Benzene bond length

Fig. 21 HF/6-31 + G -optimized structures and main geometrical parameters of the stable (T-and T/ -type complexes located on the potential energy surface (PES) of protonated sec-butyl benzene (bond lengths in Angstrpms angles in degrees (in itabc)).

Spin-coupled calculations have been performed for various azobenzenes, in a manner analogous to our first series of calculations on benzene. Bond lengths and angles were taken from the volumes edited by Katritzky and Rees [17], but where we were unable to find precise C — H bond lengths we took values from related molecules. 

The structure polycyclic hydrocarbons (eg. Fig. 2) is taken with benzene bond lengths Rq = 1.397A and bond angles 27r/3. The Ohno formula[7] for V(Rpq) is... 

Figure 2. Dependence of 9Si shift on the Si-C(benzene) bond length in the Wheland c-complex (Me3Si)3Si (CgH (,)+ involving the tris-(trimethylsilyl)silylium cation and benzene. (HF-IGLO/[7s6p2d/5s4pld/3slp] calculations from Ref. 45)...

NMR chemical shifts can be measured with high accuracy and, therefore, the reliability of IGLO chemical shifts enables the determination of molecular geometries. For example, it was found by Ottosson and Cremer [45] that the experimental shift value of 111 ppm [9] corresponds to a Si-C(benzene) bond length of 2.29 A. Such a distance is indicative of covalent bonding and, therefore, documents the loss of silylium ion character in the benzene complex. 

Estimate interelectronic repulsion in the cyclooctatetraene dianion due to the interaction of the two charges only (use the benzene bond length and a reasonable average for the interelectronic distance). 

Because the carbons that are singly bonded m one resonance form are doubly bonded m the other the resonance description is consistent with the observed carbon-carbon bond distances m benzene These distances not only are all identical but also are intermediate between typical single bond and double bond lengths... 

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

Index of aromaticity based c bond-length variation (benzene = 100). ... 

Measure the carbon-carbon bond length in benzene. Would you describe it as a single bond, a double bond, or somewhere in between Draw whatever resonance contributors are needed to justify your conclusion. 

A large body of experimental evidence confirms that covalent bonds have characteristic distances depending on bond type. Carbon-carbon single and double bond lengths are around 1.54A and 1.32A, respectively, while partial double bond distances, e.g., in benzene, are about 1.40A. 

Cyclohexatriene to benzene displays a sequence of structures from 1,3,5-cyclohexatriene (withCC single and double bonds initially set to 1.5 and 1.3 A, respectively) to benzene (witb all CC bonds set to 1.4 A) and back to cyclohexatriene. Plot energy (vertical axis) vs. CC bond length (horizontal axis). How many energy minima are there Do the minima look more like 1,3,5-cyclohexatriene or benzene What is the correct interpretation of the resonance picture ... 

E Benzene is planar and has the shape of a regular hexagon. All bond angles are 120°, all carbon atoms are sp2-hybndized, and all carbon-carbon bond lengths are 139 pm. 

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