CC bond lengths

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 ... 

Finally, examine the geometry of the lower-energy transition state. Measure all CC bond lengths. Draw a Lewis structure representing partial bonds in terms of... 

Several theoretical studies have been devoted to the ground state structure of all-trans-1,3,5-hexatriene21,25,31 and all-fraws-l,3,5,7-octatetraene18,21,26,30,31,36. Tables 4 and 5 present the values of the CC bond lengths obtained in some selected theoretical calculations. 

The introduction of electron correlation produces the same kind of effects on the CC bond lengths as those observed for butadiene. For hexatriene and octatetraene the inner C=C bonds are predicted to be longer than the outer C=C bonds. This result is in excellent agreement with experimental data corresponding to hexatriene, but differs from the experimental result in the case of octatetraene. This discrepancy has been suggested to be due to an important experimental error in the reported values42. 

The Mataga-Nishimoto parameterization (Mataga et ah, 1957) is used for the matrix elements. The CC bond length was taken to be 1.4A. The so-called was regarded as the coupling constant of the interaction of electrons. A set of localized orbitals corresponding to a Kekule structure was singled out. 

Are the carbon-carbon bond distances in allyl cation, allyl radical and allyl anion all similar, or are they significantly different The three molecules differ mainly in the number of electrons they assign to one particular molecular orbital. (This is the lowest-unoccupied molecular orbital (LUMO) in allyl cation, and the highest-occupied molecular orbital (HOMO) in allyl radical and allyl anion.) Examine the shape of this orbital. Are the changes in electron occupancy consistent with the changes in CC bond length Explain. 

The same theoretical methods have been applied to ethylidyne absorbed in the Rh(100) surface, with the conclusion, as experimentally found, that this species will be stable on the 4-fold site (237). However, the extended-Hiickel treatment, as well as the atom-superposition and electron-delocalization molecular orbital (ASED-MO) method, predict that the CC bond length will be shorter, and hence the vCC mode higher in wavenumber on the 4-fold site, whereas experimentally... 

In the end of the 1920s, the structure of hexam-ethylbenzene was solved by Lonsdale53 by means of X-ray diffraction and found to have identical CC bond lengths. Thus, a correspondence has been established between the real geometric structure and the empirical equivalence of the molecular positions deduced from the early experiments of the Kekule era. Still, the difference between benzene and cyclooctatetraene defied all existing hypotheses. The concept of delocalization has been latent still, and its connection to stability is not at all apparent. 

The value of q3 = (6) V2R (R is the CC bond length) is 0.63 A. Under pseudorotation the equatorial boat-shaped structures B (0 = 90°, = 0, 60°, 120°,.. . ) turn into a twist-boat structure TB (0 = 90°, = 30°, 90°,.. . ). The transitions between the chair and twist boat structures involve the intermediate formation of half boat (HB) and half chair (HC) structures. Quantum chemical calculations carried out by Dixon and Komornicki [1990] show that the axial structure C with symmetry D3d is stable. The energies of structures B and TB are 7.9 and 6.8kcal/mol higher than C. The barrier for transition from C to TB is 12.2-12.4 kcal/ mol. Because of the high barriers for pseudorotation, only thermally activated conformational transitions occur in cyclohexane. 

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