Trimer stmcture

Palladium(II) acetate is usually prepared by the reaction of Pd sponge in hot glacial acetic acid with ititric acid as oxidant. A slight excess of Pd sponge must be used to make sure that all the HNO3 is consumed. Several other paUadium(II) carboxylates can be obtained similarly. The crystal-structure determination of the acetate reveals a trimeric stmcture with bridging acetates in the sohd state (1). Thus, the compound is better formulated as [Pd(/u-02CMe)2]3. 

Sokolova EV, Hawthorne FC (2001) The crystal chemistry of the [Msijin.M] trimeric stmctures from hyperagpaitic complexes to saline lakes. Can Mineral 39 1275-1294 Sokolova EV, Khomyakov AP (1992) Crystal stmctme of a new mineral, NasSr (P04)(C03), from the bredliit group. Dokl Akad Nauk SSSR 322 531-535... 

Other relevant reactions of resin acids are those involving their addition to formaldehyde and/or phenols. Formaldehyde adds to abietic acid to form complex mixtures containing mono and dihydroxymethyl derivatives [28-31]. Under alkaline conditions, the formation of the mono derivative is favoured, whereas in acetic acid, the dihomologue (isolated as acetate) is the major product (Fig. 4.10). More recently, studies of the acid-catalyzed condensation of abietic acid with formaldehyde showed that, under such conditions the reactions mixtures were predominantly composed of trimeric stmctures [26, 32]. 

Single-Stack Acceptor. Simple charge-transfer salts formed from the planar acceptor TCNQ have a stacked arrangement with the TCNQ units facing each other (intermolecular distances of ca 0.3 nm (- 3). Complex salts of TCNQ such as TEA(TCNQ)2 consist of stacks of parallel TCNQ molecules, with cation sites between the stacks (17). The interatomic distance between TCNQ units is not always uniform in these salts, and formation of TCNQ dimers (as in TEA(TCNQ)2) and trimers (as in Cs2(TCNQ)Q can lead to complex crystal stmctures for the chainlike salts. 

Catalysis is usually accompHshed through the use of tertiary amines such as triethylenediamine. Other catalysts such as 2,4,6-/m(/V,/V-dimethylaminomethyl)phenol are used in the presence of high levels of cmde MDI to promote trimerization of the isocyanate and thus form isocyanurate ring stmctures. These groups are more thermally stable than the urethane stmcture and hence are desirable for improved flammabiUty resistance (236). Some urethane content is desirable for improved physical properties such as abrasion resistance. 

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

Much work has been done on the structure of the metal alkoxides (49). The simple alkaU alkoxides have an ionic lattice and a layer stmcture, but alkaline earth alkoxides show more covalent character. The aluminum alkoxides have been thoroughly studied and there is no doubt as to their covalent nature the lower alkoxides are associated, even in solution and in the vapor phase. The degree of association depends on the bulkiness of the alkoxy group and can range from 2 to 4, eg, the freshly distilled isopropylate is trimeric (4) ... 

Fig. 8. De novo designed a-hehcal proteins. Dimers of the amphiphilic helix-forming peptide a B, GELEELLKKLKELLKG (see Table 1), in which the nature of the linker connecting the individual heflces plays a critical role in the stmcture of the final protein, (a) Using a Pro residue as the linker, ie, a B-Pro-a B, three molecules aggregated to form a trimeric coded-cod. (b) Using Pro-Arg-Arg as the linker, ie, a B-Pro-Aig-Arg-a, resulted in the...

Bismuth Penta.fIuoride, Bismuth(V) fluoride consists of long white needles that have been shown to have the same stmcture as the body-centered, tetragonal a-polymorph of uranium hexafluoride. The density of the soHd is 5.4 g/mL at 25°C. The soHd consists of infinite chains of trans-bridged BiF polyhedra dimers and trimers are present in the vapor phase (22). Bismuth pentafluoride may be prepared by the fluorination of BiF or... 

FIGURE 6. Optimized stmctures of trimer (CH2=CHOLi) (5a) complex with three molecules of Me20 from PM3 computations and of the Cubic tetrasolvated tetramer (CH2=CH0Li)4(Me20)4 (5b) obtained at the PM3 level. Reprinted with permission from Reference 12. Copyright 1997 American Chemical Society... 

Also, some amide-linked dimers, sulhde-linked dimers, sulfone-linked dimers, and trimers were synthesized by Jung et al. These compounds showed potent and selective inhibition on the growth of certain human cancer cell lines (Stmcture 5-23). In particular, trimer 152 was comparable with that of clinically used anticancer dmgs."2 ... 

Special ID systems, chains made of rings, are fluorides with jarhte or usovite stmctures (Section 3.2.10). HomometaUic or bimetalhc chains with various combinations of spins (or nonmagnetic metals) allow the study of ID antiferromagnets, ID ferrimagnets, or hnear trimers. ... 

Liquid SO3 may be prepared in the laboratory by a similar method to the industrial process. When completely dry, it remains as a liquid for many months but traces of water (as low as 10 mol%) catalyze the polymerization to jS-SOs. In the liquid, the monomeric species is in equilibrium with a trimer, S3O9, which has a similar, nonplanar, stmcture to the trimer in K-SO3. 

The iminoborane. i-PrBsN-r-Bu. trimerizes to /-PrBN-r-Bulj which has a Dewar-borazinc type stmcture. x Compare this structure to that of borazine. 

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