Basic structural unit concept

Fig. 15. A schematic model illustrating the concepts of basic structural unit, BSU, and local molecular ordering, LMO [c.g., 116].

The capabilities of Oberlin and Bonnamy to see the big picture, and thus synthesize the advancements in seemingly unrelated carbon research fields, both old and new, have been amply demonstrated in previous volumes of this series. Here they bring order to the often confusing issue of disordered carbons, whose relevance has increased tremendously as a consequence of the often frustrated (and certainly incomplete) conversion of a wide variety of carbon precursors to nanotubes, fullerenes, or graphene. As they reviewed all the relevant information, unexpected new concepts emerged related to the carbonization and graphitization processes, and they were able to define more precisely the domain of existence of the various substances such as amorphous carbon and [basic structural units], as well as [local molecular orientations and] turbostratic carbons. .. by using diffraction techniques or their associations. ... 

In this chapter we will familiarize ourselves with basic concepts in molecular symmetry [17]. The presence or absence of symmetry has consequences on the appearance of spectra, the relative reactivity of groups, and many other aspects of chemistry, including the way we will make use of orbitals and their interactions. We will see that the orbitals that make up the primary description of the electronic structure of molecules or groups within a molecule have a definite relationship to the three-dimensional structure of the molecule as defined by the positions of the nuclei. The orientations of the nuclear framework will determine the orientations of the orbitals. The relationships between structural units (groups) of a molecule to each other can often be classified in terms of the symmetry that the molecule as a whole possesses. We will begin by introducing the basic termi-... 

In the synthesis of occidentalol (ref. 13), a eudesmane-type compound consisting of a cis-fused decalin containing a homoannular 1,3-diene system, dihydrocarvone was converted by a typical Robinson annellation reaction to the basic reguired bicyclic structural unit. (It is of interest that a related bicyclic methyldecalenone structure, the Wieland-Miescher ketone, has been employed for the synthesis of longifolene (ref.14), copaene (ref.15) and sativene (ref.16) by three totally different strategies outside the present concept of the semi-synthetic approach). 

The surface modeling is connected with the basic concepts, introduced by Tasker [772] in a discussion of the stabihty of surfaces of ionic or partly ionic crystals. According to classical electrostatic criteria, the stabihty of a compound surface depends on the characteristics of the charge distribution in the structural unit that repeats itself in the direction perpendicular to the surface. The surface can be studied by considering the crystal as a stack of atomic planes. Each plane consists of sublattices of nonequivalent atoms, a sum over each sublattice on each plane gives the total electrostatic potential. For perpendicular distances z greater than a few interionic spacings, the contribution from a planar sublattice reduces to the particularly simple form... 

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