Methane model structure

Methane, for example, is described as a tetrahedral molecule because its four hydrogens occupy the comers of a tetrahedron with carbon at its center as the various methane models in Figure 1.7 illustrate. We often show three-dimensionality in structural formulas by using a solid wedge ) to depict a bond projecting from the paper... 

As seen in Fig. 3, reaction of compoimd Q (modeled as structure I) with methane starts with coordination of CH4. In general, the CH4 molecule could coordinate to I via two distinct pathways O-side and N-side. The O-side pathway corresponds to the coordination of the methane molecule from the side where the two Glu (carboxylate) located, while the N-side pathway corresponds to the coordination of CH4 from the two His (imidazole) side. Our calculations show that both pathways proceed via very similar transition states and intermediates, and the N-side pathway is thermodynamically and kineticaUy more favorable than O-side. In spite of this, in this paper we base our discussions only on the O-side mechanism because it is believe to correspond to the process occurring in the protein. The coordination of CH4 to complex I leads to the methane-Q complex, structure II. The interaction between methane and structure I (compound Q) is extremely weak the complexation energy is calculated (relative to the corresponding reactants) to be 0.7 and 0.3 kcal/mol for the A and "A state, respectively. Because of unfavorable zero point energy and entropy factors, it is most Ukely that the complex II does not exist in reality, and therefore we will not discuss it. 

The s and p orbitals used in the quantum mechanical description of the carbon atom, given in Section 1.10, were based on calculations for hydrogen atoms. These simple s and p orbitals do not, when taken alone, provide a satisfactory model for the tetravalent— tetrahedral carbon of methane (CH4, see Practice Problem 1.22). However, a satisfactory model of methane s structure that is based on quantum mechanics can be obtained through an approach called orbital hybridization. Orbital hybridization, in its simplest terms, is nothing more than a mathematical approach that involves the combining of individual wave functions for r and p orbitals to obtain wave functions for new orbitals. The new orbitals have, in varying proportions, the properties of the original orbitals taken separately. These new orbitals are called hybrid atomic orbitals. 

Replace any one hydrogen atom on each of the two methane models with a halogen to form two molecules of CH3X. Are the two structures identical Does it make a difference which of the four hydrogen atoms on a methane molecule you replace How many different configurations of CH3X are possible ... 

The structural features of methane ethane and propane are summarrzed rn Ergure 2 7 All of the carbon atoms have four bonds all of the bonds are srngle bonds and the bond angles are close to tetrahedral In the next sectron we 11 see how to adapt the valence bond model to accommodate the observed structures... 

Thus, while models may suggest optimal pore spuctures to maximize methane storage, they give no indication or suggestion as to how such a material might be produced. On the other hand, simple measurement of methane uptake from variously prepared adsorbents is not sufficient to elucidate the difference in the pore structure of adsorbents. Sosin and Quinn s method of determining a PSD directly from the supercritical methane isotherm provides an important and valuable link between theoretical models and the practical production of carbon adsorbents... 

Adsorption of hard sphere fluid mixtures in disordered hard sphere matrices has not been studied profoundly and the accuracy of the ROZ-type theory in the description of the structure and thermodynamics of simple mixtures is difficult to discuss. Adsorption of mixtures consisting of argon with ethane and methane in a matrix mimicking silica xerogel has been simulated by Kaminsky and Monson [42,43] in the framework of the Lennard-Jones model. A comparison with experimentally measured properties has also been performed. However, we are not aware of similar studies for simpler hard sphere mixtures, but the work from our laboratory has focused on a two-dimensional partly quenched model of hard discs [44]. That makes it impossible to judge the accuracy of theoretical approaches even for simple binary mixtures in disordered microporous media. 

As useful as molecular models are, they are limited in that they only show the location of the atoms and the space they occupy. Another important dimension to molecular structure is its electron distribution. We introduced electrostatic potential maps in Section 1.5 as a way of illustrating charge distribution and will continue to use them throughout the text. Figure 1.6(d) shows the electrostatic potential map of methane. Its overall shape is similar to the volume occupied by the space-filling model. The most electron-rich regions are closer to carbon and the most electron-poor ones are closer to the hydrogens. 

Kee, R.J., Miller, J.A., Evans, G.H., and Dixon-Lewis, G., A computational model of the structure and extinction of strained, opposed flow, premixed methane-air flames, Proc. Combust. Inst., 22, 1479, 1988. 

Transient computations of methane, ethane, and propane gas-jet diffusion flames in Ig and Oy have been performed using the numerical code developed by Katta [30,46], with a detailed reaction mechanism [47,48] (33 species and 112 elementary steps) for these fuels and a simple radiation heat-loss model [49], for the high fuel-flow condition. The results for methane and ethane can be obtained from earlier studies [44,45]. For propane. Figure 8.1.5 shows the calculated flame structure in Ig and Og. The variables on the right half include, velocity vectors (v), isotherms (T), total heat-release rate ( j), and the local equivalence ratio (( locai) while on the left half the total molar flux vectors of atomic hydrogen (M ), oxygen mole fraction oxygen consumption rate... 

Now let us return to our discussion of the conical intersection structure for the [2+2] photochemical cycloaddition of two ethylenes and photochemical di-Jt-methane rearrangement. They are both similar to the 4 orbital 4 electron model just discussed, except that we have p and p overlaps rather than Is orbital overlaps. In Figure 9.5 it is clear that the conical intersection geometry is associated with T = 0 in Eq. 9.2b. Thus (inspecting Figure 9.5) we can deduce that... 

Construct a model of methane, CH4. The structural formula for methane, written on paper, is... 

Observing and Inferring How does the structural diagram of methane compare with the model ... 

Without any doubt, the zeolite framework porous characteristics (micropores sizes and topology) largely govern the zeolite properties and their industrial applications. Nevertheless for some zeolite uses, as for instance, host materials for confined phases, the zeolite inner surface characteristics should be precised to understand their influence on such low dimensionality sorbed systems. In that paper, we present illustrative examples of zeolite inner surface influence on confined methane phases. Our investigation extends from relatively complex zeolite inner surface types (as for MOR structural types) to the model inner surface ones (well illustrated by the AFI zeolite type). Sorption isotherm measurements associated with neutron diffraction experiments are used in the present study. 

---