Tetrahedral C atoms

Figure 2-73. The stereo part" (right) of a tetrahedral C-atom inciuded in the connection table of an SMD file.

The given structure A is confirmed by interpretation of the CH COSY and CH COLOC diagrams. All of the essential bonds of the deealin stmcture are derived from the correlation signals of the methyl protons. In this, the DEPT subspeetra differentiate between the tetrahedral C atoms which... 

Figure 29-13 (A) Structure of expected intermediate with tetrahedral C-atom in peptidyltransferase reaction with a tRNA, with a minihelix analog, or with the antibiotic puromycin. (B) Transition-state (or bisubstrate) analog formed with puromycin and a mimic of the CCA end of a tRNA. See Box 29-B.

The chiral center most frequently encountered is the asymmetric carbon atom, a tetrahedral C atom, bonded to four different substituents. Chiral centers of this type are known for many other elements (4). However, chiral centers are also found in other polyhedra, e.g., the metal atoms in octahedral compounds containing three bidendate chelate ligands. Chirality axes, present in the atrop isomers of ortho-substituted biaryls, occur in coordination chemistry in appropriately substituted aryl, pyridyl, and carbene metal complexes. Well known examples of planar chirality in organometallic chemistry are ferrocenes, cymantrenes, and benchrotrenes containing two different substituents in 1,2- or 1,3-positions relative to each other (5-5). 

The given structure A is confirmed by interpretation of the Cff COSY and Cff COLOC diagrams. All of the essential bonds of the decalin structure are derived from the correlation signals of the methyl protons. In this, the DEPT spectra differentiate between the tetrahedral C atoms which are bonded to oxygen (75.5 ppm Cf/—O 72.5 ppm C—O 66.6 ppm C// —O). The methyl protons at J.J9ppm, for example, give correlation maxima with the C atoms at 72.5 59.4 and 44.1 ppm A... 

The valence bond approach is especially useful in organic chemistry where so many molecules are built of tetrahedral C atoms, sp hybridised. The concept of hybrids is intuitively very satisfying because they fit visually with our perceived picture of the shape of a molecule with its directed bonds between pairs of atoms. Unfortimately, the VB approach is not satisfactory for species like C03 , NOj, and benzene because the VB picture does not reflect the known chemical structure. A new concept of resonance hybrids must be introduced, and C03 must now be represented by a combination of three Lewis-octet structures. Worse still, the VB approach cannot easily give a satisfactory bonding picture for either of the important molecules O2 or CO. 

The optical isomers in (a) a tetrahedral C atom represented as nonsuperimposable mirror images of one another and (b) the optical isomers of the [Co(en)3] + ion. 

Chinese gallotannin see Tannins Chirality the necessary and sufficient condition for optical activity (rotation of the plane of polarized light). C. means handedness (from the Greek Kelp = hand). Chiral molecules have no second order symmetry element (center, plane or axis of symmetry) and exist in two mirror-image forms (enantiomers) which cannot be rotated in such a way as to coincide. Most chiral compounds contain an asymmetrically substituted C-atom, i.e. a tetrahedral C-atom with 4 different substituents [E.L.EIiel, S.H.Wilen L.N.Mander Stereochemistry of Organic Compounds, Wiley Sons New York, 1994]... 

Fig. 3.2 Examples of the topological characteristics of H atoms on tetrahedral C atoms...

Spiroconjugation [6] is claimed to occur in the electronic structure of the polyspiroquinoid chain. [1] This type of through-space interaction can be considered in terms of 4 p atomic orbitals held spiro to each other about a tetrahedral C atom (see Fig. 3). These four orbitals interact to form 4 combinations of differeing symmetry with respect to the two perpendicular mirror planes dividing the tetrahedral C atom. One combination is symmetric under reflection in each mirror plane, labeled (SS), and there are also (SA), (AS) and (AA) combinations. The (AA) combination is shown, it is of the proper symmetry for a bonding spiro-type interaction. 

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