crystal studyP However, in aqueous solution, this trimer dissociates into monomeric zinc(II)-OH species lb. The zinc(II) complex 6 in CHCI3 reacted immediately with CO2, possibly to form a HCOa" complex 7 that ultimately and irreversibly gave a bridging carbonate complex 8. However, quantitative determination of the value for the zinc (II)-bound H2O or of the nucleophilicity of 6 was not reported. The HCOs complex 7 was characterized by IR spectroscopy [1675 and 1302 cm for zinc(n)-bound bicarbonate]. The formation of the bicarbonate com-... 

A series of porphyrin trimers linked into a star-shaped constract with triary-lamine in the center was synthesized and applied to BHJ SCs [125-127]. These porphyrins were used as either donors or acceptors in the BHJ SCs. The PCE of these BHJ SCs was higher if these porphyrins served as D components of the active material. The PCE of these cells did not exceed 0.4 % [125, 126]. Recently, a similar triazine-bridged ZnP triad approached PCE of 4 % (Table 27, Scheme 56) [127]. Apparentiy, this improvement was due to symmetry breaking in the porphyrin triad. That is, two ZnPs with phenyl i 5o-substituents were linked with free-base porphyrin bearing one carboxyphenyl and two 2,4,6-trimethylphenyl substituents. 

Combination techniques such as microscopy—ftir and pyrolysis—ir have helped solve some particularly difficult separations and complex identifications. Microscopy—ftir has been used to determine the composition of copolymer fibers (22) polyacrylonitrile, methyl acrylate, and a dye-receptive organic sulfonate trimer have been identified in acryHc fiber. Both normal and grazing angle modes can be used to identify components (23). Pyrolysis—ir has been used to study polymer decomposition (24) and to determine the degree of cross-linking of sulfonated divinylbenzene—styrene copolymer (25) and ethylene or propylene levels and ratios in ethylene—propylene copolymers (26). 

The tetramer exists in two-molal zirconium chloride and nitrate solutions, but it polymerizes into cross-linked chains on hydrolysis (190—191) in strong acid solutions, the hydroxyl bridges can be replaced by other anions to form trimers (192) and monomers (192—193). 

The leucine zipper DNA-binding proteins, described in Chapter 10, are examples of globular proteins that use coiled coils to form both homo- and heterodimers. A variety of fibrous proteins also have heptad repeats in their sequences and use coiled coils to form oligomers, mainly dimers and trimers. Among these are myosin, fibrinogen, actin cross-linking proteins such as spectrin and dystrophin as well as the intermediate filament proteins keratin, vimentin, desmin, and neurofilament proteins. 

The 2,6-DHMP condensation produced only one dimer and a significant amount of trimer as depicted in Scheme 8. The structure of the trimer was not reported. The reaction path is analogous to that of 2-HMP, but occurred at a faster rate. 2,6-DHMP was the only derivative to form a significant amount of trimer under the reaction conditions studied. This supports the idea that ortho-linked PF polymers should have a faster cure than others. It also points out the futility of attempting to manufacture an ortho-Ymkcd polymer under alkaline conditions. Extension of the polymerization process as depicted in Scheme 8 leads to a continual reduction in the amount of para functionality available for condensation as shown in Table 7. 

Orthorhombic HBO2 consists of trimeric units B303(0H)3 which are linked into layers by H bonding (Fig. 6.26) all the B atoms are 3-coordinate. Monoclinic HBO2 is built of chains of composition [B304(0H)(H20)] in which some of the B atoms are now 4-coordinate, whereas cubic HBO2 has a framework structure of tetrahedral BO4 groups some of which are H bonded. The increase in CN of B is paralleled by an increase in density and mp. 

The linked MeF6 octahedron structure remains unchanged when moving from solid to gaseous phase [200]. Various researchers indicate that the gaseous phase above niobium pentafluoride, NbFs, predominantly consists of trimers, (NbF5)3 with a D3h symmetiy [201-203]. 

In a very recent study, it has been demonstrated116 that zinc 5,15-bis(3,5-di-tert-butylphenyl)-porphyrin (13) without any activating halogen atoms at the chromophore can be directly linked in a very simple oxidative coupling reaction with silver(I) hexafluorophosphate to a mixture of porphyrin dimers, trimers and tetramers. The separation of the product mixture was achieved by gel-permeation chromatography based on the molecular weights of the oligomers. The dimer when re-exposed to the same reaction conditions yielded 25% of the tetramer.116... 

As an approach to biomimetic catalysis, Sanders and colleagues [67] synthesized a series of 1,1,2-linked cyclic porphyrin trimers that allow the stereo- and regiochemistry of the Diels-Alder reaction of 84 and 85 within the molecular cavity to be controlled, thereby producing prevalently or exclusively the endo 86 or the exo 87 adduct. Two examples are illustrated in Scheme 4.18. At 30 °C and in the absence of 88, the reaction furnishes a mixture of diastereoisomers, while the addition of one equivalent of trimer 88 accelerates the reaction 1000-fold and the thermodynamically more stable exo adduct 87 is the sole detectable product. 

In contrast, the trimer 89 with ethyne and butadiyne links stabilizes the thermodynamically disfavored endo transition state, and the endo adduct 86 is rapidly and almost exclusively formed. 

An X-ray crystal-structure determination (370) revealed that mixed, nickel coordination exists in the trimeric complex, [Ni(S2CPh)2]3. The trimeric structure is similar to that found (371) in [Pd(PhCS2>]. The structure contains one molecule of type A linked centrosymmetrically through short Ni-S bridges to two molecules of type B, the three molecules being closely parallel (XXVII). Bonamico and co-workers (372)... 

Furthermore, C-type procyanidins include trimers composed of fiavan-3-ol units linked mainly through C4 C8 (Fig. 4). 

Third, metallocene units, such as ferrocene or ruthenocene, have been linked to phosphazene cyclic trimers or tetramers and these were polymerized and substituted to give polymers of the type mentioned previously (41). Polyphosphazenes with ferrocenyl groups can be doped with iodine to form weak semiconductors. Polymer chains that bear both ruthenocenyl and ferrocenyl side groups are prospective electrode mediator systems. 

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