Bonded and nonbonded electron pairs

In particular, their study of the H2O molecule (C2v point symmetry) revealed that the VSCC of the oxide anion displays four maxima that correspond with the two lone pair. Ip, and the two bond pair, bp, domains as predicted by VSEPR model (Gillespie and Hargitti 1991 Bader and MacDougall 1984). The two bp domains were found to be symmetrically disposed in the plane of the HOH angle (105.6°) on the same side of the anion as the two H atoms whereas the two Ip domains were found to be disposed on the opposite side of the molecule in a perpendicular plane that bisects the HOH angle. Each Ip domain was found to be located 0.33 A from the anion, making an IpOlp angle of 

In general, the more electron rich the Ip domains, the more susceptible they are to 

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Figure 4.12 Representation of the bonding and nonbonding electron pair domains in the ammonia molecule, an AX3E molecule.

The nonequivalence in the size and shape of bonding and nonbonding electron pair domains can alternatively be expressed in terms of the relative magnitude of their mutual Pauli repulsions, which decrease in the following order ... 

As usual, these diagrams depict the atomic connectivity and assignment of bonding and nonbonding electron pairs of the localized Lewis-like wavefunction, but not (necessarily) the molecular shape or symmetry. 

Exercise 2-1 Draw the Lewis electron-pair structure of 2-propanone (acetone) clearly showing the bonding and nonbonding electron pairs in the valence shell of each atom. Draw structural formulas for other compounds having the composition C3H60 that possess... 

Both valence bond (VB) and molecular orbital (MO) theories have been used to explain the observed shapes of molecules. What we wish to know here is the shape of a transition state containing m atoms and n electrons. Fortunately, the preferred shapes of the simple species are known or can be guessed from the numbers and kinds of bonding and nonbonding electron pairs (Gillespie, 1967). Therefore, we must examine the preferred shape of clusters of three, four or more atoms. For, to envision the topology of a transition state is tantamount to a description of the stereochemical result of an elementary process. 

Compound Geometry/ symmetry Xe-F (pm) Xe-O (pm) Arrangement in the bonded and nonbonded electron pairs in Fig. 17.5.2 ... 

A search for extrema in — V2p reveals maxima in the VSCCs that can be ascribed to bonded and nonbonded electron pairs. The different and distinctive properties of sulfides and oxides are examined in terms of the number and the positions of the electron pairs and the topographic features of the Laplacian maps. The evidence provided by p and its topological properties indicates that the bonded interactions in sulfides are more directional, for a given M-cation, than in oxides. The value and the length of a given M-S bond are reliable measures of a bonded interaction the greater the accumulation of p and the shorter the bond, the greater its shared (covalent) interaction. 

This means that the bonding and nonbonding electron pairs (lone pairs) around a given atom are positioned as far apart as possible. 

Use VSEPR theory to decide what symmetrical three-dimensional figure will accommodate all bonded and nonbonded electron pairs. 

When related calculations are carried out for molecules (and also solids) [107,108], one either plots color-coded two-dimensional ELF(r) slices to represent the local ELF values, or one draws three-dimensional isosurfaces referring to a chosen ELF value, usually around 0.8 or close to it. With the advent of powerful computer graphics, such representations produce aesthetically compelling color figures from which ELF "attractors" (bonded and nonbonded electron pairs as well as atomic shells) show up and reflect the underlying molecular symmetry. All-electron calculations are generally preferable for this purpose [106]. 

Table 7-2 relates the number of bonding and nonbonding electron pairs to the electron-pair geometry and molecular shape.)... 

The electron localization function (ELF) is another tool that has been used with considerable success in highlighting domains in p(r) of strong electron localization (Becke and Edgecombe 1990). These domains, like those defined by the maxima in VSCC, have likewise been associated with the bonding and nonbonding electron-pairs of the VSEPR model (Becke and Edgecombe 1990 Bader et al. 1996 Bader et al. 1996 Savin et al. 1997). The ELF has also been found to reveal the shell structure of an atom in a clear and faithful fashion. 

The central atom hybridizes suitably to accommodate both bonding and nonbonding electron pairs. 

We can predict the geometry of simple molecules using valence-shell electron-pair repulsion (VSEPR) theory. This theory is based on the idea that bonded and nonbonded electron pairs around a central atom repel one another. Hence, they are arranged in a geometry that provides maximum separation in space, and therefore rninimum electron repulsion. For bonds to carbon, the following rules apply ... 

Next we will consider molecules that have both bonded and nonbonded pairs of electrons in the valence shell of the central atom. Water and ammonia have four electron pairs around the central atom. Some of the electron pairs in water and ammonia are bonded to hydrogen atoms, but the central atom also has unshared electron pairs. VSEPR theory describes the distribution of bonded and nonbonded electron pairs. However, molecular structure is defined by the positions of the nuclei. The four pairs of electrons in both water and ammonia are tetrahedrally arranged around the central atom. Water, with only three atoms, is angular, and ammonia, with four atoms, is pyramidal (Figure 1.7). 

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