Structure determination graphite

In a similar way Kekule s theory of the benzene structure has been very completely established by the whole development of aromatic chemistry. The direct physical verification of the presumably planar arrangement was in this case more delayed. The crystal structure of graphite was examined almost as soon as that of diamond, but the early results were inconclusive. The structure is not determined by the symmetry alone, and the later detailed investigation by Bernal (1924) showed that the carbon atoms in the hexagonal net must be coplanar to within at least 0-38 A. Later work by Ott (1928) narrowed this limit still further. Although it is generally assumed that the atoms are coplanar, the exactness with which this can be established depends on... 

If we consider hydrocarbons at surfaces, we must set the saturated ones aside. These do not chemisorb at metal surfaces, but rather physisorb at low temperatures. No detailed structural determination has been performed for saturated hydrocarbons on metals. However, some structures have been obtained using the graphite basal plane as a substrate (cf. Suzanne etc) which give information about Van der Waals packing of such molecules, but no chemical reactions are involved. 

In Group 14, only carbon and tin exist as allotropes under normal conditions. For most of recorded history, the only known allotropes of carbon were diamond and graphite. Both are polymeric solids. Diamond forms hard, clear, colorless crystals, and was the first element to have its structure determined by x-ray diffraction. It has the highest melting point and is the hardest of the naturally occurring solids. Graphite, the most thermodynamically stable form of carbon, is a dark gray, waxy solid, used extensively as a lubricant. It also comprises the lead in pencils. 

In general, aromatic hydrocarbons and presumably fossil fuels containing aromatic molecules do not form extended stacks of molecules as embodied by a column of poker chips. H-H and H-C interactions are significant structural determinants for hydrocarbon molecules (59-60) and can lead to arrangements other than the graphitelike stack. To see a diffraction peak in the range of 3.3-3.8 A and infer a graphite structure is potentially naive. 

X-ray Structure Determination. A three-dimensional single-crystal structure was determined on crystals obtained by recrystallization of NiZn(BAA)2en from pyridine. The crystals were monoclinic I 2/c with lattice parameters of a = 28.403(6) A, b = 8.465(3) A, c = 30.220(9) A, == 105.86(2)°, and Z = 8. Intensity data were collected by the 6 20 scan technique with graphite-monochromated Mo-Ka radiation on a Syntex P2i diffractometer. Of the 5055 data with sin 0/ < 0.54, 2559 had I > 3(t(I) and these were used in the solution and reflnement of the structure. The structure was solved by Patterson-Fourier methods and reflned by full-matrix (isotropic) and block-diagonal (anisotropic) least squares reflnement. The conventional discrepancy factors at the present state of refinement are R = 0.056 and Rw = 0.071. 

Though fairly simple, the structure of graphite points to the importance of both chemical bonding and intermolecular forces in determining the properties of a solid. Next we turn our attention to the various types of forces that can exist between molecules and the way those forces influence physical properties. 

Besides the expected glassy carbon structure, the resulting product often contained considerable amounts of needle-like material that was determined to have a three-dimensional graphitic structure. The graphitic whiskers resulting from the experiments were usually a few microns long and about 1 (tm thick. Electron diffraction and transmission electron microscopy (TEM) revealed the twinned structure of the whiskers. The angle between the layers was measured to be about 135°. 

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