Resonance allyl type

The energies of this Cl and of the other ones calculated in this work are listed in Table III. The calculated CASSCF values of the energies of the two lowest electronically states are 9.0 eV (5i, vertical) and 10.3 eV ( 2, vertical) [99]. They are considerably higher than the expenmental ones, as noted for this method by other workers [65]. In all cases, the computed conical intersections lie at much lower energies than the excited state, and are easily accessible upon excitation to Si. In the case of the H/allyl Cl, the validity confirmation process recovered the CHDN and 1,3-CHDN anchors. An attempt to approach the third anchor [BCE(I)] resulted instead in a biradical, shown in Figure 43. The bhadical may be regarded as a resonance hybrid of two allyl-type biradicals. 

The allylic-type furylic radical 6 is resonance stabilized to such a degree that its reactivity in promoting propagation by adding onto another furan ring is minimal. The fate of these radicals will simply be to couple with another radical present in the reaction medium (primary or secondary) or to disproportionate to regenerate the furan character of the ring26. ... 

Fig. 8. (a) Homoaromatic lit-electron conjugation in silyl cation 3. (b) Allyl-type resonance in Sit-electron radical 26. ... 

In a polymerization system, not only tertiary alkyl ions but also ions of the allyl type, because of their stabilizing resonance, would be formed readily. Hence, some hydrogenation and dehydrogenation of the primary polymer (e.g., RCH2CH2CH=CHR ) would occur in the following manner ... 

Problem 6.42 Use the concepts of (a) resonance and b) extended tt orbital overlap (delocalization) to account for the extraordinary stability of the allyl-type radical. 

Two products are formed because an allylic type of carbocation is a resonance hybrid... 

Allyl-type monomers do not yield high polymers. The substituent on the carbon in the / position with respect to the double bond is easily eliminated (especially hydrogen, halogenides, etc.). The generated radical is resonance-stabilized. It reacts much more readily with growing radicals than with the monomer. The low probability of long chain formation is a consequence of these terminating and transfer reactions. 

This is called allyl type resonance because it can be drawn for allylic carbocations, allylic carb-anions, and allylic radicals. 

Because this is an example of an allyl-type system p<=Y—Z ), a second resonance structure can be drawn that moves the lone pair and the jt bond. To delocalize the lone pair and make the system conjugated, the labeled carbon atom must besp hybridized with the lone pair occupying ap orbital. 

These kinetic parameters take into account the formation of a five-membered ring intermediate, the H-abstraction reaction of two H-atoms of allyl type and the formation of the resonantly stabilized l-hexen-3-yl radical. These facts explain why the reverse isomerization reaction requires greater activation energy. As clearly shown in Fig. 6, there is a new class of important reactions, i.e. ring decomposition (e.g., cyclo-hexyl to form hexenyl radical) and the reverse cyclo-addition reaction. The activation energies of ring decomposition to form primary radicals are 31,500 and 28,000 kcal/kmol respectively for the... 

This particular carbocation is a resonance-stabilized one of the allylic type. It is a cyclo-hexadienyl cation (often referred to as an arenium ion). 

A and B characterized by the VB-like structures shown in Scheme 2.4. State A is an allyl-type structure, whereas state B has a bond between 1 and 1 . States A and B will be degenerate at geometries where the sides of the triangle formed by the three centers are approximately equivalent. This geometry will be our model conical intersection. We use two additional resonance structures, Loc and Loc which have bonds between centers 1 and 2, and 1 and 2, respectively. These resonance structures or states can be constructed as linear combinations of the two states A and B. 

When a gas-phase thermal reaction involves molecules of the type YH which are likely to form free radicals of the allylic type Y, which are stabilized by resonance, by the abstraction of an H atom, the terminations involving these radicals must be taken into account, i.e. ... 

The decay behavior of allenes (1,2-propadienes) is quite different from that of the conjugated 1,3-dienes. Figure 8 shows the decay of ArS in cyclohexane for the reaction with methyl-substituted allenes [46]. By adding allene, the decay of ArS is accelerated even in the degassed solution, suggesting that the reaction proceeds irreversibly. Such irreversibility occurs when the incipient C atom-centered radical becomes a resonance stable allyl-type radical by rotation of the C-C bond, as shown in Scheme 9. In the aerated solution, the decay of ArS is further accelerated, indicating that the irreversibility due to the rotation is not completely established the addition of O2 further shifts the equilibrium to the peroxy radical side by trapping the incipient short-lived C atom-centered radicals. 

Similarly, homocoupling and coupling of two different allyl moieties (cross coupling) can be efficiently run with Ni or Pd complexes, the reaction proceeding by jr-allyl type intermediates formation through the oxidative addition of the allyl halide to the metal centre. A mixture of isomers is usually obtained as a result of the allylic resonance, but the reaction is however of interest for the preparation of macrocycles the selectivity being in such cases mediated by geometrical and conformational factors. 

As shown in Figure 27, an in-phase combination of type-V structures leads to another A] symmetry structures (type-VI), which is expected to be stabilized by allyl cation-type resonance. However, calculation shows that the two shuctures are isoenergetic. The electronic wave function preserves its phase when tr ansported through a complete loop around the degeneracy shown in Figure 25, so that no conical intersection (or an even number of conical intersections) should be enclosed in it. This is obviously in contrast with the Jahn-Teller theorem, that predicts splitting into A and states. 

C 1 IS more reactive because the intermediate formed by electrophilic attack there IS a relatively stable carbocation A benzene type pattern of bonds is retained m one nng and the positive charge is delocalized by allylic resonance... 

Resonance theory can also account for the stability of the allyl radical. For example, to form an ethylene radical from ethylene requites a bond dissociation energy of 410 kj/mol (98 kcal/mol), whereas the bond dissociation energy to form an allyl radical from propylene requites 368 kj/mol (88 kcal/mol). This difference results entirely from resonance stabilization. The electron spin resonance spectmm of the allyl radical shows three, not four, types of hydrogen signals. The infrared spectmm shows one type, not two, of carbon—carbon bonds. These data imply the existence, at least on the time scale probed, of a symmetric molecule. The two equivalent resonance stmctures for the allyl radical are as follows ... 

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