Structure of methane

The tetrahedral structure of methane has been verified by electron diffraction (Fig. 2.1c), which shows beyond question the arrangement of atoms in such simple molecules. Later on, we shall examine some of the evidence that led chemists to accept this tetrahedral structure long before quantum mechanics or electron diffraction was known. 

We shall ordinarily write methane with a dash to represent each pair of electrons shared by carbon and hydrogen (I). To focus our attention on individual electrons, we may sometimes indicate a pair of electrons by a pair of dots (II). Finally, when we wish to represent the actual shape of the molecule, we shall use a simple three-dimensional formula like III or IV. 

In three-dimensional formulas of this kind, a solid wedge represents a bond coming toward us out of the plane of the paper a broken wedge, a bond going away from us behind the plane of the paper and an ordinary line, a bond lying in the plane of the paper. Thus formulas III and IV represent methane as in Fig. 2.2c and Fig. 2.2b, respectively. 

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Hollenstein H, Marquardt R, Quack M and Suhm M A 1994 Dipole moment function and equilibrium structure of methane In an analytical, anharmonic nine-dimenslonal potential surface related to experimental rotational constants and transition moments by quantum Monte Carlo calculations J. Chem. Phys. 101 3588-602... 

FIGURE 2 7 Structures of methane ethane and propane showing bond distances and bond angles... 

The axes of the sp orbitals point toward the corners of a tetrahedron Therefore sp hybridization of carbon is consistent with the tetrahedral structure of methane Each C—H bond is a ct bond m which a half filled Is orbital of hydrogen over laps with a half filled sp orbital of carbon along a line drawn between them... 

Active Figure 1.11 The structure of methane, showing its 109.5° bond angles. Sign in at www.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

At this stage, it looks as though electron promotion should result in two different types of bonds in methane, one bond from the overlap of a hydrogen ls-orbital and a carbon 2s-orbital, and three more bonds from the overlap of hydrogen Is-orbitals with each of the three carbon 2/ -orbitals. The overlap with the 2p-orbitals should result in three cr-bonds at 90° to one another. However, this arrangement is inconsistent with the known tetrahedral structure of methane with four equivalent bonds. 

Seshadri, K. and Peters, N., The inner structure of methane-air flames. Combust. Flame 81 96 1990. 

The unstable organic molecules have fixed structures because of the nature of their bonds. The structure of methane is a tetrahedron of covalent single —C — H bonds. Where the structure of even a simple compound is based on double bonds, such as in an ethylene, there are two forms labelled trans and cis geometric isomers ... 

Figure 1.2 The tetrahedral structure of methane. Bonding electrons in methane principally occupy the space within the wire mesh.

Figure 1.16 (a) In this structure of methane, based on quantum mechanical... 

Figure 9.14 The structure of methanal changes following photo-excitation, from a flat ground-state molecule to a bent structure in the first photo-excited state...

On the Internet, research one possible structure of methane hydrate. Create a physical model or a three-dimensional computer model to represent it. Use your model to explain why methane hydrates are unstable at temperatures that are warmer than 0°C. 

Once we know the primary structure for a protein, we have the basic aspects of the structure in hand. We know the sequence of amino acids along the polypeptide backbone. Even more basically, we know which atom is bonded to which for all atoms in the protein. It is like knowing that the carbon atom is bonded to four hydrogen atoms in the structure of methane, but on a vastly larger scale. 

Machida, S. Hirai, H. Kawamura, T. Yamamoto, Y. Yagi, T. (2006). A new high-pressure structure of methane hydrate surviving to 86 GPa and its implications for the interiors of giant icy planets. Physics of the Earth and Planetary Interiors, 155 (1-2), 170-176. 

Like ammonia, the structure is similar to the tetrahedral structure of methane. The two lone pairs repel each other in order to be as far apart as possible. The squeezing of the hydrogens in water is even greater than that in ammonia. The H-O-H bond angle in water is 104.5°. 

Fig. 6J (a) The molecular structure of methane. (b) The molecular structure of ammonia showing (he reduction of bond angles, (c) The molecular structure of water showing the greater reduction of the bond angle by two lone pairs. 

FIGURE 3.30 Field emission scanning electron microscopy image of the submicron porous structure of methane hydrate after 2 weeks of reaction at 60 bar, 265 K. (Reproduced from Staykova, D.K., Kuhs, J. Phys. Chem. B, 107, 10299 (2003). With permission from the American Chemical Society.)... 

In contrast to the four tetrahedrally oriented elliptic orbits of the Sommer-feld model, the new theory leads to only three, mutually orthogonal orbitals, at variance with the known structure of methane. A further new theory that developed to overcome this problem is known as the theory of orbital hybridization. In order to simulate the carbon atom s basicity of four an additional orbital is clearly required. The only possible candidate is the 2s orbital, but because it lies at a much lower energy and has no angular momentum to match, it cannot possibly mix with the eigenfunctions on an equal footing. The precise manoeuvre to overcome this dilemma is never fully disclosed and appears to rely on the process of chemical resonance, invented by Pauling to address this, and other, problems. With resonance, it is assumed that, linear combinations of an s and three p eigenfunctions produce a set of hybrid orbitals with the required tetrahedral properties. 

In Chapter 3 you met some of the spectroscopic methods frequently used by organic chemists to determine the shape of molecules. Spectroscopy would reveal the structure of methane, for example, as tetrahedral—the carbon atom in the centre of a regular tetrahedron with the hydrogen atoms at the corners. In this chapter we are going to discuss why compounds adopt the shapes that they do. 

Let us start by using the hybrid orbital method to predict the structure of methane. Methane, CH4, is composed of a carbon atom and four hydrogen atoms. The carbon atom has an electron configuration of Is2 2s2 2p2. Each hydrogen atom has an electron configuration of Is1. Experiments showed that the geometry of the... 

Ethylene has the well-known classical >2/1 structure with a barrier to rotation. The next in complexity of the simple hydrides is the methyl radical CH3. The obvious (sp2) planar arrangement can only accommodate six of the seven valence electrons. The electronic configuration of this molecule can therefore not be described in terms of either atomic wave functions or hybrid orbitals. An alternative approach is to view the structure of the methyl radical as a reduced-symmetry form, derived from the structure of methane, to be considered next. 

Hsu, H., Dong, Y., Shu, L.,Young, J., V. G., and Que, J., L., 1999, Crystal structure of a synthetic high-valent complex with an Fc2(p-0)2 diamond core. Implications for the core structures of methane monooxygenase intermediate Q and ribonucleotide reductase intermediate X, J. Am. Chem. Soc. 121 5230n5237. 

The molecular structure of methane. The tetrahedral arrangement of electron pairs produces a tetrahedral arrangement of hydrogen atoms. 

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