Natural Lewis structure model

For hydrogen fluoride, which is well described by a single Lewis structure (cf. Example 1.6), the localized natural Lewis-structure model wavefunction gives... 

In this case the energy of the natural Lewis-structure model... 

Figure 2.4 Components of the Li-F potential-energy curve E R) = E (R) + E(NL>(R), showing the localized natural-Lewis-structure model energy E(L> (circles, left-hand scale) and delocalized non-Lewis correction ,(NL) (squares, right-hand scale). The classical electrostatic estimate E (dotted line) is shown for comparison.

Age-old questions concerning the nature of the bonds between atoms in molecules culminated in the remarkable Lewis structure model of G. N. Lewis (1916). The notion that such bonds were formed from directed hybrids was subsequently developed by Linus Pauling (1932), shortly after the discovery of quantum mechanics. Although many theoretical advances have ensued, it is fair to say that the underlying concepts of valence-shell hybridization, shared-electron pair bonds, and Lewis structural dot diagrams continue to dominate chemical thinking and pedagogy to this day. 

The Lewis-type (L) contribution is considered the easy part of chemical wavefunction analysis, because it corresponds closely to the elementary Lewis structure model of freshman chemistry. Nevertheless, controversy often arises over the magnitude of steric or electrostatic effects that are associated with the Lewis model itself [i.e., distinct from the resonance-type effects contained in (NL)]. The NBO program offers useful tools for quantifying both steric and electrostatic interactions in terms of the space-filling (size and shape) and dielectric properties (charge, dipole moment, etc.) of the electron pair bonds and lone pairs that comprise the Lewis structure model. This chapter discusses the physical nature and numerical quantitation of these important chemical effects, which are often invoked in a hand-waving manner that reflects (and promotes) significant misconceptions. 

Since the LE model is based on pairs of electrons, it does not handle odd-electron cases in a natural way, although Lewis structures are sometimes written for these species. To treat odd-electron molecules accurately, we need a more sophisticated model. 

The two most characteristic properties of the stannylenes are their behaviour as Lewis acids and Lewis bases, and their ready oxidative addition to give Sn(IV) compounds. The amphoteric nature provides the model for the structure of the dimers, the distannenes, and for the oligomerisation that occurs with the less hindered examples two dative bonds are assumed to be formed between the doubly-occupied sp2 orbital and the vacant p orbital, affording a trans non-planar distannene (equation 21-9). These structures are discussed in more detail below. 

Approaches to pharmaceutically apphcable inhibitors of selectin-hgand interactions began with target structures modeled on the entire sialyl-Lewis tetrasaccharide. However, the scope of these studies was soon focused when structure-activity relationships emerged from consideration of the structures of naturally occurring ligands of the selectins (Section V). 

Chemists have discovered that part of the reason the small difference in structure leads to large differences in properties lies in the nature of covalent bonds and the arrangement of those bonds in space. This chapter provides a model for explaining how covalent bonds form, teaches you how to describe the resulting molecules with Lewis structures, and shows how Lewis structures can be used to predict the three-dimensional geometric arrangement of atoms in molecules. 

The large-dimension limit has recently resolved at least some of the difficulties of the molecular model. The molecule-like structure falls out quite naturally from the rigid bent triatomic Lewis configuration obtained in the limit D — oo [5], and the Langmuir vibrations at finite D can be analyzed in terms of normal modes, which provide a set of approximate quantum numbers [6,7]. These results are obtained directly from the Schrodinger equation, in contrast to the phenomenological basis of some of the earlier studies. When coupled with an analysis of the rotations of the Lewis structure, this approach provides an excellent alternative classification scheme for the doubly-excited spectrum [8]. Furthermore, an analysis [7] of the normal modes offers a simple explanation of the connection between the explicitly molecular approaches of Herrick and of Briggs on the one hand, and the hyperspherical approach, which is rather different in its formulation and basic philosophy. 

Similarly, the orbitals determined in both semiempirical and Hartree-Fock calculations may be transformed into natural bond orbitals (NBOs), which provide pictures of localized bonds and lone pairs that correspond closely with Lewis structure bonding models. " For example. Figure 4.53 shows the NBOs for ethene cch/ o ccr ai d ncc bonds. 

It is important to recognize at this point that VBT Is only a model. As such, it is useful only to the extent that it helps us visualize and better understand the nature of the chemical bonding in molecules. Given the simplicity of Lewis structures, the intricacies of the VSEPR model, and the relatively straightforward concept of hybridization, it Is remarkable how far the valence bond model can be pushed in the first place. In the next section, MOT will be introduced as an alternative and complementary model to VBT. In many ways, MOT will prove itself to be the more powerful of the two theories in that it can explain most of the shortcomings of the... 

Our purpose in this chapter is not to try to master theories of covalent bonding in all their details. We want simply to discover how these theories provide models that yield deeper insights into the nature of chemical bonding than do Lewis structures alone. 

Nitrones are a rather polarized 1,3-dipoles so that the transition structure of their cydoaddition reactions to alkenes activated by an electron-withdrawing substituent would involve some asynchronous nature with respect to the newly forming bonds, especially so in the Lewis acid-catalyzed reactions. Therefore, the transition structures for the catalyzed nitrone cydoaddition reactions were estimated on the basis of ab-initio calculations using the 3-21G basis set. A model reaction indudes the interaction between CH2=NH(0) and acrolein in the presence or absence of BH3 as an acid catalyst (Scheme 7.30). Both the catalyzed and uncatalyzed reactions have only one transition state in each case, indicating that the reactions are both concerted. However, the synchronous nature between the newly forming 01-C5 and C3-C4 bonds in the transition structure TS-J of the catalyzed reaction is rather different from that in the uncatalyzed reaction TS-K. For example, the bond lengths and bond orders in the uncatalyzed reaction are 1.93 A and 0.37 for the 01-C5 bond and 2.47 A and 0.19 for the C3-C4 bond, while those in... 

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