Bond angle, carbon

The chemical and physical properties of DMFL contrast most sharply with those of FL. The geometrical features, in particular the carbene-carbon bond angle, of these two carbenes are expected to be identical. The most important difference between DMFL and FL is that the spin multiplicities of the lowest electronic states appear to have been inverted. The experiments indicate that the ground-state of DMFL is the singlet. This conclusion is outlined in the reactions shown in Scheme 4. 

Since there is a difference in reactivity between the different cyclopar-affins, we will begin with the least reactive—cyclohexane. If cyclohexane existed as a flat hexagon, the carbon-carbon bond angle would be 120°. However, cyclohexane takes on a puckered structure as shown in the two conformations in Figure 1-10. The bonds in the conformations of cyclohexane have bond angles of 109.5°. Thus, the stability of the bonds is the same as in the straight-chain alkanes. 

The five carbon atoms of cyclopentane form a flat regular pentagon with internal angles of 108°, which are very close to normal carbon bond angles. Actually, cyclopentane molecules are not exactly flat however, the bond angles are close enough to the normal bond angle that these bonds are substantially as stable as those of cyclohexane. 

Fig. 1. Symmetric and distorted geometries for allyl radical, benzene, and singlet cyclobutadiene. The distortions keep constant nuclear repulsions between carbons. Bond angles are the same for the distorted and undistorted structures...

The carbon-carbon bond angle of the Zr-CH2-CH2-Zr structure (compound 4) is unusually narrow—only 76° (64). Compound 6 shows a similar carbon-carbon bond angle of 75° for the Zr-CH2-CH-Al structure. (i-Agostic hydrogen bonds can explain these angles and have been suggested to be a reason for the stabilization of the active site during the polymerization when methylaluminoxane is used as a cocatalyst. 

The central carbon of allene forms two a bonds and two n bonds. The central carbon is sp-hybridized, and the carbon-carbon bond angle is 180°, indicating linear geometry for the carbons of allene. The terminal =CH2 units are oriented 90° with respect to each other. 

When cyclooctatetraene accepts two electrons, it becomes a An + 2) n electron aromatic ion. Cyclooctatetraenyl dianion is planar with a carbon-carbon bond angle of 135° (a regular octagon). 

In contrast, the lability of carbon bond angles rises when carbon is bonded to 7t-acceptor o-donor atoms (Li, Be, B, and so on)... 

X-ray crystal structures of both 97 (all-tra/is isomer) and 99 trans is,trans isomer) have been obtained [39], and they confirm the relief of angle strain at the acetylenic carbon atoms that the phosphorus atoms provide in these small-ring pericyclynes (actual geometries shown in Fig. 9-28 average phosphorus atom endocyclic bond angles = 96° in 97 and 91° in 99 average acetylene carbon bond angles = 174° in 97 and 163° in 99). 

Bond-angle strain Relative instability of a molecule dtie to distortion of the tetrahedral carbon bond angle away from 109.5°. 

For n-alkenyl compounds, the position of the double bond will affect half-lives or GC retention times. Its position relative to both the polar and non-polar end of the molecule is significant. How close the double bond can be to the polar end without mutual interaction occurring is uncertain, but because of carbon bond angles, interaction should be negligible for double bonds at tv or greater. For series such as Z7-12 Ac, Z9-14 Ac,... 

In a chair conformation (Fig. 4.11), all of the carbon—carbon bond angles are 109.5°, and are thereby free of angle strain. The chair conformation is free of torsional strain, as well. When viewed along any carbon—carbon bond (viewing the structure from an end, Fig. 4.12), the bonds are seen to be perfectly sta ered. Moreover, the hydrogen atoms at opposite corners of the cyclohexane ring are maximally separated. 

X-ray crystallographic studies of cyclodecane reveal that the most stable conformation has carbon—carbon-carbon bond angles of 117°. This indicates some angle strain. The wide bond angles apparently allow the molecules to expand and thereby minimize unfavorable repulsions between hydrogen atoms across the ring. 

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