Electron delocalization, transition

FIGURE 10.6 Conformations and electron delocalization in 1,3-butadiene. The s-cis and the s-trans conformations permit the 2p orbitals to be aligned parallel to one another for maximum TT electron delocalization. The s-trans conformation is more stable than the s-cis. Stabilization resulting from tt electron de-localization is least in the perpendicular conformation, which is a transition state for rotation about the C-2—C-3 single bond. The green and yellow colors are meant to differentiate the orbitals and do not indicate their phases. 

As is outlined for ene reactions of singlet oxygen in Scheme 15, the prototypical ene reaction starts with the electron delocalization from the HOMO of propene to the LUMO of X=Y. The delocalization from the HOMO, a combined n and orbital with larger amplitude on n, leads to a bond formation between the C=C and X=Y bonds. Concurrent elongation of the bond enables a six-membered ring transition stracture, where partial electron density is back-donated from the LUMO of X=Y having accepted the density, to an unoccupied orbital of propene localized on the bond. As a result, the partial electron density is promoted (pseudoex-cited) from the HOMO (it) to an unoccupied orbital (ct n ) of alkenes. This is a reaction in the pseudoexcitation band. 

Fukui applied the orbital mixing rule [1,2, 59] to the orbital hybridization or the deformation of the LUMO of cyclohexanone to explain the origin of the Jt-facial selectivity in the reduction of cyclohexanone. Cieplak [60] proposed that electron delocalization occurs from the bonds into the o orbital of the incipient bonds at the transition state. 

Extensive electron delocalization along the chain direction leads to an electronic transition energy for one-photon absorption (E0) of typically 15 000-16 000 cm-l for an unstrained backbone (.7). This value is very close to that of polyacetylene (or... 

All three levels of theory predict the ring expansion of singlet phenylcarbene ( A -la) to cycloheptatetraene (3a) to occur in two steps, via bicy-clo[4.1.0]hepta-2,4,6-triene (2a) as an intermediate. The first step is addition of the carbene carbon to an adjacent 7t bond of the ring. The second step involves a six-electron, disrotatory, electrocyclic ring opening, which is allowed by orbital symmetry67 and thus proceeds by a highly delocalized transition state. Fig. 4... 

In a similar fashion the bonding in H2 might be formally regarded as a complementary pair of one-electron donor-acceptor interactions, one in the ot (spin up ) and the other in the 3 (spin down ) spin set.8 In the long-range diradical or spin-polarized portion of the potential-energy curve, the electrons of ot and (3 spin are localized on opposite atoms (say, at on HA and 3 on HB), in accordance with the asymptotic dissociation into neutral atoms. However as R diminishes, the ot electron begins to delocalize into the vacant lsB(a) spin-orbital on HB, while (3 simultaneously delocalizes into Isa on HA, until the ot and (3 occupancies on each atom become equalized near R = 1.4 A, as shown in Fig. 3.3. These one-electron delocalizations are formally very similar to the two-electron ( dative ) delocalizations discussed in Chapter 2, and they culminate as before (cf. Fig. 2.9) in an ionic-covalent transition to a completely delocalized two-center spin distribution at... 

Simple 2c/2e bonds to the transition metals commonly are weaker than the corresponding sigma bonds from the p-block elements, resulting in lower-lying acceptor ctml antibonds and increased electronic delocalization. 

In the ground state, aminomethylenemalonates possess an essentially planar geometry, which maximizes the electron delocalization in the molecules. In the heteropolar transition state, the plane of the groups R3 and NR R2 and the plane of the two carbonyl groups occupy orthogonal positions. More details of the dynamic and static stereochemistry of push-pull ethylenes, as in compounds 1 and 2, are discussed in two excellent reviews (73TS295 83TS83). 

For the elements highlighted by the diagonal strip there is an indication that the / and d electrons may be balanced between being localized and itinerant. According to Smith and Kmetko (1983), materials close to this localization-delocalization transition can have their properties modified appreciably by small... 

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. 

Roth and coworkers23 reported NMR data of the orthogonal butadiene (Z,Z)-3,4-dimethylhexa-2,4-diene. (Z,Z)-13 having the planes of the double bonds at a dihedral angle not far from 90°. This diene serves as the model for conjugated diene lacking rr-electron delocalization and for the transition state for interconversion of antiperiplanar (trans) and synperiplanar (cis or gauche) butadiene. 

The reactivity of carbenes is strongly influenced by the electronic properties of their substituents. If an atom with a lone pair (e.g. O, N, or S) is directly bound to the carbene carbon atom, the electronic deficit at the carbene will be compensated to some extent by electron delocalization, resulting in stabilization of the reactive species. If both substituents are capable of donating electrons into the empty p orbital of the carbene, isolable carbenes, as e.g. diaminocarbenes (Section 2.1.6), can result. The second way in which carbenes can be stabilized consists in complexation. The shape of the molecular orbitals of carbenes enable them to act towards transition metals as a-donors and 71-acceptors. The chemical properties of the resulting complexes will also depend on the electronic properties of the metallic fragment to which the carbene is bound. Particularly relevant for the reactivity of carbene complexes are the ability of the metal to accept a-electrons from the carbene, and its capacity for back-donation into the empty p orbital of the carbene. 

As to the cation-radical version of this isomerization, there are testimonies on the transition of the norcaradiene carcass into the cycloheptatriene skeleton. Calculations at the B3LYP level shows that cycloheptatriene cation-radical is more stable than norcaradiene cation-radical by ca. 29 kJ mol (Norberg et al. 2006). Hydrocarbon ion-radicals with strained ring structures have a tendency to undergo facile rearrangement to enforce the unpaired electron delocalization and release their strain energy. 

Figures 3a and 3a depict the weak bond of an O2 molecule with the lattice. It is formed by an electron being drawn from an ion of the lattice to an O2 molecule. Owing to the greater electron aflSnity of the O2 molecule, the electron may be considered completely transferred from the lattice to the molecule as a result, a molecular ion 02 is formed and a localized hole appears in the lattice attached to the ion Oi, The entire system (the adsorbed O2 molecule + adsorption center) acquires a noticeable dipole moment with negative pole directed outward, but remains electrically neutral as a whole. The bond is effected without the participation of a free lattice electron. The transition to a strong acceptor bond entails the localization of an electron, or, what amounts to the same thing, the delocalization of a hole. Such a strong acceptor bond is depicted in Figs. 3b and 3b. ...

The formation of the products could be explained by hemiacetal formation followed by Prins cyclization and subsequent Ritter amidation. A tentative reaction mechanism to realize the cis selectivity is given in Fig. 20 and could be explained by assuming the formation of an (L )-oxocarbenium ion via a chair-like transition state, which has an increased stability relative to the open oxocarbenium ion owing to electron delocalization. The optimal geometry for this delocalization places the hydrogen atom at C4 in a pseudoaxial position, which favors equatorial attack of the nucleophiles. 

The reducing character of the ligand is linearly related to the relative position of the first spin-allowed transition and the first electron transfer transition. Jorgensen 233) calculated the optical electronegativity of ethyl-dsep as 2.6 which is close to that of ethyl-dtp, 2.7. Jorgensen 233) has also discussed electron delocalization in M(X2P)3 (X = S, Se) chromophores in terms of a molecular orbital model. 

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