Symmetric geometrical configuration

It was demonstrated in Section 7.2.1.3 that the efficiency of orbital hyperconjugation in the spiro center depends both on the geometric configuration of the LEPs of the heteroatoms and adjacent bonds and on the electronic properties of the heteroatoms. Since symmetrical spiropyrans contain the same heteroatoms in the spiro center, efficiency of n-a interactions depends mainly on the electronic state of the heteroatoms as determined by the nature and positions of substituents in the benzopyran moiety. 

The structures observed in the mass spectra of fullerene molecules covered with alkaline earth metals, as described in the previous section, all seem to have a geometric origin, resulting in particularly stable cluster configurations every time a highly symmetrical layer of metal atoms around a central fullerene molecule was completed. When replacing the alkaline earth metals by an alkali metal (i.e., Li, Na, K, Rb, or Cs), a quite different situation arises. 

The observed spectrum of HNO is that of a nearly symmetric top, and shows that the molecule is bent in both the upper and lower states. Figure 7 shows the geometrical data. Both states are unquestionably singlet states which, in all probability, are derived from the A state of the linear conformation. Since HNO is isoelectronic with 02, its lowest electron configuration for the linear conformation must be. .. 7r2 which, as in 02, must give rise, in addition to JA, to a 3E and a 1S+ state. As for 02, the 3S is expected to be the ground state as long as the HNO molecule is linear. But it is difficult to predict whether the... 

Tris-chelate complexes exist in enantiomeric configuration A and D about the metal atom, and when the chelating ligand is unsymmetrical, there are also geometrical isomers, cis and trans. Each geometrical isomer exists in enantiomeric forms thus there are four different molecules. In the case of tris complexes with symmetrical ligands, the process of inversion (interconversion of enantiomers) is important. When the metal ions are of the inert type, it is often possible to resolve the complex then the process of racemisation can be followed by measurement of optical rotation as a function of time. Possible pathways for racemisation fall into two broad classes those without bond rupture and those with bond rupture. 

Figure 4. Geometric description of the C3-symmetry transform for 6 points. The centroid of the points is marked by . (a) The original points shown as two sets of 3 points So = (Po, P2, Pd and Si = (Pi, P3, P5. (b) The obtained C3-symmetric configuration.

The geometrical data on these bonds are given in Thble 8.3. They constitute about one quarter of the total number of O-H- -O bonds in the sample of crystal structures surveyed. They range from almost symmetrical bonds1 with r r 2.1 A and 6 d 135° to very unsymmetrical configurations where r-r 0.6 A and 6-6 70°. 

The chemical structure of a polymer determines whether it will be crystalline or amorphous in the solid state. Both tacticity (i.e., syndio-tactic or isotactic) and geometric isomerism (i.e., trans configuration) favor crystallinity. In general, tactic polymers with their more stereoregular chain structure are more likely to be crystalline than their atactic counterparts. For example, isotactic polypropylene is crystalline, whereas commercial-grade atactic polypropylene is amorphous. Also, cis-pol3nsoprene is amorphous, whereas the more easily packed rans-poly-isoprene is crystalline. In addition to symmetrical chain structures that allow close packing of polymer molecules into crystalline lamellae, specific interactions between chains that favor molecular orientation, favor crystallinity. For example, crystallinity in nylon is enhanced because of... 

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