Bonding sequence

Fig. 9. Molecular model of the crystal conformation of cyclotetraeicosane at —160 °C according to P. Groth, ref. 19. The numbers at the bonds give the rotational angles. The assignment of the CP-MAS 13C-NMR shifts to conformational four bond sequences is given at the carbon atoms of the upper left corner...

Recently, Rankin et al. [84] obtained the X-ray structure of the stable pnictinyl radical and observed a very unusual conformational arrangement, i.e., a syn-syn arrangement of the H-C-P-C-H bond sequence (Fig. 7). 

The condensation reaction of two alcohol molecules to eliminate water and form a C—O—C bond sequence is yet another linkage reaction. A molecule that contains a C—O—C linkage is called an ether ... 

They contain different atom-to-atom bonding sequences. Stereoisomers, on the other hand, are isomers that differ only in the way that atoms are oriented in space, and therefore involve only one coordination sphere and the same ligands and donor atoms. 

Since the two structures are chemically and energetically equivalent, it is possible that in long chains one part tends to assume one version of the structure and another part the other version with a transition zone in between. This zone may extend over a few carbon atoms and one may assume that from one side the double bond gradually become longer and the single bond accordingly shorter, in order progressively to reverse the alternation of the bond sequence ... 

Fig. 9.6 The two different bond sequences which are possible in the trans configuration of polyacetylene.

Figure 9.3 (a) Duplex 3 4 consisting of two different strands with complementary H bonding sequences, (b) Duplex 5 5 consisting of two identical single strands with a self-complementary H bonding sequence. Interstrand NOE contacts revealed by NOESY are indicated by arrows. 

The two most used reversible covalent reactions are disulfide exchange and palladium-catalyzed olefin metathesis. We first probed the incorporation of olefin units into the H bonded duplexes by subjecting the modified duplexes to a Pd (Gmbb s) catalyst. Based on a duplex template with the same unsymmetrical H bonding sequence used for directing the formation of the /3-sheet structures, we prepared two groups (strands 17 and 18) of five olefins covalently linked to the two template strands (Fig. 9.13). Mixing each one of components 17 with each one of components 18 in a 1 1 fashion results in a small library of 25 (5 x 5) members. 

The formation of the cross-linked products demonstrated an apparent correlation to H bonding sequences. What is the mechanism behind the observed sequence specificity One of two possible mechanisms, one kinetic and the other thermodynamic, may be involved in aqueous media. The kinetic mechanism involves intermolecular... 

The above observations are consistent with a thermodynamically controlled process shown in Figure 9.17. Thus, when two strands, A and B, with complementary H bonding sequences and termini that can be reversible cross-linked, are present in the same solution under redox conditions, products A-A (or self-cyclized A ), B-B (or self-cyclized B ), and A-B, may be generated. Among these products, A-B gains the most stabilization from the newly generated, complementary intramolecular H bonds because of the formation of the two disulfide bonds. Thus,... 

Figure 9.17 Two components bearing complementary H bonding sequences can reversibly form disulfide cross-linked products. A-B represents the most stable product because of the stabilization fi om its fully matched, interstrand (now intramolecular) H bonds.

In a recent report, Shi et al. developed a valuable tool for the synthesis of 2,6-trans substituted morpholines by addition of water and alcohol to epoxy alkynes [109]. The procedure involved a domino three-membered ring opening, 6-exo-cycloisomerization, and subsequent intra-or intermolecular nucleophilic addition or a double-bond sequence. 

A general theory is presented on the effect of elongation of a polymer chain on the apportionment of its bonds and bond sequences among various rotational Isomeric states. Numerical calculations are presented for PE chains and for syndiotactlc vinyl polymers. 

Diad CPd) and interdiad

priori probabilities for two-bond sequences (for 400 K5... 

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