Linear arrangement shapes

Section 1 10 The shapes of molecules can often be predicted on the basis of valence shell electron pair repulsions A tetrahedral arrangement gives the max imum separation of four electron pairs (left) a trigonal planar arrange ment is best for three electron pairs (center) and a linear arrangement for two electron pairs (right)... 

Valence shell electron pair repulsion (VSEPR) model (Section 110) Method for predicting the shape of a molecule based on the notion that electron pairs surrounding a central atom repel one another Four electron pairs will arrange them selves in a tetrahedral geometry three will assume a trigo nal planar geometry and two electron pairs will adopt a linear arrangement... 

The above discussion points out the difficulty associated with using the linear dimensions of a molecule as a measure of its size It is not the molecule alone that determines its dimensions, but also the shape in which it exists. Linear arrangements of the sort described above exist in polymer crystals, at least for some distance, although not over the full length of the chain. We shall take up the structure of polymer crystals in Chap. 4. In the solution and bulk states, many polymers exist in the coiled form we have also described. Still other structures are important, notably the helix, which we shall discuss in Sec. 1.11. The overall shape assumed by a polymer molecule is greatly affected... 

Fig. 3.44 Intermittent contact AFM of two virions in linear arrangement on graphite (reproduced with permission from [96]). The measured height of 18 nm corresponds to the true diameter, while the width (ca. 85 nm (fwhm)) is attributed to the AFM tip shape. Reproduced with permission from [96]. Copyright 2004. American Chemical Society...

Of all the porphyrin isomers, porphycene and its derivatives have been studied most thoroughly. DFT calculations predict [33] that porphycene 1 as a free base is the most stable of all isomers, including porphyrin 2. The reason for this exceptional stability of 1 is a very strong intramolecular double NH---N hydrogen bond (HB), due to, first, the rectangular shape of the inner cavity that leads to a nearly linear arrangement of the three atoms, and, second, a small N-N distance (2.63 A in 1 [21] as compared to 2.90 A in 2 [34]). In metal complexes, the relative stabilities of porphycene and porphyrin are reversed, since the larger, square-shaped cavity in the latter is much better suited to the accommodation of a metal ion. 

The Molecular Shape with Two Electron Groups (Linear Arrangement)... 

When two electron groups attached to a central atom are oriented as far apart as possible, they point in opposite directions. The linear arrangement of electron groups results in a molecule with a linear shape and a bond angle of 180°. Figure 10.4 shows the general form (top) and shape (middle) with VSEPR shape class (AXa), and the formulas of some linear molecules. 

Fig. la shows spherical particles adhering to each other in more or less linear arrangement, Fig. lb rodlet shaped particles building up a similar continuous framework. In Fig. Ic the case of linear macro-molecules forming a framework consisting of molecular chains with junction points of a crystalline nature is represented. Fig. Id shows a typical case of gel formation by chemical cross-linking of dissolved linear macromolecules. Such a case can, e. g., be reali2 ed if a rubber solution is vulcanised... 

Lewis structures can be used to explain and predict the shapes of molecules. The basic assumption is that, if the core of an atom is effectively spherical (as for most atoms it is), groups of electrons in the Lewis shell (single bond pairs, double-bond quartets, fractional bond pairs, etc.) get as far apart as possible. Thus, two groups take up a linear arrangement, three a trigonal-planar one, four tetrahedral, five trigonal-bip5Tamidal, six octahedral, and so on. 

Block copolymers are polymers constituted of at least two different monomers arranged in a specific manner - they could be diblock, triblock, multi-block, linear, star shaped, etc. Those based on styrene and butadiene, SB or SBS, are the earliest to be applied and studied, as well as the largest as far as the volume of production is concerned (Holden et al. 1967). 

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