Lone pairs, and bonding

RAMSES is usually generated from molecular structures in a VB representation. The details of the connection table (localized charges, lone pairs, and bond orders) are kept within the model and are accessible for further processes. Bond orders are stored with the n-systems, while the number of free electrons is stored with the atoms. Upon modification oF a molecule (e.g., in systems dealing with reactions), the VB representation has to be generated in an adapted Form from the RAMSES notation. 

Rule 3 All regions of high electron density, lone pairs and bonds, are included... 

Figure 4.14 AX4E2 molecules have a planar structure with two trans lone pairs and bond angles of 90°. This geometry allows the lone pair domains to occupy a maximum of space in the valence shell and the bond angles to attain a maximum value of 90°.

In the previous sections, we focused our attention upon nonbonded interactions and their effect upon the relative stability of conformational and geometric isomers. We shall now use OEMO theory in order to examine how geminal lone pair-lone pair, bond pair-lone pair and bond pair-bond pair interactions can affect the shape of a molecule264. ... 

The basic features of the model are the following a) Lone pair and bonds are classified according to their intrinsic donor and acceptor abilities. Tables 36 and 37 summarize the relative intrinsic donor and acceptor strengths of various lone pairs and bonds. 

Table 36. Intrinsic donor ability of lone pairs and bonds )...

Count the number of lone pairs and bonding partners an atom actually has within a molecule. You can do this by looking at the Lewis structure. 

A drawing of a two-dimensional, electron-domain model of a conventional Lewis lone pair is shown in Fig. 23. The lone pair and bonding pairs are structurally equivalent they have identical van der Waals envelopes. Such seems to be nearly the case for lone pairs in the valence-shells of small-core, non-octet-expanding atoms (carbon, nitrogen, oxygen and fluorine). 

For trigonal bipyramidal geometry, there are two possible locations of lone pairs, axial and equatorial. If there is a single lone pair, for example in SF4, the lone pair occupies an equatorial position. This position provides the lone pair with the most space and minimizes the interactions between the lone pair and bonding pairs. If the lone pair were axial, it would have three 90° interactions with bonding pairs in an equatorial position it has only two such interactions, as shown in Figure 3-11. The actual structure is distorted by the lone pair as it spreads out in space and effectively squeezes the rest of the molecule together. 

Using the same sequence of atoms, it is possible to have more than one correct Lewis structure when a molecule or polyatomic ion has both a double bond and a single bond. Consider the polyatomic ion nitrate (N03 ) shown in Figure 9-11a. Three equivalent structures can be used to represent the nitrate ion. Resonance is a condition that occurs when more than one valid Lewis structure can be written for a molecule or ion. The two or more correct Lewis structures that represent a single molecule or ion are often referred to as resonance structures. Resonance structures differ only in the position of the electron pairs, never the atom positions. The location of the lone pairs and bonding pairs differs in resonance structures. The molecule O3 and the polyatomic ions N03, N02, 803 and C03 commonly form resonance structures. 

The localized character of the geminals can be displayed by introducing the concepts of charge centroids [22] and charge ellipsoids [23, 24] of the geminal one-electron densities. By using these localization measures, we can classify the geminals as core electron pairs, lone pairs and bond pairs [25]. 

The perfect-pairing (PP) orbitals of this wave function clearly show the "lone-pairs" and "bond pairs" which are part of the language of the experimental chemist. This is in contrast to the molecular orbital description or to the GVB description with (7-7T restrictions where the lone pairs and "7T" bonds are not discernable from contour plots of the orbitals (2 ). It is somewhat reassuring that the wave function which gives the lowest variational energy (that of Figures 1 and 2a) also most closely coincides with the experimental chemist s traditional view of the bonding ( 3) ... 

It is also interesting to use molecular orbitals localized in core, lone pairs, and bond regions, rather than fully delocalized canonical orbitals. A good choice of the remaining n(n - l)/2 Lagrangian multipliers should lead to those localized orbitals, but such an a priori choice is hardly feasible. Most often, localized orbitals are determined by applying an adequate unitary transformation on previously obtained canonical orbitals ... 

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