Intermediate, locally symmetric

A locally symmetric intermediate may be represented in the LCFMOF approximation in the form of an equilateral triangle having a pair of energetically degenerate or quasidegenerate frontier orbitals ... 

The symmetry group of permutation S(3) [27] among the three fragments may be considered to be a symmetry group of the locally symmetric intermediate. This group is isomorphic to the point symmetry group C3V. 

Note. In the case of free-radical polymerization, the locally symmetric intermediate being formed will be extremely unstable because of the Jahn-Teller effect [28]. 

On the other hand, in the case of anionic-coordination polymerization on paramagnetic catalytic centers, the existence of free spins in the active center can lead to the stabilization of the triplet intermediate by involving unpaired electrons of the transition metal in the exchange interaction with those of the intermediate. This should increase the effect of spin exclusion. However, the final result will depend on the competition of two groups of factors stabilizing (pseudosymmetry and exchange interaction) and destabilizing (electric and quantum-chemical fields of the counterion) a locally symmetric triplet intermediate. 

Moreover, Fig. 2 does not contradict the well-known Murrel-Laidler theorem [35,36] which forbids taking the locally symmetric intermediate for the transition state because for the reason of symmetry at least two independent paths exist for its isomerization, i.e., for the insertion of the monomer into the polymer chain. In other words, the bifurcation of the reaction coordinate proceeds in the locally symmetric intermediate. Nevertheless, this is forbidden for the transition state by the Murrel-Laidler theorem which asserts that the matrix of force constants in the transition state has a single negative value, i.e., that the transition state corresponds to a single reaction coordinate. 

Let us assume that the polymerization of quasisymmetric dienes proceeds in accordance with the principle of local symmetry, Le., via the stage of formation of a locally symmetric intermediate. The structure of the locally symmetric diene intermediate (Scheme 11) is tentative. However, this is not very important since the train of thoughts is based only on the hypothesis of the existence of the Jahn-Teller effect in this case. 

The modem theory of chemical reaction is based on the concept of the potential energy surface, which assumes that the Born-Oppenheimer adiabatic approximation [16] is obeyed. However, in systems subjected to the Jahn-Teller effect, adiabatic potentials have the physical meaning of the potential energy of nuclei only under the condition that non-adiabatic corrections are small [28]. In the vicinity of the locally symmetric intermediate, these corrections will be very large. The complete description of nuclear motion, i.e. of the mechanism of the chemical reaction, can be obtained only from Schroedinger s equation without applying the Born-Oppenheimer approximation in the vicinity of the locally... 

Although Fig. 2 is not quite correct in the above sense, the conclusion is probably physically justified that the free energies of the locally symmetric intermediate and the nearest transition state are very close to each other. First, the Jahn-Teller system, not being at the minimum of the adiabatic potential, is hardly long-lived in the condensed liquid phase, in other words it will hardly be at any considerable minimum of free energy. Secondly, this system cannot be at the maximum of free energy (in this case it would be the transition state) because of the Murrel-Laidler theorem [36], Hence, the concept of a plateau in Fig. 2 is actually physically meaningful. 

As already mentioned, bifurcation of the reaction coordinate takes place in the locally symmetric Jahn-Teller intermediate. Let us assume that one branch of the reaction coordinate leads to the fixation of the 1,4 (or 4,l)-structure in the penultimate chain unit (Scheme 12a) and the other branch causes the formation of the vinyl structure of this unit (Scheme 12b). Then it should be expected that the relative content of these structures in the polymer will not depend on the conditions of the process, in particular on the polymerization temperature (because a plateau exists in Fig. 2). 

Let us consider the interaction between a symmetric vinyl monomer and a free anion. According to the local symmetry principle, in this case a locally symmetric intermediate is formed being in the ground triplet state (Schemes 5 and 15). In accordance with the spin exclusion principle, it should be expected that the polymerization of symmetric vinyl monomers on free anions proceeds very slowly. 

In this case the spin exclusion principle is not valid because the locally symmetric intermediate (there are intermediates but not of this type) is absent. The correspondence principle is also weakened to a considerable extent. For all these reasons, the dependence of the propagation rate constant kp on the polarity of the carbon-metal bond is approximately similar to that represented qualitatively by curve 2 in Fig. 3. 

In the interaction of the symmetric diene with its own free anion, in case of an attack at the a-carbon atom of the chain, according to the local symmetry principle, a triplet locally symmetric intermediate should be formed (Scheme 18a). In accordance with the spin exclusion principle, this reaction path, i.e., the attachment of the molecule of a quasisymmetric diene to the a-carbon atom, is suppressed. It should be noted that it is the assumption of the triplet character of the locally symmetric intermediate that has a physical meaning rather than its specific structure proposed in Scheme 18a (it should be established experimentally). 

In the case of the vinyl monomer, its polymerization would stop at the stage of the formation of the locally symmetric intermediate. In contrast, for dienes the attack at the y-carbon atom is possible and results in the formation of an intermediate of lower symmetry. Hence, it becomes possible to eliminate or at least to weaken the spin exclusion principle. Consequently, the polymerization at the y-carbon atom of a free anion should proceed more intensively than at the a-carbon atom. This should lead to the predominant formation of the vinyl structure of chain units. 

Hence, it should be expected a priori that in the tri det locally symmetric intermediate considered tere the value of the Fermi correlation ene will not be lower than 1 eV in the order of magnitude. Rigoroumess of spin exclusion should probably de d on the value of the singlet-trii t splitting in the intermediate A = Ef — AE where AE characterL the degree of degeneration of frontier orbitals (SclKme 6b) and in contrast to E J is a structure-... 

Nilj (A,a ) there is a natural identification of the intermediate e-symmetric L-theory localization exact sequence... 

An examination of these systems revealed that certain types of localized orbitals occurred over and over, so a classification of all occurring localized ir orbitals into a small number of different types proved possible. This classification is embodied in Fig. 13 (see p. 89). There are four pure types the orbitals denoted by ir 2, 7r 3, h 4, and 7r 2. The types 7r 2 and rr 3 are the two symmetric types found in the benzene molecule. The two intermediate types denoted by 7r 23 were also found in benzene. The type 7r 4 occurs in aromatic molecules which have a joint and extends over four atoms, the joint atom being at the center and carrying most of the charge. 

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