Ordering of the Carbon Atoms

It is not possible in such reactions to have a transition state with a high degree of bond-formation and a low degree of bond-rupture. The total bond order of the entering and leaving groups must be less than or equal to unity, otherwise the total bond order of the carbon atom at the reaction centre will exceed 4 and it is generally accepted that this does not occur. 

As mentioned, the carbons resulting from the pyrolysis of stabilized PAN precursor fibers are non-graphitizable. As a result, further heating of PAN based carbon fibers at very high temperatures (>2500 C) and without any mechanical stretching only slightly improves the ordering of the carbon atoms and the fiber stiffness. 

They are primarily non-stoichiometric phases, and ordering of the carbon atoms is common... 

Ta2C has two crystal structures. The low temperature form shows an ordering of the carbon atoms such that the hexagonal metal layers are separated by alternately completely filled and completely empty carbon layers. This is the cadmium iodide antitype structure found by A. L. Bowman et al. (1965). However, the difference between the x-ray pattern of this phase and an L3 structure is very slight. Near 2000° (Rudy and Harmon, 1965) the carbon lattice presumably disorders to give the L 3 structure. 

Non-graphitic carbons arc all varieties of solids consisting mainly of the element carbon with two-dimensional long-range order of the carbon atoms in planar hexagonal networks, but without any measurable crystallographic order in the third direction (c-direction) apart from more or less parallel stacking. 

However unlike H which is the most abundant of the hydrogen isotopes (99 985%) only 1 1% of the carbon atoms m a sample are Moreover the intensity of the signal produced by nuclei is far weaker than the signal produced by the same number of H nuclei In order for NMR to be a useful technique in structure deter mination a vast increase in the signal to noise ratio is required Pulsed FT NMR pro vides for this and its development was the critical breakthrough that led to NMR becoming the routine tool that it is today... 

The single-bond covalent radius of C can be taken as half the interatomic distance in diamond, i.e. r(C) = 77.2pm. The corresponding values for doubly-bonded and triply-bonded carbon atoms are usually taken to be 66.7 and 60.3 pm respectively though variations occur, depending on details of the bonding and the nature of the attached atom (see also p. 292). Despite these smaller perturbations the underlying trend is clear the covalent radius of the carbon atom becomes smaller the lower the coordination number and the higher the formal bond order. 

It is thus anticipated that compressive stress inhibits while tensile stress promotes chemical processes which necessitate a rehybridization of the carbon atom from the sp3 to the sp2 state, regardless of the reaction mechanism. This tendency has been verified for model ring-compounds during the hydrogen abstraction reactions by ozone and methyl radicals the abstraction rate increases from cyclopropane (c3) to cyclononane (c9), then decreases afterwards in the order anticipated from Es [79]. The following relationship was derived for this type of reactions ... 

If a cyclic form is to be named, the locants of the anomeric centre and of the carbon atom bearing the ring oxygen atom must be given (in that order) (cf. 2-Carb-6.4). If there is more than one ring size designator, they are placed in alphabetical order (e.g. furanose before pyranose). 

These carbides, also known as interstitial carbides, are crystalline compounds of a host metal and carbon. The host-metal atoms are generally arranged in a close-packed structure and the carbon occupies specific interstitial sites in that structure. Such a structure sets size restrictions on the two elements in order for the carbon atom to fit into the available sites and the population of these sites (if all are occupied) determines the stoichiometry of the carbide. 

Table 4 Correlation of the number of classical structures with calculated bond lengths for Ceo and Cn. The bond environment column describes the arrangements of the carbon atoms which have the bond in common. The column labelled with a f is the bond order calculated using resonance theory as described in the text.

In order to calculate the orbitals for a methane molecule, the four Lv functions of the four hydrogen atoms and the functions 2s, 2px, 2py and 2pz of the carbon atom are combined to give eight wave functions, four of which are bonding and four of which are antibonding. The four bonding wave functions are ... 

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