Bonding benzene

To derive the chromium-benzene bond dissociation enthalpy, the values of several stepwise bond dissociation enthalpies in chromium hexacarbonyl are... 

There are alternative ways of viewing the previous problem that are closer to the idealized concept of chemical bond strength. Consider reaction 5.20, where all the chromium-ligand bonds are cleaved simultaneously. The enthalpy of this disruption reaction at 298.15 K, calculated as 497.9 10.3 kJ mol-1 by using enthalpy of formation data [15-17,31], can be given as a sum of three chromium-carbonyl and one chromium-benzene bond enthalpy contributions (equation 5.21). 

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)).

Calculation of m — 0.90. For an aromatic carbon linked to a nonaromatic C, aj,i is 2flcc(Ar) + flcc(nAr), where Occ(Ar) is used for benzenic bonds and flcc(nAr) is for the bond formed with the nonaromatic carbon. The change in charge at the aromatic C is Aqc (relative to benzene), contributing Aqc flki to A a- On the other hand, this Aqc is also part of qc, hence of and contri-... 

Many bond structures have been proposed for benzene, and no single one may be accepted as fully satisfactory. Probably the best explanation is that the electrons are delocalized over all the six carbon nuclei. Thus benzene does not contain three carbon-carbon single bonds and three carbon-carbon double bonds. Rather, the benzene molecule contains six identical bonds, each one intermediate between a single and a double bond. This type of bond has been called a hybrid bond, a one and one-half bond, or simply a benzene bond. We normally draw the benzene ring as below. Each of the six comers represents a carbon atom, and each carbon atom is bonded to one hydrogen atom. 

Benzene—Benzene Bonds—Nomenclature of Aromatic Hydrocarbons — Physical and Chemical Properties of the Aromatic Hydrocarbons... 

In considering the properties of the polynuclear hydrocarbons relative to benzene, it is important to recognize that we neither expect nor find that all the carbon-carbon bonds in polynuclear hydrocarbons are alike or correspond to benzene bonds in being halfway between single and double bonds. 

In solution the bridging fluorines are rapidly transferred between adjacent silicons, such that the 29Si spectrum features a triplet due to two equivalent fluorines (Vsi-F = 127 Hz). This triplet did not change to a doublet of doublets at the low-temperature limit in toluene-ds solution, and hence freezing out of the fluorine transfer between vicinal silicons could not be achieved. When the temperature was raised, all six fluorines became equivalent, causing the silicon resonance to be a septet ( /si-F = 43 Hz), presumably due to exchange by correlated rotation178 of the silicon substituents about the Si-benzene bonds. This rotational process was stopped when a molybdenum complex was formed (152), presumably as a result of increased steric hindrance for rotation. 

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. 

S. Ziller, J. X-ray diffraction evidence for a cyclohexatriene motif in the molecnlar structure of tris(bicyclo[2.1.1 ]hexeno)benzene bond alternation after the refutation of the Mills-Nixon theory, Angew. Chem. Int. Ed. Engl 1995, 34, 1454-1456. 

Each carbon atom in benzene bonds to three atoms, so their electrons are in three sp orbitals and one p orbital as we ve seen for C2H4. The p orbitals are shown as the shaded shapes below on the left (only the C-C bonds are shown). The p atomic orbitals combine to form molecular orbitals with delocalized electrons as shown in the bonding tt molecular orbital below to the right. 

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