Basicity Chain structure

Benzimidazole (BIm) crystallizes in hydrogen-bonded chains with nearly linear N-H---N donor-acceptor contacts.185 Viewed down the chain axis, the benzene rings form a sawtooth alternation. This motif can accommodate substitution in the 2-position and on the benzene ring, while still retaining the basic chain structure, as seen in the large majority of structures from the CSD. Lahti recently summarized186 magnetostructural studies carried out on radicals linked to neutral aza-arenes. This chapter will focus only on BIm-functionalized radicals. 

Long-range interactions, i.e. those interactions between non-bonded groups which are separated in the basic chain structure by many valence bonds. These interactions cause the molecule to pervade a larger volume ... 

Besides synthesis, current basic research on conducting polymers is concentrated on structural analysis. Structural parameters — e.g. regularity and homogeneity of chain structures, but also chain length — play an important role in our understanding of the properties of such materials. Research on electropolymerized polymers has concentrated on polypyrrole and polythiophene in particular and, more recently, on polyaniline as well, while of the chemically produced materials polyacetylene stih attracts greatest interest. Spectroscopic methods have proved particularly suitable for characterizing structural properties These comprise surface techniques such as XPS, AES or ATR, on the one hand, and the usual methods of structural analysis, such as NMR, ESR and X-ray diffraction techniques, on the other hand. 

The known family of heterometaUic perrhenate structures provides a basic picture of the local structural units and their connectivity patterns. Shown in Fig. 17.1, these include molecular clusters such as M(H20)4(Re04)2 (M = Fe, Co, Cu, Zn) [21-23], and Fe(H20)3(Re04)3 [24], the chain structure of Mn(H20)2(Re04)2 or the... 

Unfortunately, complexes 39 and 40 are still more prone to decomposition than catalyst 16. Therefore, Grubbs sought to investigate a series of new ruthenium catalysts bearing NHCs with varying degrees of iV-heterocyclic backbone and aryl side chain substitution, and catalysts 16 and 30a were chosen as basic catalyst structures [57]. In 2009, complexes 41a-c and 42a-c were prepared to attempt to understand how the degree of substitution on the backbone influences catalyst activity and lifetime (Fig. 3.15). 

A polymeric chain structure is frequently observed for zinc carboxylates. Basic zinc carboxylates form a common tetrameric species, and altering conditions can result in an interconversion between the basic and regular carboxylates.369 Mass spectrometry studies have been carried out... 

There are numerous variations on the basic linear structure of polymers. Returning to our example of polyethylene, we find short chain branches and long chain branches, as shown in Figs. 1.2 and 1.3, respectively. The number and type of these branches strongly influences the way that the molecules pack in the solid state, and hence affect the physical properties. Long... 

Figure 3.5 The basic polynucleotide structure as shown in (a) outline form and (b) in chemical detail. The 5 end of the chain is defined by lacking a nucleotide attached to the first sugar s carbon number 5 the 3 end lacks a nucleotide attached to the carbon number 3 of the last sugar in the backbone...

The domains (as we have defined them) within a subunit very often resemble each other (as discussed in Section IV,B), and those similar domains are frequently related by an approximate 2-fold axis. The 2-fold relationship often occurs even in cases in which it involves considerable inconvenience because the start and end of the chain are on opposite sides of the basic domain structure, so that a long additional... 

The zeolites are generally thought of as having three-dimensional rather than sheet or chain structures. The existence of tetrahedral ring linkages that produce chainlike substructures characteristic of the natural zeolitic species found as needles or fibers reinforces the earlier statement fibers can be constructed from any number of basic units that grow in a preferential direction. 

The very idea of an intermolecular chemical bond, in conjunction with crystal periodicity, leads to the concept of chain formation. One is never too careful, however, because crystal structures are by definition periodic in space and, vice versa, the concept of chain can surreptitiously sneak in without the need of a real chemical bond. Once again, there is a dichotomy between geometrical appearance -a ribbon of molecules close to each other - and objective physics, or the presence of an effective electronic interaction between the partners in the supposed periodic structure. The basic question is, how and when is one allowed to trace a chemical bond, and thus legitimately to draw a chain structure The answer - or at least what in this author s opinion is the only objective and physically sound answer - must come from quantitative evaluation of the involved contact energies, as discussed in the next paragraphs. 

Lactam antibiotics are natural or semisynthetic compounds whose basic nuclear structure consists of a -lactam ring coupled to a thiazolidine (five-membered) or a dihydrothiazine (six-membered) ring to form the penicillin or cephalosporin nucleus, respectively. To those nuclei, various side chains that determine most of the properties of the different -lactams are attached. All members of this group of antibiotics are readily attacked at the -lactam nitrogen by nucleophilic reagents such as hydroxyl ions, alcohols, and primary amines, as well as by secondary amines. They are also susceptible to electrophilic attack at both the... 

Figure 3.8. Basic Four—polypeptide chain structural unit of an immunoglobulin. <- - represents the antibody-combining sites.(From Butler 1969. Reprinted with permission of the American Dairy Science Association.)...

The phenomenon of increased hardness occurs principally in minerals of sheet and chain structures, which link together through the cations (silicates and aluminosilicates, as well as hydrated sheet minerals, such as glauconite, melilite and gypsum—M ranging from 0 to about 1.25), and also in minerals of skeletal structures (borates, phosphates, sulphates, nitrates, carbonates, such as calcite, dolomite and others—Ah from 0 to about 1.15). For this reason, the hardness analysis of minerals with weak bonds demands consideration of the fact that just as the basic crystallo-chemical factors, so is hardness influenced by the form of domains (component parts of structures) in all anisodesmic minerals of chain, sheet or skeletal structure. Depending on the form of domain (and also according... 

Taking into consideration die stability of structures involving C-Si bonds, it is evident that the basic chain itself must be comparable in its stability to that of silica and the silicate minerals, and if the R substituents contain no carbon-to-earbon bonds, as for example with methyl groups, the combination should have excellent thermal stability and chemical resistance. 

---