Tr-Delocalization

Recently, Kochi et al. described a novel photochemical synthesis for a-nitration of ketones via enol silyl ethers. Despite the already well-known classical methods, this one uses the photochemical excitation of the intermolecular electron-donor-acceptor complexes between enol silyl ethers and tetranitrometh-ane. In addition to high yields of nitration products, the authors also provided new insights into the mechanism on this nitration reaction via time-resolved spectroscopy, thus providing, for instance, an explanation of the disparate behavior of a- and (3-tetralone enol silyl ethers [75], In contrast to the more reactive cross-conjugated a-isomer, the radical cation of (3-tetralone enol silyl ether is stabilized owing to extensive Tr-delocalization (Scheme 50). 

Scheme 10 shows the distortion energies of the ground state (GS) of benzene in part a in comparison with situations where tr-delocalization is turned off in parts b—d. All the distortions are carried out along the localizing b2U mode with a small correction which... 

The polymers show extensive tr-delocalization and they are of various types,101 such as... 

On the other hand, although the Yo values for most cations (cf Tables 16 and 17) were obtained from alkene basicities AG(cc)h+ which correspond to the kinetic protonation of the C=C double bond in the solution phase, those Yq values are nevertheless identical with the r values for the corresponding solvolyses rather than with those for the C=C protonations or acid-catalysed hydrations which are noticeably smaller than that obtained for the solvolytic processes. Accordingly, the discrepancy between the r values for hydration and the gas-phase basicities leads us to the conclusion that, in contrast to SnI solvolysis, the structure of the transition state of acid-catalysed hydration is appreciably different from that of the corresponding stable cationoid intermediates with respect to Tr-delocalization of the positive charge. 

In contrast, an ESR study of the aryl-substituted cyclopropenyl radical 2 shows that it has apparent Tr-delocalization. " ... 

Table 2. Grabowski Tr-delocalization energies (kJ/mol) and resonance parameters for the O-H- O bridges of some malonaldehyde derivatives, calculated at B3LYP/6-31G level (reprinted from Ref. 158 with permission from Elsevier)...

Values in parentheses refer to the open conformation = ting closure energy Grabowski Tr-delocalization energy Gilli s parameter... 

B3LYP/6-311++G(d,p) calculations have shown that in diketones with two resonance-assisted hydrogen bonds, as 2,4-dihydroxy-but-2-ene-4-diol, no rise of the resonance effect is noted and the Tr-delocalization energy of the closed-open conformations is greater than in the most stable conformers with two closed rings [210]. 

The models of tr-delocalized ions, contrary to the aliphatic carbocations made it possible to measure directly the rates of 1,2-shifts of such migrants as, e.g., a chlorine atom and aryl groups cf. Ref. and to establish the... 

The carbocation is stabilized by delocalization of the -tr electrons of the double bond, and the positive charge is shared by the two CH2 groups. Substituted analogs of allyl cation are called allylic carbocations. 

Conjugated diene (Section 10.5) System of the type C=C—C=C, in which two pairs of doubly bonded carbons are joined by a single bond. The -tr electrons are delocalized over the unit of four consecutive ip -hybridized carbons. 

Delocalization (Section 1.9) Association of an electron with more than one atom. The simplest example is the shared electron pair (covalent) bond. Delocalization is important in conjugated -tr electron systems, where an electron may be associated with several carbon atoms. 

