Structure, biradicaloid

Using a valence bond scheme parametrized with an effective Hamiltonian technique, it was shown that the mechanistic preference for a synchronous pathway with an aromatic transition state versus a non-synchronous mechanism via biradicaloid intermediate can be controlled by two factors (1) the stability of the long bond in the Dewar valence bond structure, and (2) the softness of the Coulomb interaction between the end methylene groups in the 1,5-diene chain. This means that the mechanism of rearrangement (equation 153) can strongly depend on substituents218. 

Recent DFT calculations by Sarzi-Amade and his collaboratorsmay well have resolved this mechanistic difference between a biradicaloid TS (Scheme 7, path b) and a mechanism involving discrete long-lived free radicals (Scheme 8). Oxygen insertion into the C—H bond of isobutane by DMDO was studied computationally at the unrestricted B3LYP level. Transition structures that were diradicaloid in nature were found to lead to... 

While the majority of trapping experiments favors a biradicaloid structure, evidence for a bicyclic intermediate (butalene, 31) was also found. " Breslow et al. " reported the formation of 31 (the possible existence of which had been pointed out by Dewar before ) upon dehydrohalogenation of 3-chloro[2.2.0]bicyclohexadiene... 

In connection with Eq. (22), yet another important factor differentiates our approach from usual quantum chemical analyses of reaction mechanisms. This difference concerns the fact that while a quantum chemical approach is in principle independent of any external information (all participating species appear automatically as various critical points on the PE hypersurface), in our model that is more closely related to classical chemical ideas some auxiliary information about the structure of the participating molecular species is required. This usually represents no problem with the reactants and the products since their structure is normally known, but certain complications may appear in the case of intermediates. This complication is not, however, too serious since in many cases the structure of the intermediate can be reasonably estimated either from some experimental or theoretical data or on the basis of chemical intuition. Thus, for example, in the case of pericyclic reactions that are of primary concern for us here, the intermediates are generally believed to correspond to biradical or biradicaloid species with the eventual contributions of zwitterionic structures in polar cases. 

The structural dependence of biradicaloid minima discussed in Section 4.3.3 on an example of twisting of a double bond A=B can be extended to take solvent effects into account. Not only the nature of the atoms A and B but also polar solvents and counterions affect the stability of zwitterionic states and states of charged species. Then, depending on the solvent, a biradicaloid minimum can represent either an intermediate or a funnel for a direct reaction. 

If all atoms involved in the reaction lie in the same plane, the unpaired electron of the acyl radical may be either in an orbital that is symmetric with respect to this plane or in an orbital that is antisymmetric—that is, either in a 0 or in a r orbital, whereas only a o orbital is available for the unpaired electron of radical R. Instead of just one singlet and one triplet covalent biradicaloid structure (Figure 4.5), there are now two of each, which may be denoted as B and B respectively. Similarly, there are also different zwitterionic structures to be expected. The increase in complexity and the number of states that results from the presence of more than two active orbitals on the atoms of a dissociating bond has been formalized and used for the development of a classification scheme for photochemical reactions ( topicity ), as is outlined in more detail in Section 6.3.3. 

Similar arguments apply to the S, state. Since twisted ethylene is a ho-mosymmetiic biradicaloid, this state is described by configurations that correspond tolcharge-separated structures in the VB picture, such as... 

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