Trigonal molecules

Just as there are enantiotopic and diastereotopic atoms and groups, so we may distinguish enantiotopic and diastereotopic faces in trigonal molecules. Again we have three cases (1) In formaldehyde or acetone (G), attack by an achiral reagent A from either face of the molecule gives rise to the same transition state and product the two faces are thus equivalent. (2) In butanone or acetaldehyde (H), attack by an achiral A at one face gives a transition... 

However, non-centrosymmetry does not automatically imply a dipolar molecule, or, more generally, vectorial properties. Also molecules without a dipole moment can exhibit second-order nonlinear optical properties. Tetrahedral molecules, such as CC14, and trigonal molecules, such as BC13, also lack centrosymmetry. However, they cannot be oriented in an electric field, due to the absence of a dipole moment. Therefore, they can simply not be measured by EFISHG. Also ionic species cannot be measured, since these migrate, rather than rotate, under the influence of an applied field. 

On the other hand, if the molecule has a very symmetric structure or if the polarities of different bonds cancel each other, the molecule as a whole may be nonpolar, even though the individual bonds are quite polar. Tetrahedral molecules such as CH4 and CCI4 and trigonal molecules and ions such as SO3, N03, and C03 are all nonpolar. The C — H bond has very little polarity, but the bonds in the other molecules and ions are quite polar. In all these cases, the sum of all the polar bonds is zero because of the symmetry of the molecules, as shown in Figure 3-18. 

The molecular orbitals of other trigonal species can be treated by similar procedures. The planar trigonal molecules SO3, N03, and C03 are isoelectronic with BF3, with three cr bonds and one tt bond, as expected. Group orbitals can also be used to derive molecular orbital descriptions of more complicated molecules. The simple... 

Fig. 4. The alternative conformation of the terminal oxygen atom for trigonal molecules (XO3) coordinated to the copper(II) ion in the plane a) Out-of-the-plane bonding, b) No out-of-the-plane bonding...

Fig. 11. aj wavefunctions for tetrahedral and trigonal molecules as a function of distortion. 

The bonding in group 13 molecules is similar. The incomplete electron shell of the atoms contains one p and two s electrons. The three covalent bonds are formed with a hybridized sp bonding configuration. Thus, a planar, trigonal molecule is formed with the three ligands separated by angles of 120°. 

As each cryptand core inherits threefold symmetry, there arises two possibilities by which these trigonal molecules can crystallize (Fig. 34) a planar centrosymmetric hexagonal lattice formed by the interactions between identical groups resulting in SHG inactive molecules or noncentrosymmetric trigonal lattice formed by the interaction between different groups leading to the SHG... 

The theory outlined here for trigonal molecules is readily generalized to pentagonal and hexagonal molecules (Zgierski and Pawlikowski, 1979b). Most of the basic conclusions reached remain valid for these systems, except that Jahn-Teller and Renner-Teller activity is now distributed over different e modes in most systems. Thus in D, 1,2 x 2,1 is Jahn-Teller active, while ,2 x ei 2 is Renner-Teller active. The same holds for except that 2 x 2 is both Jahn-Teller and Renner-Teller active. 

The positions of most of the hydrogen atoms were determined. Secondary hydroxyl groups of the hexagonal cyd molecules 0 (2) H and 0 (3) H are respe cti vely acceptor and donor- In the trigonal molecule, the two secondary hydroxyl groups are donors in one glucose residue and acceptors in the other residue of the asymmetric unit. 

Scheme 5 The HTT molecule (14), together with two other rigid, trigonal molecules (15 and 16), for directly reacting with metal species (e.g., PtClj for 15 and BiClj for 16) to produce covalent metal-organic networks with porous and semiconductive properties...

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