Skeletal conformation

The helical parameters corresponding to the various skeletal conformations of the blsphenol A polycarbonate chain are calculated. Combining these results with the conformational energy calculations shows that flat-helical and extended conformations are of equal energy for this chain. In addition, cyclic structures are also found to be stereochemically possible. The small values of the characteristic ratio of the unperturbed end-to-end distance and its temperature coefficient are attributed to the equal energy of the flat-helical and extended-helical, as well as the nonhelical, conformers. 

One final consideration that was addressed is the possibility that rotations of the primary alcohol group at C6 could account for the spectral differences seen in the spectra of celluloses I, II, and III and in the spectra of the amyloses. The normal coordinate analyses of the hexoses showed that rotations about the C5-C6 bond can result in minor variations in the region below 600 cm but that the major impact of such rotations is expected in the spectral region above 700 cm . With all of the above considerations in mind, it became clear that the only plausible rationalization of the differences between the Raman spectra of celluloses I, II, and III had to be based on the possibility that differences between the skeletal conformations were the key. The key considerations have been presented elsewhere in greater detail and need not be repeated here. 

Other workers (19-20) have interpreted these differences in the NMR spectra and other data in alternative ways. They believe that celluloses I and II have the same skeletal conformation but are packed in different lattices. In this theory, the differences within the cellulose I family are derived from the size of the unit cells. Valonia contains a larger 8 chain unit cell, whereas ramie contains a mixture of the 8 chain unit cell and the smaller Meyer and Misch unit cell. Therefore the interpretation of the NMR spectra remains controversial. 

FIGURE 2.12 Three recent examples of natural product-hke libraries from Schreiber s group. In the first, the library is prepared in mono- and bicyclic format to probe the importance of skeletal conformation. The next two examples illustrate natural product-hke hbraries made using 1,3-dipolar cycloaddition processes pioneered for total synthesis by Padwa and WUhams, respectively. 

The O-C-C-O-H side-chain conformation is CiGG (gauche,gauche) in all cases. The most stable hydroxyl conformation was determined for each skeletal conformer by conformational search (Appendix) and was used for these calculations. 

Figure 3.8 and Figure 3.9 show the equatorial and meridional X-ray diffraction profiles of cellulose polymorphs [22]. These modifications are said to have the same skeletal conformation as cellulose I that is, a fairly extended zigzag conformation. However, the chain packing, chain stacking, chain direction, and intra/inter hydrogen bonds are different from one another, which is reflected in the diffraction profiles. 

We review here theoretical and experimental results that probe the influence of this micro-environment - i.e., axial ligands, hydrogen bonds, orientation of substituents, and skeletal conformations - on the photochemical and photophysical properties of photosynthetic chromophores. 

Extension of these calculations[17] to the different BChl a conformers of P. aestuarii yields optical maxima ranging from 733 to 842 nm, larger than the spread observed in solution, 793-825 nm, but nonetheless suggesting that different conformations can influence the optical spectra of BChls (the discrepancies may reflect uncertainties in the crystallographic data used in the calculations for the skeletal conformations and the orientations of substituents). 

Let us briefly return to the skeletal conformation given in Figures 2.2b and 2.3, where the bond angles 0i and the dihedral angles (pi are depicted. Let us also imagine a hypothetical situation where there are no restrictions on 0 and (pi. Of course, this is a violation of fundamental laws of chemical bonds. Nevertheless, this imagination allows us to map a polymer conformation to a trajectory of a... 

Table 7 Separate Interaction Energy Parameters for the Skeletal Conformation of PAVE...

Similarly, the skeletal conformation of a three atom helix is uniquely determined by specifying the following quantities bi = ri = = l il =... 

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