Water bent bonds

The groundwork for the application of MO theory to the water molecule has been carried out to a large extent in Section 2.2.1. The procedure is to identify the point group to which the molecule belongs. To demonstrate the power of MO theory, both of the extreme geometries of the molecule, the bent (bond angle, 90°) and linear (bond angle, 180°) forms, are treated. 

Several models for the structure of liquid water have been proposed in the past. Some of them, like the bent-bond model with all water molecules four-coordinated as in ice, can be discarded based on the molecular simula-... 

Water (H2O) 105 H V. 0— / Oxygen has two bonded pairs + two unshared pairs Tetrahedral Bent <4... 

There is little experience with the von Niessen method, but for most molecules the remaining three schemes tend to give very similar LMOs. The main exception is systems containing both a- and vr-bonds, such as ethylene. The Pipek-Mezey procedure preserves the cr/yr-separation, while the Edmiston-Ruedenberg and Boys schemes produce bent banana bonds. Similarly, for planar molecules which contain lone pairs (like water), the Pipek-Mezey method produces one in-plane cr-type lone pair and one out-of-plane yr-type lone pair, while the Edmiston-Ruedenberg and Boys schemes produce two equivalent rabbit ear lone pairs. 

Creating the Lewis structures of molecules is a method for determining the sequence of bonding within a molecule and its three-dimensional shape. This works best for covalently bonded molecules, but can also work for ionic compounds. For example, this method can be used to explain why the sequence of bonding in water is H-O-H, rather than H-H-O, and why it has a bent structure, rather than linear. 

Figure 11.10 Lewis structures of water (H20). (a) shows two possible configurations of water, but only H-O-H satisfies the electronic requirements of the oxygen atom, (b) shows three possible bond distributions for this structure, but only one (with a single bond to each of the hydrogens and two lone pairs on the oxygen) meets the requirements of all three atoms, (c) shows the bent structure of H-O-H which follows from the need to separate the two lone pairs and two single bonds as far as possible in the three-dimensional molecule, (d) shows a space-filling version of this arrangement, where the oxygen is black and the two hydrogens white.

This determination of the molecular geometry of carbon dioxide and water also accounts for the fact that carbon dioxide does not possess a dipole and water has one, even though both are composed of polar covalent bonds. Carbon dioxide, because of its linear shape, has partial negative charges at both ends and a partial charge in the middle. To possess a dipole, one end of the molecule must have a positive charge and the other a negative end. Water, because of its bent shape, satisfies this requirement. Carbon dioxide does not. 

Note added in proof, (viii) Suppose liquid water is excited by a short intense pulse of frequency selected infra-red radiation. Let the frequency be chosen to coincide with OH stretching in one of the inferred subcomponents (linear hydrogen bonds, bent hydrogen bonds, etc.). Finally, suppose the incident pulse is intense... 

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