Complexes 79 show several types of chemical reactions (87CCR229). Nucleophilic addition may proceed at the C2 and S atoms. In excess potassium cyanide, 79 (R = R = R" = R = H) forms mainly the allyl sulfide complex 82 (R = H, Nu = CN) (84JA2901). The reaction of sodium methylate, phenyl-, and 2-thienyllithium with 79 (R = R = r" = R = H) follows the same route. The fragment consisting of three coplanar carbon atoms is described as the allyl system over which the Tr-electron density is delocalized. The sulfur atom may participate in delocalization to some extent. Complex 82 (R = H, Nu = CN) may be proto-nated by hydrochloric acid to yield the product where the 2-cyanothiophene has been converted into 2,3-dihydro-2-cyanothiophene. The initial thiophene complex 79 (R = R = r" = R = H) reacts reversibly with tri-n-butylphosphine followed by the formation of 82 [R = H, Nu = P(n-Bu)3]. Less basic phosphines, such as methyldiphenylphosphine, add with much greater difficulty. The reaction of 79 (r2 = r3 = r4 = r5 = h) with the hydride anion [BH4, HFe(CO)4, HW(CO)J] followed by the formation of 82 (R = Nu, H) has also been studied in detail. When the hydride anion originates from HFe(CO)4, the process is complicated by the formation of side products 83 and 84. The 2-methylthiophene complex 79... 

The most satisfactory route to the synthesis of the ri -borole complexes is the reaction of dihydroboroles (2-borolenes and 3-borolenes) with metal carbonyls. An alternative method of synthesis includes formation of the borole adducts with ammonia, 320 (R = Me, Ph) [87JOM(336)29]. Thermal reaction of 320 (R = Me, Ph) with M(C0)6 (M = Cr, Mo, W) gives 321 (M = Cr, R = Me, Ph M = Mo, W, R = Ph). There are data in favor of the Tr-electron delocalization over the borole... 

We can understand the differences in properties between the carbon allotropes by comparing their structures. Graphite consists of planar sheets of sp2 hybridized carbon atoms in a hexagonal network (Fig. 14.29). Electrons are free to move from one carbon atom to another through a delocalized Tr-network formed by the overlap of unhybridized p-orbitals on each carbon atom. This network spreads across the entire plane. Because of the electron delocalization, graphite is a black, lustrous, electrically conducting solid indeed, graphite is used as an electrical conductor in industry and as electrodes in electrochemical cells and batteries. Its... 

The orbital phase theory is applicable to the singlet diradicals [20]. The electron configuration of the singlet states of the cross- (TMM) and linear (BD) conjugate diradicals is shown in Scheme 9, where the mechanism of the delocalization of a and P spins between the radical centers through the double bond are separately illustrated by the arrows. The cyclic [-a-Tr-b-T -] interaction is readily seen to occur for the spin delocalizations. The p orbital a) in one radical center and the n orbital are occupied by a spins, and therefore, electron-donating orbitals. The p orbital (b) in the other radical center and the ii orbital are not occupied by a spins. 

Comparison of the TR spectra for HPDP in MeCN solvent to results from density functional theory calcnlations for the triplet state of HPDP indicates the triplet state has a qninoidal strnctnre with the carbonyl group about 16° out of the quinoidal plane and a delocalized nn character. Figure 3.29 compares ps-TR spectra of... 

A positive charge in an empty rc-orbital can be stabilized by the + M effect exerted by the free electron pair on an adjacent atom X, or by a filled tr-orbital of a double bond C = D. In delocalizing a positive charge into a double bond C = D, the larger orbital coefficient is again on atom D. The higher the electronegativities of the orbitals on X or on D, the less they are available for donation of electron density into the... 

It was expected that participation of the dipolar forms b in the ground-state hybrid of 7/8 might introduce the special electronic stability of the delocalized 2 tr-configura-tion to compensate for the high strain energy estimated for these molecules, (e.g. 8 ... 

There is also substantial stabilization of [4+] by electron delocalization from the cyclic a-vinyl group. This is shown by a comparison of the thermodynamic driving force (p Tr lies between —7.8 and —8.5) and absolute rate constant (ks = 1 -6 x 107 s 1) for the reaction of [4+] in 25% acetonitrile in water with the corresponding parameters for reaction of the resonance-stabilized l-(4-methoxyphenyl)ethyl carbocation in water (p Tr = — 9.4and s= 1 x 108 s Table 5). 

These TR results demonstrate that the localized model of Ru(bpy) + is valid on the timescales of electronic motions and molecular vibrations. It is virtually certain that delocalization (via, for example, intramolecular electron transfer or dynamic Jahn-Teller effects) occurs on some longer timescale. 

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