Benzene molecular bonding

Having just seen a resonance description of benzene, let s now look at the alternative molecular orbital description. We can construct -tt molecular orbitals for benzene just as we did for 1,3-butadiene in Section 14.1. If six p atomic orbitals combine in a cyclic manner, six benzene molecular orbitals result, as shown in Figure 15.3. The three low-energy molecular orbitals, denoted bonding combinations, and the three high-energy orbitals are antibonding. 

O Use a molecular model set to build a model of the benzene ring. Examine your model. Does your model give an accurate representation of benzene s bonding system Explain your answer. 

Fig. 20. (a) One component of the bonding et metal-benzene molecular orbital set. Diagrams (b) and (c) illustrate the effect of tilting the p orbitals of the benzene and cyclopropenyl rings. 

Mulliken attributed the strong absorption band of the system to the excitation of the ground-state complex to the CT state with the aromatic molecule acting as the electron donor and the iodine as the acceptor, that is, Bz+ I2. Several spectroscopic and theoretical studies have predicted that the Bz I2 ground state has a C(,v axial structure with the 1—I bond being perpendicular to the benzene molecular plane. The heat of formation of this complex in the gas phase was determined by spectrometric methods to be on the order of 2-3 kcal/mol and our ab initio calculations support these values. 

Give the degree of the polynomial equation that arises in calculating the molecular orbitals for the following species in their ground states (cr or n bonding, as indicated) (a) carbon dioxide (cr only) (b) benzene (n bonds only). 

The molecular structure of arsabenzene,49) 4-methylarsabenzene 50) and 4-methyl-stibabenzene 50) have been determined from an NMR study of liquids oriented in a liquid crystal phase. Unfortunately, no structural data are yet available for bisma-benzene. Selected bond lengths and angles of the complete family of group 5 heterobenzenes are illustrated in Fig. 1. 

Most organic chemists are familiar with two very different and conflicting descriptions of the 7r-electron system in benzene molecular orbital (MO) theory with delocalized orthogonal orbitals and valence bond (VB) theory with resonance between various canonical structures. An attitude fostered by many text books, especially at the undergraduate level, is that the VB description is much easier to understand and simpler to use, but that MO theory is in some sense more fundamental . 

Figure 5 Gradient and bond paths in the molecular plane of benzene. The bond and ring critical points are indicated by asterisks and solid points, respeaively.

For benzene, molecular orbital theory shows that the six p-orbitals combine to give six molecular orbitals. The three lower-energy molecular orbitals are bonding molecular orbitals, and these are completely filled by the six electrons (which are spin-paired). There are no electrons in the (higher-energy) antibonding orbitals, and hence benzene has a closed bonding shell of delocalised Jt-electrons. 

None of the three, though, is a completely accurate depiction. As with benzene, the bonds that are shown in the Lewis structure as one double and two single are actually three equivalent bonds. Use a combination of valence bond theory and molecular orbital theory to explain the bonding in CO 3. ... 

Asphaltenes are obtained in the laboratory by precipitation in normal heptane. Refer to the separation flow diagram in Figure 1.2. They comprise an accumulation of condensed polynuclear aromatic layers linked by saturated chains. A folding of the construction shows the aromatic layers to be in piles, whose cohesion is attributed to -it electrons from double bonds of the benzene ring. These are shiny black solids whose molecular weight can vary from 1000 to 100,000. 

A is a parameter that can be varied to give the correct amount of ionic character. Another way to view the valence bond picture is that the incorporation of ionic character corrects the overemphasis that the valence bond treatment places on electron correlation. The molecular orbital wavefimction underestimates electron correlation and requires methods such as configuration interaction to correct for it. Although the presence of ionic structures in species such as H2 appears coimterintuitive to many chemists, such species are widely used to explain certain other phenomena such as the ortho/para or meta directing properties of substituted benzene compounds imder electrophilic attack. Moverover, it has been shown that the ionic structures correspond to the deformation of the atomic orbitals when daey are involved in chemical bonds. 

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