Bonding overlap

Aldol condensation offers an effective route to a p unsaturated aldehydes and ketones These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group As shown m Figure 18 6 the rr systems of the carbon-carbon and carbon-oxygen double bonds overlap to form an extended rr system that permits increased electron delocalization... 

Woodward and Hoffmann speeulated that the preferred motion was that whieh involved constructive (bonding) overlap between the terminal lobes of the highest-occupied molecular orbital (HOMO). 

Display the HOMO for cis-l,3,5-hexatriene. Which motion (conrotatory or disrotatory) insures bonding overlap Examine the geometry of the transition state for ring closure (hexatriene to cyclohexadiene). Is it consistent with the anticipated (conrotatory or disrotatory) motion of the terminal methylenes ... 

Woodward and Hoffmann pointed out that the Diels-Alder reaction involved bonding overlap of the highest-occupied molecular orbital (HOMO) on the diene and the lowest-unoccupied molecular orbital (LUMO) on the dienophile. Display the HOMO for 2-methoxybutadiene. Where is it localized Display the LUMO for acrylonitrile. Where is it localized Orient the two fragments such that the HOMO and LUMO best overlap (A clearer picture is provided by examining-the HOMO map for 2-methoxybutadiene and the LUMO map for acrylonitrile.) Which product should result ... 

FIGURE 18. (a) C—S bond overlap population as a function of molecular geometry (b) the same for the S—O BOP. (c) Total charge on sulphur, and (d), p-part of the charge. The black dots are the values for the S02 molecule itself. 

Bond overlap population 14 Boron compounds, as reducing agents 928, 929... 

Fig.4- Crystal overlap population of M0O3 (negative part antibonding overlap positive part bonding overlap)...

When two p orbitals overlap in a side-by-side configuration, they form a pi bond, shown in Figure 7.7. This bond is named after the Greek letter 7t. The electron clouds in pi bonds overlap less than those in sigma bonds, and they are correspondingly weaker. Pi bonds are often found in molecules with double or triple bonds. One example is ethene, commonly known as ethylene, a simple double-bonded molecule (Figure 7.8). The two vertical p orbitals form a pi bond. The two horizontal orbitals form a sigma bond. 

When the 7r-systerns of two or more double bonds overlap, as in conjugated dienes and polyenes, the 7r-clccIrons will be delocalized. This has chemical consequences, which implies that the range of possible chemical reactions is vastly extended over that of the alkenes. Examples are various pericyclic reactions or charge transport in doped polyacetylenes. A detailed understanding of the electronic structure of polyenes is therefore of utmost importance for development within this field. We will first discuss the structure of dienes and polyenes based on theoretical studies. Thereafter the results from experimental studies are presented and discussed. 

The two C atoms join with a sigma bond overlap of sp2 on the left-hand C with sp on the right-hand C. The three bonds between C and N consist of a sigma bond (sp on C with sp on N), and two pi bonds (2p on C with 2p on N). 

Figure 4.25 Bent 7t(o)-like (left) and 6(h)-like (right) metal-metal bonding overlap in HReReH, showing tilting of bonding NHOs with respect to the bond axis (the contour plane is perpendicular to one of the two Re—Re—H groups).

The bonding overlap between the hydrogens will be greater, in the eclipsed geometry relative to staggered geometry. However, due to the small hydro-... 

The Prediction of Gross Atomic Charges and Bond Overlap Populations... 

The factors which determine bond overlap population are as follows ... 

The relative energies, atomic charges (in parenthesis) and bond overlap populations of cis and trans N2H2 are shown in Scheme 14. 

The various atomic charges and bond overlap populations of CH2 = NH are shown below and confirm our expectations based on consideration of sigma conjugative effects. However, an anomaly is noted in the case of the 4—31G calculation of the C—Hb and C—Hc overlap populations. Since the STO—3G optimization leads to a longer C—Hc bond, as predicted, the anomaly represents most likely a computational artifact. 

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