Methane structural representation

Representations of the three-dimensional structure of methane, CH4. (a) Tetrahedral methane structure, (b) Ball and stick model of tetrahedral methane, (c) Three-dimensional representation of structure (b). 

The tetrahedral molecular structure of methane. This representation is called a ball-and-stick model the atoms are represented by balls and the bonds by sticks. The dashed lines show the outline of the tetrahedron. 

It is instructive to compare the results of simulations obtained using spherical representations of the sorbates with those firom nonspherical (structured) representations. We need also to consider the influence of the rigidity of the lattice, which can be constrained to be completely rigid or flexible. June et al. were able to reproduce experimental self-diffusivities and isosteric heats of adsorption of methane and xenon in silicalite by employing a rigid but structured methane molecule despite the neglect of the Coulomb term in their poten-tial.244 Even when methane is treated as a nonstructured sphere, it was still possible to predict experimentally observed diffusion coefficients well. " ... 

Fig. 1.3. Valence bond structural representation of methane resulting from overlap of H Is orbitals with four equivalent sp orbitals of carbon.

In the mid 1970s, Ugi and co-workers developed a scheme based on treating reactions by means of matrices - reaction (R-) matrices [16, 17]. The representation of chemical structures by bond and electron (BE-) matrices was presented in Section 2.4. BE-matrices can be constructed not only for single molecules but also for ensembles of them, such as the starting materials of a reaction, e.g., formaldehyde (methanal) and hydrocyanic add as shown with the B E-matrix, B, in Figure 3-12. Figure 3-12 also shows the BE-matrix, E, of the reaction product, the cyanohydrin of formaldehyde. 

Schematic representation of carbon filaments of different structure produced by metal-catalyzed decomposition of methane, (a) Platelet structure, (b) "herringbone" structure, and (c) ribbon structure. MP denotes a nanosized metal particle.

Mechanism. Of great importance in permitting chemist lo explain rationally the reactivity of carbonyl compounds towattl di m -methane was the elucidation of its electronic structure. It is now accepted that a diazoalkane can be represented by a set of canonical structure ( A M C ), none of which Is an adequate representation by Itself Evidence for the linear nature of diaxoalkanee has been obtained hy various types of physical measurements, including electron diffraction and infrared fipectroeoopy.u,> 1WZ... 

The polymer was imprinted with dibenzoyl-methane (42), a 1,3-diketone able to be held in place by formation of a complex with the Co(II) ion, and structurally similar to the product of the cross-aldol condensation. A schematic representation of this approach is given in Scheme 7. 

Drawing the structure of a chemical compound is probably one of the first basic requirements of any chemist. It requires knowledge of the chemical composition of the structure to be drawn, an understanding of the type of bonding, and frequently a mental visualization of the arrangement of atoms (or ions). Once this has been assimilated it is not uncommon to draw a representation of the structure on paper. What is often lacking is the realization that the molecule should be represented in three dimensions. To some extent it is possible to represent a three-dimensional chemical structure on a piece of paper. Fig. 42.1 shows the structure of methane, CH4, where standard symbols e.g. the hatched hne, are used to imply a direction of the bond, and one that is different to, for example, the solid hne. This simple notation is commonly used to give a molecule the perception of three-dimensionality. 

Figure 10 Theoretical (a) and practical (b) representation of QSARs. Panel b describes a QSAR for the methanotrophic oxidation (activity of methane monooxygenase) of 6>r /i6>(Ci2)-substituted biphenyls. The structural backbone was biphenyl, and the substituents considered included all halogens, methyl-, methoxy-, hydroxyl-, nitro-, and amino-moieties (Lindner et al, 2003). The molecular descriptors used in (b) are (charge on the ortho-csubon), (Taft s steric parameter), and log ow...

This means that methane is CH4, a saturated, symmetrical compound, in which carbon is tetra-valent, and all of the hydrogens are alike. It should be emphasized again that such a formula, which we call a structural or constitutional formula, does not represent the arrangement of the atoms in space, but is simply a plane representation of the most important facts in regard to methane as shown by definite reactions. The structural formulas for methane and the four chlor methanes are then as follows ... 

Figure II.4 In a molecule like methane, CH4, the four valency arms of the carbon atom point symmetrically to the four comers of a tetrahedron. This is usually not shown in 2-D representations of chemical structures, but, to emphasise the point, the bonds can be drawn so that the ones coming towards you (out of the page) seem to be getting thicker, and the ones going away into the page taper into the distance, getting smaller.

FIGURE 2.19 Different representations of the methane (CH4) molecule. Structural formulas, perspective drawings, bali-and-stick models, and spacefilling modeis correspond to the molecuiar formuia, and each heips us visualize the ways atoms are attached to each other. 

The bond angles for any atom that has a regular tetrahedral structure are 109.5°. A representation of these angles in methane is shown in Fig. 1.34. 

The aim of many surface science experiments is to provide the fundamental detail of a reaction over a well-characterized single crystal surface in order to establish structure-reactivity relationships. Supported catalytic particles, on the other hand, may have various exposed surface facets along with defect sites. A choice then has to be made as to which single crystal surface will provide the most accurate representation of the active surface facets of the support particles. In order to address the similarities or differences in the rate over ideal surfaces and those over supported particles, Kelly and Goodmanl l compared the rate of methane formation from CO and H2 catalyzed by an Ni(lOO) single crystal with that over a supported catalyst taken under the same conditions (see Fig. 2.12). 

Alkanes all have the molecular formula C H.2 +2- The simplest alkane is methane, CH4. Vast numbers of related molecules can be constructed from methane by replacing one or more of its hydrogens with other carbons and their attendant hydrogens. Linear arrays can be made, as well as branched structures and even rings (ring compounds called cycloalkanes, have a slightly different formula C H2 ). Figure 2.1 shows a few schematic representations for these molecules. 

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