Tetrahedra rings

Poggemann, et al. [43] managed to obtain high spatial resolution AMF pictures of a fresh fracture surface of the amorphous silica, like that shown in Fig. 5.10. Seen are 0-0 and Si-O atomic distances, the Si04 tetrahedra, rings of such tetrahedra, as... 

Fig. 4.33 Crystal structure of apophyllite showing the four-fold Si tetrahedra rings as well as Ca...

TABLE 5. Tetrahedron rings observed in silicates, phosphates and germanates... 

Figure 6 18-membered tetrahedron ring of KNae[SigO 8(OI )9l l 9H2O. 

Roth H D, Weng H and Herbertz T 1997 CIDNP study and ab initio calculations of rigid vinylcyclopropane systems evidence for delocalized ring-closed radical cations Tetrahedron 53 10 051-70... 

Birch Reductions reduction of aromatic rings Organic Reactions 1976, 23, 1. Tetrahedron 1986, 42, 6354. Cornprehensice Organic Synthesis 1991, voJ. 8, 107. 

The compound anisatin is an example of a naturally occur ring 3 lactone Its isolation and structure determination were described in the journal Tetrahedron Letters (1982) p 5111... 

Fig. 1. Methods for representing SiO and AlO tetrahedra by means of (a) baH-and-stick model, (b) soHd tetrahedron, (c) skeletal tetrahedron, and (d) spare-filling of packed spheres (1). (e) Linking of four tetrahedra in a four-membered ring, (f) Secondary building unit called tmncated octahedron as...

C), and is much less reactive it is therefore safer and easier to handle, and is essentially non-toxic. The amorphous material can be transformed into various crystalline red modifications by suitable heat treatment, as summarized on the right hand side of Fig. 12.3. It seems likely that all are highly polymeric and contain three-dimensional networks formed by breaking one P-P bond in each P4 tetrahedron and then linking the remaining P4 units into chains or rings of P atoms each of which is pyramidal and 3 coordinate as shown schematically below ... 

In certain small-ring systems, including small propellanes, the geometry of one or more carbon atoms is so constrained that all four of their valences are directed to the same side of a plane (inverted tetrahedron), as in 98. An example is 1,3-... 

Wehle, D. Fitjer, L. Tetrahedron Lett., 1986, 27, 5843, have succeeded in producing two conformers that are indefinitely stable in solution at room temperature. However, the other five positions of the cyclohexane ring in this case are all spiro substituted with cyclobutane rings, greatly increasing the barrier to chair-chair interconversion. 

For definitions of axial, equatorial, and related terms for rings of any size, see Anet, F. A.L. Tetrahedron Lett., 1990, 31, 2125. 

For an example of OCOR as a neighboring group where the ring size is seven-membered, see Wilen, S.H. Delguzzo, L. Saferstein, R. Tetrahedron, 1987, 43, 5089. 

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