Two-plane orientation complex

Spectroscopic analysis revealed that the thermally initiated [3 + 2] polycycloaddition produced 1,4- and 1,5-substituted triazole isomers in an approximately 1 1 ratio. This ratio appears to be statistic and dependant on the bulkiness of the organic moieties. For example, hfr-r-P[30(4)-20] with butyl spacers contained slightly more 1,4-triazole isomers than did hb-r-P[30(6)-20] with hexyl spacers. This becomes clearer if we look at the proposed transition states a and b of the [3 + 2]-dipolar cycloaddition (Scheme 16). Because of their molecular orbital symmetry, the acetylene and azide functional groups arrange in two parallel planes, a so-called two-plane orientation complex [48], which facilitates a concerted ring formation. If the monomer fragment or the polymer branch ( ) attached to the functional groups are bulky, steric repulsion will come into play and transition state a will be... 

The intermediate 6.11 cannot be planar. Interaction of the ethene n molecular orbital of styrene can take place with the MO of the sydnone 1,3-dipole only if the o-bond systems of these reagents are located in two approximately parallel planes. Therefore, Huisgen (1963b) suggested the two-plane orientation complex 6.13 for the transition state of this cycloaddition. This transition state is feasible not only for... 

The photocycloadditions show all the characteristics of concerted reactions, including stereospecificity and regioselectivity. Concerted 1,3-dipolar additions are known to proceed via a two-plane orientation complex. For the case of diphenylazirine and methyl acrylate, there are two possible orientation complexes (26 or 27). The interaction of substituent groups in the syn complex 26 can be of an attractive (tc overlap, dipole-dipole interaction) or of a repulsive nature (van der Waals strain). Both effects are probably negligible in the anti complex 27. The ratio of the two steric courses syn and anti) functions... 

Figure 12.19 Schematic diagrams illustrating the arrangement of hacteriochlorophyll molecules in the light-harvesting complex LH2, viewed from the periplasmic space, (a) Eighteen hacteriochlorophyll molecules (green] are hound between the two rings of a (red) and p (blue) chains. The planes of these molecules are oriented perpendicular to the plane of the membrane and the molecules are bound close to the periplasmic space, (b) Nine hacteriochlorophyll molecules (green) are bound between the p chains (blue) with their planes oriented parallel to the plane of the membrane. These molecules are bound in the middle of the membrane.

In addition to the charge control over the reaction discussed above, there is also a marked element of conformational control over alkylation reactions. This is seen clearly in the methylation of the nickel(n) complex of the tetraaza macrocyclic ligand, cyclam (Fig. 5-32). Reaction of the nickel complex with methylating agents allows the formation of a A, A V",A "-tetramethylcyclam complex. In this product, each of the four nitrogen atoms is four-co-ordinate and tetrahedral, and specific configurations are associated with each. Of the four methyl groups in the product, two are oriented above the square plane about the nickel, and two below it. 

Fluorescence in INpOH-aliphatic amine hydrogen-bonded systems in nonpolar rigid matrices like polyethylene films at 77 K is dual in nature [200-202], The ESPT in the hydrogen-bonded complex results in two types of ion pairs, one with in-plane orientation between excited naphtholate and ammonium ions (fluorescence maximum at 395 nm with a lifetime of 5.3 nsec) and the other with an... 

In the case of the complex 29 36, the bromine atoms influence the recognition process as the diol moiety assumes, for the first time, two different orientations in the crystal that are rotated of about 20° in the a-c plane (Figure 39). A ribbon like H-bonded motif for the core in which only two amino groups are fully engaged in H-bonding is evident. The CPK representations of complexes 29 35 and 29 36 are shown in Figure 40. 

Two points should be emphasized. First, according to classical structure theory, all the equivalent positions of a given set should be occupied and moreover they should all be occupied by atoms of the same kind. In later chapters we shall note examples of crystals in which one or both of these criteria are not satisfied an obvious case is a solid solution in which atoms of different elements occupy at random one or more sets of equivalent positions. (The occupation of different sets of equivalent positions by atoms of the same kind occurs frequently and may lead to quite different environments of chemically similar atoms. Examples include the numerous crystals in which there is both tetrahedral and octahedral coordination of atoms of the same element—in the same oxidation state—as noted in Chapter 5, and crystals in which there is both coplanar and tetrahedral coordination of Cu(ii), p. 890, or Ni(ii), p. 965.) The second point for emphasis is if a molecule (or complex ion) is situated at one of the special positions it should possess the point symmetry of that position. A molecule lying on a plane of symmetry must itself possess a plane of symmetry, and one having its centre at the intersection of two planes of symmetry must itself possess two perpendicular planes of symmetry. If, therefore, it can be demonstrated that a molecule lies at such a position as, for example, would be the case if the unit cell of Fig. 2.13 contained only one molecule, (a fact deducible from the density of the crystal) this would constitute a proof of the symmetry of the molecule. Such a conclusion is not, of course, valid if there is any question of random orientation or free rotation of the molecules. Moreover, there is another reason for caution in applying this type of argument to inorganic crystals. 

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