Cyclic structures, definition

Nucleosides are compounds formed when a base is linked to a sugar via a gly-cosidic bond (Figure 11.10). Glycosidic bonds by definition involve the carbonyl carbon atom of the sugar, which in cyclic structures is joined to the ring... 

The free amino group of the amino ester may then react analogously with another molecule of the monomer, etc. The kinetics of the polymerization are in harmony with a mechanism of this sort. The final polypeptide may contain up to 300 or more structural units. While the polymerization of N-carboxyanhydrides is closely analogous to the addition polymerizations of ethylene oxide and of other cyclic substances, definition unfortunately classifies it as a condensation polymerization inasmuch as carbon dioxide is eliminated in the process. 

The major results of this work are (1) the definite establishment of the presence of dimers in the three alcohols (2) the determination of heats of formation of the dimers and (3) the s,-imort for a cyclic structure of the alcohol dimer. 

The acetaldehyde complex takes a dimeric form in benzene solution. Three possible structures, two oxygen-bridged and one alkyl-bridged four-membered cyclic structures, are conceivable for the dimeric form of this complex. X-ray structure analysis of the single crystal of this complex (Fig. 6) demonstrated definitely the structure to be the oxygen-bridged four-membered cyclic structure [XIX]. This structure supports the assumed structure [XVIII] (46]. 

The above definition excludes a number of types of heteroatom cyclofunctionalization reactions from discussion in this chapter. Examples include neighboring group participation in which the cyclic structure is only a transient intermediate cyclization reactions where the C—C tr-bond is internally activated... 

Consider a particular closed loop in such a diagram. In general, a certain niimber of interaction lines are attached to this loop at definite points. At each such point the momentum along the loop changes discontinuously. Thus with each interaction line ending at the loop we may associate a certain momentum transferred to the loop. From the cyclic structure of the loop it follows immediately that the siun of the transferred momenta must be zero (i.e., momentum is conserved around a closed loop). Two kinds of momentum transfer are possible (a) momentum transfer from other loops (or eventually from the same loop) with dotted lines (b) momentum transfer from the external field, i.e., from the positive lattice (wiggly lines). 

A cycloaddition reaction is actually a type of pericydic reaction, but the term peri-cyclic includes other types of reactions. The textbook definition of pericydic is a reaction whose transition state has a cyclic structure (i.e., the electrons are flowing in a closed loop). In addition to cycloaddition reactions (which exchange two Jt-bonds for two o-bonds, or vice versa), pericydic reactions include sigmatropic reactions, electro-cyclic reactions, and cheletropic reactions (and a few others which we ll ignore). 

Latajka and Scheiner analyzed their results also on the basis of a more rigorous definition of many-body effects in which relative geometries are frozen in the cyclic structure moreover, their prescription deals explicitly with BSSE [103]. They found that the three-body term is not negligible in either trimer. This term is approximately half as large as the two-body interaction energy in (HF)3 while about V4 as large in (HC1)3. The three-body term appears to be adequately described at the SCF level since correlation does not change it appreciably. 

Although in a strict sense the term metallacycle applies to cyclic complexes containing two Ni-G <7-bonds, in this section we use a broader definition of the term, to comprise also those compounds that contain an Ni-G and an Ni-heteroatom bond where the heteroatom has a formal negative charge. The two types of metallacycles can be distinguished by the terms carbometallacycles and heterometallacycles, but their synthesis and reactivity are closely related, and will be treated together (Table 9). Those complexes displaying cyclic structures closed by one... 

The definitive structures of the a- and (3-dextrins were shown by French and Rundle [9] to be composed of six and seven D-glucose residues, each linked a-1 4 in a cyclic structure, respectively. Freudenberg and Cramer [10] and French, et al. [11] showed that 7-dextrin had eight D-glucose residues. Figures 8.1 and 8.2 show the structural formulas and molecular models of these cyclic dex-... 

The first workers, while attempting to elucidate the structure of iron pentacarbonyl, treated Fe(CO)s with butadiene in a sealed tube at 150° C for several hours and obtained a product which analyzed as C4HjFe(CO)3. Although they were not able to assign a definite structure to this material they considered that the iron atom was bonded to the terminal carbon atoms of the diene and to three carbonyl groups. The cyclic structure (I) was proposed however, they stated that the alternative structure (II) could not be ruled out. 

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