Bonding orbital view

To answer this question, it is necessary to consider the shape or spatial distribution of the orbitals filled by bonding electrons in molecules. From this point of view, we can distinguish between two types of bonding orbitals. Ihe first of these, and by far the more common, is called a sigma bonding orbital. It consists of a single lobe ... 

FIGURE 4. Top left three hybrids, hf, on sulphur and two s orbitals on hydrogen forming bond orbitals, b and b below, combination of BOs to give again the orbitals of H 2S (compare with Figure 3d). In this view, n and lone pair orbitals. 

Lewis structures are blueprints that show the distribution of valence electrons in molecules. However, the dots and lines of a Lewis structure do not show any details of how bonds form, how molecules react, or the shape of a molecule. In this respect, a Lewis structure is like the electron configuration of an atom both tell us about electron distributions, but neither provides detailed descriptions. Just as we need atomic orbitals to understand how electrons are distributed in an atom, we need an orbital view to understand how electrons are distributed in a molecule. 

In this section, we present a unified picture of the different electronic effects that combine to determine methyl rotor potentials in the S0, Sp and D0 electronic states of different substituted toluenes. Our approach is based on analysis of ab initio wavefunctions using the natural bond orbitals (NBOs)33 of Weinhold and cowork-ers. We will attempt to decompose the methyl torsional potential into two dominant contributions. The first is repulsive steric interactions, which are important only when an ortho substituent is present. The second is attractive donor-acceptor interactions between CH bond pairs and empty antibonding orbitals vicinal to the CH bonds. In the NBO basis, these attractive interactions dominate the barrier in ethane (1025 cm-1) and in 2-methylpropene (1010 cm-1) see Figure 3. By analogy, donor-acceptor attractions are important in toluenes whenever there is a substantial difference in bond order between the two ring CC bonds adjacent to the C-CH3 bond. Viewed the other way around, we can use the measured methyl rotor potential as a sensitive probe of local ring geometry. 

The chemistry of nitroxide radicals has been extensively reviewed (e.g. Forrester et al., 1968 Rozantsev, 1970 Rozantsev and Scholle, 1971 Aurich and Heiss, 1976 Berliner, 1976 Keana, 1978). They are -radicals, the unpaired electron occupying a if orbital between the oxygen and nitrogen atoms [4]. Since there is also an N—O o-bond, and two electrons fill a -bonding orbital between these atoms, the effective N—O bond order is 1.5. The nitroxide function is frequently represented as a hybrid of the two structures [5a] and [5b]. The result of this electron delocalization is a relatively stable structure. An alternative view of the absence of any significant tendency... 

Fig. 3.2-2. Molecular formula (top view) and plot [9a] of the 3c BBB natural bonding orbital (NBO) [9b] (side view) for the triborirane dianion (M) as well as homo- (N), bishomo-...

Under the Born-Oppenheimer approximation, two major methods exist to determine the electronic structure of molecules The valence bond (VB) and the molecular orbital (MO) methods (Atkins, 1986). In the valence bond method, the chemical bond is assumed to be an electron pair at the onset. Thus, bonds are viewed to be distinct atom-atom interactions, and upon dissociation molecules always lead to neutral species. In contrast, in the MO method the individual electrons are assumed to occupy an orbital that spreads the entire nuclear framework, and upon dissociation, neutral and ionic species form with equal probabilities. Consequently, the charge correlation, or the avoidance of one electron by others based on electrostatic repulsion, is overestimated by the VB method and is underestimated by the MO method (Atkins, 1986). The MO method turned out to be easier to apply to complex systems, and with the advent of computers it became a powerful computational tool in chemistry. Consequently, we shall concentrate on the MO method for the remainder of this section. 

Alkynes have two v and two it orbitals that can potentially interact with metal orbitals, and in some instances, it is thought that all of these are involved at the same time in a mononuclear complex. An extended Hiickel calculation on Mofmexo-tetra-p-tolylporphyrinXHC=CH) supports this view (Fig. I5.25).8" Thus both bonding orbitals of the alkyne (6, and at) can donate electron density to molybdenum to form (he 16, and lo, MOs. and both anti bonding orbitals (b2 and a2) can accept electron density to form the lf>2 and Ui2 MOs. Notice that both it bonding orbitals (a, and bt) of acetylene interact significantly with metal d orbitals of the same symmetry. 

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