Methane molecules, orientation

Figure 7. Methane molecules oriented so that their bonds point in appropriate directions for (a) ethylene or (b) acetylene molecule formation.

In Faujasites. Bezus et al. (49) reported in 1978 statistical calculations on the low-coverage adsorption thermodynamics of methane in NaX zeolite (Si/Al = 1.48). As for single-atom adsorbates described earlier, the agreement between their calculated values and a range of experimental values was excellent. Allowing for different orientations of the molecule, they calculated a value of 17.9 kJ/mol for the isosteric heat of adsorption at 323 K. Experimental values available for comparison at that time (134-136) ranged from 17.6 to 18.8 kJ/mol. Treating the methane molecule as a hard-sphere particle, with a radius of 2 A, resulted in a far lower heat of adsorption (12.6 kJ/mol). Further calculations (99) yielded heats of adsorption of 19.8 and 18.1 kJ/mol for methane in NaX and NaY zeolites, respectively. 

First, this mechanism involves a very precisely oriented four-center collision between chlorine and methane that would have a low probability of occurrence (i.e., a large decrease in entropy because a precise orientation means high molecular ordering). Second, it requires pushing a chlorine molecule sufficiently deeply into a methane molecule so one of the chlorine atoms comes close enough to the carbon to form a bond and yield chloromethane. 

In a compound composed of molecules, the individual molecules move around as independent units. For example, a methane molecule is represented in Fig. 2.15 using a space-filling model. These models show the relative sizes of the atoms, as well as their relative orientation in the molecule. Figure 2.16 shows other examples. Ball-and-stick models are also used to represent molecules. The ball-and-stick model of methane is shown in Fig. 2.17. 

Table II shows that the approach of undistorted methane toward the Ni surface is repulsive for all orientations of methane. For a C-Ni surface (perpendicular) distance of 2.4 A, and a 3-fold adsorption site, the three symmetric CH. orientations are (in order of increasing energy) 3 H s down, 1 H down, and 2 H s down. The latter is unfavorable due to interactions of hydrogens with the Ni-Ni bridge site. Thus, these data show the expected repulsive interaction between the saturated methane molecule and the surface. Figure 3b shows the most favorable orientation of the hydrogens for tetrahedral CH. ...

The distances between the C atom of methane and O and H atoms of the water molecules of type A (C—O and C—H) are almost equal to each other (see Table 3) and are tangentially oriented toward the surface of the methane molecule, as was also found experimentallyHowever, the B water molecules have a different orientation (see Table 3). Another important characteristic of the water molecules in the vicinity of a... 

There must be forces between the molecules of a non-polar compound, since even such compounds can solidify. Such attractions are called van der Waals forces. The existence of these forces is accounted for by quantum mechanics. We can roughly visualize them arising in the following way. The average distribution of charge about, say, a methane molecule is symmetrical, so that there is no net dipole moment. However, the electrons move about, so that at any instant of time the distribution will probably be distorted, and a small dipole will exist. This momentary dipole will affect the electron distribution in a second methane molecule nearby. The negative end of the dipole tends to repel electrons, and the positive end tends to attract electrons the dipole thus induces an oppositely oriented dipole in the neighboring molecule ... 

Finally, in addition to being sufficiently energetic, the collisions must occur when the particles are properly oriented. At the instant of collision, the methane molecule must be turned in such a way as to present a hydrogen atom to the full force of the impact. In the present example, only about one collision in eight is properly oriented. 

K by the forcible pressure swing adsorption method (ca. 13 MPa). The adsorbed methane molecules are located in the pocket-like narrow corners of the necks of the ID channel [20]. Because the thermal motion of the pseudo-spherical methane molecules seems to be effectively suppressed in its translation mode but rotation is allowed, the forcible adsorption of methane gas produces an inclusion plastic crystal [20], which can be regarded as a mesophase between the fluid and solid state of the phase of a guest incorporated in a crystal host the guest molecules are randomly oriented, but their alignment follows the crystal periodicity. 

At this point let s summarize the bonding in the methane molecule. The experimentally known structure of this molecule can be explained if we assume that the carbon atom adopts a special set of atomic orbitals. These new orbitals are obtained by combining the 2s and the three 2p orbitals of the carbon atom to produce four identically shaped orbitals that are oriented toward the corners of a tetrahedron and are used to bond to the hydrogen atoms. Thus the four sp orbitals on carbon in methane are postulated to account for its known structure. 

Figure 15.5. Arrangement of water molecules around two methane moleeules. Water moleeules around the methane molecules are oriented to maximize the number of HBs among the water molecules.

There remains one more type of symmetry element, called an alter7ialing axis of symmetry, best illustrated by an example. If a methane molecule (Fig. 5-2) is oriented so that two hydrogen atoms are in one horizontal plane and the other two hydrogen atoms arc in another, lower, horizontal plane, then there exists a vertical fourfold alternating axis of symmetry through the carbon atom. If the molecule is rotated one quarter of a revolution about the axis and then reflected through a plane... 

Triphenol 5 was capped with tribromo compound 6 by intermolecular reaction in DMF under high dilution conditions and yielded the capped fl(//-homocalixarene 7 [14]. Preliminary results of the x-ray structure analysis are shown in Fig. 12. A trichloro-methane molecule is oriented over one of the benzene rings in a distance of 338 pm in the crystal [14]. 

Of course, the surface molecules of methane (in liquid state) obviously will exhibit symmetry in comparison to water molecule. This characteristic can also be associated to the force field resulting from induced dipoles of the adsorbed molecules or spread lipid films (Adamson and Cast, 1997 Birdi, 1989). The surface potential arises from the fact that the lipid molecule orients with polar part toward the aqueous phase. This gives rise to a change in dipole at the surface. There would thus be a change in snrface potential when a monolayer is present, as compared to the clean surface. The surface potential, AV, is thus... 

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