Electron delocalization, equation

The observed difference in the parameters is due most likely to the electron delocalization in the aromatic ring and, hence, to the increase in the triplet repulsion. The difference between the Eeo values for the reactions considered amounts to 13kJmol-1. With allowance for the examples considered above, Equation (6.39) can be extended and written in the following form ... 

There are extensive data for the acid-catalyzed protiodesilylation of XCgELrSiMes in methanol-aqueous perchloric acid or acetic acid-aqueous sulphuric acid at 50°C225. Correlation analysis of the partial rate factors (relative rate constants) by means of the Yukawa-Tsuno equation (Section n.B) finds p = —5.3 and r+ = 0.65. These values are consistent with a relatively low demand for stabilization of the transition state by electron delocalization, i.e. the transition state is early along the reaction coordinate, p-NO2 is highly deactivating with / = 14 x 10 but 0-NO2 is even more deactivating, with / = 6.8 x 10-5. This contrasts with the deactivation order discussed above for nitration and chlorination (Table 6), and may be explained in terms of the early transition state, well removed from the Wheland intermediate. 

The admixture of singly-excited configurations O, to Og to represent the HF wave function indicates electron transfer from bonding to antibonding orbitals and, accordingly, can be interpreted in terms of electron delocalization, which can be measured by the ratio CjlCG. Alternatively, one can define interbond populations by equation 1647 80 ... 

Further studies on the cholesteryl/i-cholesteryl system were reported by Dodson and Reigel4 and particularly by Winstein and his coworkers5-6. This early work showed that the interconversion of the two cholesteryl derivatives, one of which has an open or homoal-lyF form 1 and the other a cyclopropylcarbinyl form 3, could be understood in terms of the intermediate 2 (equation 1). Electron delocalization in 2 was suggested to occur across the intervening carbon atom rather than between adjacent carbon atoms as in normal conjugated systems. 

From bond energies (Table 4-6) we know that the weakest C—H bonds of propene are to the allylic hydrogens, H2C=CHCH2—H. Therefore, in the first step of radical-chain chlorination of propene, an allylic hydrogen is removed by a chlorine atom (Equation 14-1). The allylic C-H bonds are weaker than the alkenic C-H bonds because of the extra stabilization of the radical obtained on hydrogen abstraction (Equation 14-1). Two equivalent valence-bond structures (1a and 1b) can be written for the 2-propenyl radical the electron delocalization enhances the stability of the radical (see Section 6-5C) ... 

The higher exothermicity, by 13 kcalmol 1, of the hydrogenation energy of the silicon atom in 76d (equation 25) compared to that of 75d (equation 26)98a and also in the hydrogenation energy of the C=C double bond in 77a (equation 27) compared to that of 75d (equation 28)98b are also believed to be manifestations of the cyclic 67r-electron delocalization in 75d98a,b. 

The reaction of cyclic silane 23 with lithium metal in THF opens an unexpected way to c-(MenSi6)Li (24). Whereas in tri-, tetra- and penta-cyclosilanes a Si—Si bond is cleaved under these conditions (Section VLB.3), 23 reacts with cleavage of a exocyclic Si—C bond (equation 35)85. The reasons for the different behavior of 23 toward lithium remain unclear. It was, however, suggested that the facile formation of 24 is due to its thermodynamic stabilization by electron delocalization into the silicon framework85. 

The cyclic peralkylsilane oligomers, (R2Si) with n = 4-6, manifested especially strong electron delocalization.5 These rings are structurally analogous to those of the cycloalkanes, since the silicon atoms form four sigma bonds. However, the electronic properties of the cyclosilanes more nearly resemble those of aromatic hydrocarbons such as benzene. One example of such behavior is their reduction to anion radicals. Aromatic hydrocarbons such as naphthalene can be reduced, electrolytically or with alkali metals, to deeply colored anion radicals in which an unpaired electron occupies the lowest unoccupied molecular orbital (LUMO) of the hydrocarbon (equation (2)). 

Fig. 4.23 Hiickel s rule says that cyclic n systems with An + 2 n electrons ( = 0, 1, 2,. .. An + 2 = 2, 6, 10,. ..) should be especially stable, since they have all bonding levels full and all antibonding levels empty. The special stability is usually equated with aromaticity. Shown here are the cyclopropenyl cation, the cyclobutadiene dication, the cyclopentadienyl anion, and benzene formal structures are given for these species - the actual molecules do not have single and double bonds, but rather electron delocalization makes all C/C bonds the same...

The reaction of 1,3,2-diazasilole 83 with several organometallic and organic radical sources has been studied by EPR spectroscopy (Equation 9). They form neutral radical adducts 118 with the unpaired electron delocalized over the five-membered ring. Several of these radicals persist at room temperature for days <2004JA7786>. 

The higher rate of HC1 elimination of 4-chloro-l-phenyl-1-butanone with respect to 5-chloro-2-pentanone by a factor of 9.9 confirmed the participation of the C=0 group. The 7r-electron delocalization of the benzene ring increases the basicity and nucleophilicity of the carbonylic oxygen and led to greater stabilization of the transition state through assistance to the C—Cl bond cleavage in the transition state (equation 80). 

The combination of Eq. [134] with Eq. [144] provides an effective formalism for the band shape analysis of CT spectra when a substantial degree of electronic delocalization is involved. Equation [134] is exact for a TS donor-acceptor complex and, therefore, can be used for an arbitrary degree of electronic delocalization as long as the assumption of decoupling of the vibrational and solvent modes holds. Figure 18 illustrates the application of the band shape... 

Both electronic delocalization and polarizability of the donor-acceptor complex lead to a significant asymmetry between the absorption and emission optical lines as is often observed in experiment.The importance of this effect can be assessed by comparing the dependence of the observed spectral width on solvent polarity with the prediction of MH theory. Equations [6] and [12] can be combined to give... 

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