Two-dimensional molecules

The retrosynthetic analysis described above outlines a triply convergent strategy towards the synthesis of zaragozic acid A (1). The most pleasing aspect of the retrosynthetic strategy is the rapid and enantioselective manner in which the two-dimensional molecule... 

Thermoplastics are generally long-chain, linear, two-dimensional molecules, while thermbsets are generally three-dimensional long chains, connected by cross-linking chemical bonds. 

Compared to small two-dimensional molecules, for example the planar benzene, the structures of these three-dimensional systems are aesthetically appealing. The beauty and the unprecedented spherical architecture of these molecular cages immediately attracted the attention of many scientists. Indeed, Buckminsterfullerene CgQ rapidly became one of the most intensively investigated molecules. For synthetic chemists the challenge arose to synthesize exohedrally modified derivatives, in which the properties of fullerenes can be combined with those of other classes of materials. The following initial questions concerned the derivatiza-tion of fullerenes What kind of reactivity do the fullerenes have Do they behave like a three-dimensional superbenzene What are the structures of exohedral fullerene derivatives and how stable are they ... 

Is the linear dimensionality of biological molecules essential Or can a monomer collection or two-dimensional molecules support Darwinian evolution ... 

More sophisticated methods have been developed for the assembly of very complex two dimensional molecules (i.e., palytoxin, Mwt. 2680 or vitamin B12) or three dimensional architecture (i.e., cryptates/carcerands) involving a convergent covalent approach. The resulting assemblies of covalently linked atoms are precise... 

In graphite, which can be considered as a giant two-dimensional molecule from the series of condensed rings, the bonding between the separate layers is very weak, being due, as in molecular lattices, to Van der Waals-London interaction. The now infinite system of n electrons results in metallic conduction, only, however, in the plane of the rings Boron nitride has perhaps also a diamond-like form as well as the common graphite-like modification (p. 235). 

With molecular modeling becoming more common, the QSAR paradigm that traditionally used physiccK hemical dc-.scriptors on a two-dimensional molecule can be adapted to... 

Fig. 9. A depiction of the scaling that is necessary to get a pair of two-dimensional molecules to fit exactly into the ar taken up by one of the pair. This diagram derives from the work of Mezey [42]...

Arsenic, antimony and bismuth are all isomorphous, and in the structure of these elements each atom is covalently bound, to three others to form an indefinitely extended puckered sheet of the type shown in fig. 7.05. Each such sheet may be regarded as a two-dimensional molecule of infinite extent analogous to the one-dimensional molecules in the structure of selenium, and the whole structure is formed by the superposition of these sheets to give a rhombohedral arrangement. 

The structure of HgCl2 is of interest in that it consists of discrete Cl-Hg-Cl molecules, in contrast to the infinite two-dimensional molecules found in Hgl2 ( 8.27). These linear molecules are arranged in the crystal in a manner similar to that of the I2 molecules in iodine ( 7.03), and are bound to one another only by van der Waals bonds. In this compound the mercury atom acquires the configuration 2, 8, 18, 32,... 

The exceptional properties of graphite are due to its unique structure which is the most perfect example known of layer lattices. It consists of sheets of carbon atoms linked hexagonally like wire netting (fig. 4) each sheet representing a gigantic, two-dimensional molecule. Adjacent atoms are 1 421 A apart and statistically rather more than three valencies out of the four of each carbon atom are absorbed in the C—C bonds in each layer. The valency forces left over are absorbed in holding the various layers... 

We have just seen that a chiral object is defined in a three-dimensional space. However, it is possible to reduce the space to two dimensions and to define chiral objects in a plane. To say that a molecule is two-dimensional appears at first sight to be an abuse of language. In effect, no molecule, even benzene, can be reduced to a plane. We will nevertheless treat molecules such as benzene and ethylene as two-dimensional molecules. These molecules are achiral in three-dimensional space since they possess at least the plane of symmetry in which they lie (Figure 2.15). 

For a linear macromolecule in space, a restriction to only one dimension does not correspond to reality. One must consider that in addition to one-dimensional, longitudinal vibrations of N vibrators, there are two transverse vibrations, each of N frequencies. Naturally, the longitudinal and transverse vibrations should have different 0,-values in Eq. (5). For a two-dimensional molecule, there are two longitudinal vibrations, as described by Eq. (7) in Fig. 2.38, and one transverse vibration with half as many vibrations, as given in Eq. (6). As always, the total possible number of vibrations per atom must be three, as fixed by the number of degrees of freedom. 

Three-dimensional versions of these essentially two-dimensional molecules have now also been made, using both oxygen and other heteroatoms, such as sulfur and nitrogen, as the Lewis bases. These host molecules are called cryptands and can incorporate positive ions into the roughly spherical cavity within the cage (Fig. 6.62). 

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