Methane molecular shape

Having introduced methane and the tetrahedron, we now begin a systematic coverage of the VSEPR model and molecular shapes. The valence shell electron pair repulsion model assumes that electron-electron repulsion determines the arrangement of valence electrons around each inner atom. This is accomplished by positioning electron pairs as far apart as possible. Figure 9-12 shows the optimal arrangements for two electron pairs (linear),... 

Figure 9-16 shows the molecular shapes of methane, ammonia, and water, all of which have hydrogen ligands bonded to an inner atom. These molecules have different numbers of ligands, but they all have the same steric number. 

This stiffness also has an influence on the shape of the dendrimers, when different core building units are employed. The biphenyl core 9 leads to the dumbbell shaped molecule whose most stable conformers show a twist between 20° and 60° around the central biphenyl unit. 2, based on the tetrahedral core 4, with a diabolo-like molecular shape which resembles the shape of the core very well. Due to the large number of benzene rings around the central methane unit, the branches are hindered in their rotation (Scheme 4), lowering the internal mobility of the molecule compared to 15. 

It is important to realize that methane is not tetrahedral because carbon has sp3, hybrid orbitals. Hybridization is only a model—a theoretical way of describing the bonds that are needed for a given molecular structure. Hybridization is an interpretation of molecular shape shape is not a consequence of hybridization. 

The spherically shaped cryptophanes are of much interest in particular for their ability to bind derivatives of methane, achieving for instance chiral discrimination of CHFClBr they allow the study of recognition between neutral receptors and substrates, namely the effect of molecular shape and volume complementarity on selectivity [4.39]. The efficient protection of included molecules by the carcerands [4.40] makes possible the generation of highly reactive species such as cyclobutadiene [4.41a] or orthoquinones [4.41b] inside the cavity. Numerous container molecules [A.38] capable of including a variety of guests have been described. A few representative examples of these various types of compounds are shown in structures 59 (cyclophane) 60 (cubic azacyclophane [4.34]), 61a, 61b ([4]- and [6]-calixa-renes), 62 (cavitand), 63 (cryptophane), 64 (carcerand). 

How does valence bond theory describe the electronic structure of a polyatomic molecule, and how does it account for molecular shape Let s look, for example, at a simple tetrahedral molecule such as methane, CH4. There are several problems to be dealt with. 

Make your own clay. Find an area in your community where the soil is tightly packed and resembles clay. Dig out a ball of this soil. Add just enough water to make the soil plastic. Sculpt your clay into a molecular shape. It might be a bent water molecule or a tetrahedral-shaped methane molecule. Let your molecular model dry in sunlight. Paint your molecular model sculpture. 

When all the rotations are possible in the solid state the symmetry increases to hexagonal. This form corresponds to the close packing of spheres or cylinders and the molecule is in a rotational crystalline state, characterized by rigorous order in the arrangement of the center (axes) of the molecules and by disordered azimuthal rotations [118]. If the chain molecules are azimuthally chaotic (they rotate freely around their axes), their average cross sections are circular and, for this reason, they choose hexagonal packing. The ease of rotation of molecules in the crystal depends merely on the molecular shape, as in molecules of an almost spherical shape like methane and ethane derivatives with small substituents, or molecules of a shape close to that of a cylinder (e.g., paraffin-like molecules). 

In the examples we have discussed so far, the shape of the molecule is defined by the coordination geometry thus the carbon in methane is tetrahedrally coordinated, and there is a hydrogen at each corner of the tetrahedron, so the molecular shape is also tetrahedral. 

The isoelectronic molecules CH4, NH3, and H2O (Figure 3-10) illustrate the effect of lone pairs on molecular shape. Methane has four identical bonds between carbon and each of the hydrogens. When the four pairs of electrons are arranged as far from each other as possible, the result is the familiar tetrahedral shape. The tetrahedron, with all H—C — H angles measuring 109.5°, has four identical bonds. 

In attempting to predict molecular shapes, it is often useful to consider the oversimplified vi of molecules with independent electron orbitals (e.g., s andp orbitals). In the case of methane (CH4), the greatest distance in space that can separate the four carbon-hydrogen bonds around the central carbon atom occurs when the bonds are pointed at the corners of a tetrahedron. When the bonds are pointed toward the corners of a tetrahedron the bond angles are 109.5° and the molecule is said to be a tetrahedral molecule. 

The best way to keep the negative charges for the four covalent bonds in a methane molecule as far apart as possible is to place them in a three-dimensional molecular shape called tetrahedral, with angles of 109.5° between the bonds. 

The VSEPR model, simple as it is, does a surprisingly good job at predicting molecular shape, despite the fact that it has no obvious relationship to the filling and shapes of atomic orbitals. For example, we would like to understand how to account for the tetrahedral arrangement of C—H bonds in methane in terms of the 2s and 2p orbitals of the central carbon atom, which are not directed toward the apices of a tetrahedron. How can we reconcile the notion that covalent bonds are formed from overlap of atomic orbitals with the molecular geometries that come from the VSEPR model ... 

After preaching to you, I can proceed to a more challenging case with four electron pairs around a central atom. We have already constructed methane many times, so let us skip the click step and focus on the molecular shape. As shown in Scheme 7.2a, methane has four pairs around the carbon atom. We might have thought that the molecule CH4 could be planar and have HCH angles of 90°. But this is not the... 

Water is known to have the geometric shape known as bent or V-shaped. Carbon dioxide exhibits a linear shape. BF3 forms a third molecular shape called trigonal planar since all the atoms lie in one plane in a triangular arrangement. One of the more common molecular shapes is the tetrahedron, illustrated by the molecule methane (CH4). 

Lewis formulas do not directly convey information about molecular shape. For example, the Lewis formula of methane, CH4, is written as the flat (two-dimensional) formula... 

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