Double selected structural data

Figure 15.12 The structure of 34-Tbt (a) and [34-Bbt] (b). Hydrogen atoms are omitted for clarity. Table 15.2 Selected structural data for compounds with Pn=Pn double bonds (Pn = Sb, Bi).

Single- and double-selective relaxation-rates, together with n.O.e. experiments, have been used to examine the configuration and conformation of asperlin (1) in benzene solution." " Comparing experimental distances for the proton pairs H-4,H-7 and H-5,H-7 with those obtained from molecular models, it was possible to confirm earlier evidence that the oxirane ring is trans, and also to show that, of the two possible diastereoisomeric forms (49a and 49b), the data are more fully compatible with structure 49a, the... 

In this review we will concentrate on the metrical aspects of this interaction as derived from a considerable body of structural data. We wall not attempt a comprehensive review rather, we use selected examples and discuss the results within the framework of current theories of bonding in these systems. These examples will be limited to nonring molecules interacting through one double bond, that is, olefins, acetylenes, diazenes, ketones, and imincs. Consequently, the emphasis in this review differs from that of other recent reviews (40, 46, 55, 58, 72) on metal-olefin and related complexes. 

Current methods take root in the early 1960s, when the conformational analysis of macromolecules became of general interest [29-30]. Anderson et al. [31] used model building and X-ray diffraction studies to determine the double helical structures of polysaccharides using crystalline structure data as an initial set of coordinates followed by computational sampling of new structures by rotation around selected covalent bonds. The details of these so-called hard-sphere calculations are described in Rees and Skerrett [32] and Rees and Smith [33]. This approach was also applied to carbohydrate conformations in the analysis of bacteria and polysaccharidic structures and linkages [34-35]. 

Revised Values of Double-Bond Covalent Radii.—This investigation has led to the value 1.34 A. for the carbon-carbon double-bond distance, 0.04 A. less than the value provided by the table of covalent radii.111 4 Five years ago, when this table was extended to multiple bonds, there were few reliable experimental data on which the selected values for double-bond and triple-bond radii could be based. The single-bond radii were obtained -from the study of a large number of interatomic distances found experimentally by crystal-structure and spectroscopic methods. The spectroscopic value of the triple-bond radius of nitrogen (in N2) was found to bear the ratio 0.79 to the single-bond radius, and this ratio was as-... 

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