Configuration head-to-tail

The structure of the dimers from mero-substituted derivatives was initially determined by comparison of the observed and calculated dipole moments. For a head-to-tail dimer the dipole moments resulting from the 9 and 9 substituents should cancel each other and the resultant dipole moment should be essentially zero. For a head-to-head arrangement the dipoles would be in the same direction and the resultant should be considerably greater than zero. The dimers produced upon irradiation of 9-chloro and 9-bromoanthracene solutions were observed to be 0.36 and 0.60 D, respectively. Since these values are much less than expected for a head-to-head arrangement for these derivatives (3.8 D), it was concluded that both of these dimers were formed in a head-to-tail configuration/30 ... 

Propagation, or chain growth, takes place in a head-to-tail configuration as a result of resonance stabilization and steric factors by carbocation (M+) addition to another monomer molecule. The head stabilizes the cation best so it is the growing site while the least sterically... 

Figure 1.6. Portion of PP illustrating the head-to-tail configuration, which is typical for polymers.

Proposed organization of the enzymatic activities of fatty acid synthase from animal liver. Fatty acid synthase exists as a dimer of two giant identical peptides (Mr = 272,000). Each subunit has one copy of acyl carrier protein (ACP) and each of the enzyme activities involved in fatty acid synthesis is covalently linked. The two peptides are organized in a head-to-tail configuration in such a way that it is possible to make two fatty acid molecules at the same time. 

Scheme 1.12 The preparation of 13, in which the ligands adopt a head-to-tail configuration.

The head-to-head and head-to-tail configuration of the isomeric ketones (7) and (8) were discernable. A slight distortion of the cyclobutane ring in compound (7) suggests an imbalance in steric strain <81CJC3136>. 

When heated at 150 C in xylene for 24 h with an excess of phospholes, Santini showed that complexes of type XVI gave sandwich complexes of type XVIII with a head-to-tail configuration which were fully characterized including an X-ray crystal structure analysis. Depending on the starting materials, a number of mixed homo- and hetero-bimetallic sandwiches could be obtained. On the basis of structural and NMR data, it appears that there is a strong through-space interaction between the non-bonded iron and phosphorus atoms in these complexes. 

Poly (vinyl chloride) (PVC) is essentially a linear polymer with a head-to-tail configuration. Theoretical calculations and model compound studies predicted that PVC of ideal... 

Propagation. Propagation or chain growth takes place by successive addition of monomer molecules in a head-to-tail configuration. At low temperatures, the chain growth takes place rapidly, and the rate constant is essentially the same for all propagation steps. 

The remaining reactions in fatty acid synthesis take place on the fatty acid synthase multienzyme complex. This complex, the site of seven enzyme activities and ACP, is a 500-kD dimer. Because the enormous polypeptides in the dimer are arranged in a head-to-tail configuration, two fatty acids can be constructed simultaneously. A proposed mechanism for palmitate synthesis is shown in Figure 12.13. 

A head-to-head configuration would be represented by -(CH2CHXCHXCH2)n-Even with a head-to-tail configuration, a variety of possible structures exists. For illustrative purposes, we will consider possible combinations derived from the homopolymerization of A and the copolymerization of A with B. The following are types of polymers that can be prepared. Homopolymerization (involves one monomeric unit in the chain) ... 

Even with the head-to-tail configuration, a variety of structures are possible. These include a simple hnear homopolymer structure... 

The behavior of bimetalated rhodium(II) compounds with head-to-tail configuration (type Vila) in catalytic transformation of a-diazo compounds has been also studiedJ " Results have confirmed that by changing the carboxylate groups and the substituents on the metalated and non-metalated rings of the phosphine the activity and the regio and chemoselectivity of these compounds can be substantially modified. 

We also tested the reactivity and selectivity of mono metalated complexes (Type V). These catalysts, although reactive, were in general less selective than bimetal-ated compounds with head-to-tail configuration. Finally, preliminary studies of reactivity of doubly metalated compounds with the head-to-head configuration (Type Vllb) showed that these compounds are not promising catalysts for C-H insertion reactions. 

The CHX group is considered the head of the monomer and the CH2 group is the tail. If only reaction (1) occurs, the resulting polymer Is said to have a head-to-tail configuration, where the X group is attached on every other carhon in the main chain, as shown in Fig. 3.23. 

CHj-CH- CHj-CH- CHj-CH- CH2-CH-Figure 3.23 Vinyl polymer with regular head-to-tail configuration... 

If both reactions (1) and (2) take place, both head-to-head and tail-to-tail configurations occur randomly in the molecule, along with head-to-tail configurations, as shown in Fig. 3.24. 

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