Biradicals biradicaloids

The concept of biradicals and biradicaloids was often used in attempts to account for the mechanism of photochemical reactions [2,20,129-131]. A biradical (or diradical) may be defined as [132] an even-electron molecule that has one bond less than the number permitted by the standard rules of valence. 

Isomerizations in which C—C bonds are cleaved homolytically have been chosen several times as probes for the study of substituent effects on radical stabilization. The nature of the intermediates—in some cases there may not even be intermediates but only biradical-like transition states—is often not known in detail. It may thus be uncertain whether the radicals include fully evolved radical centres, especially in the case of intramolecular isomerizations where biradicaloids might be involved. On this basis it is not expected that stabilization energies which derive from rate measurements for isomerizations will be identical to those obtained by other procedures. 

In view of the above exposed facts, to date there is no direct experimental evidence of the intermediary 1,4-dioxy biradical in the decomposition of 1,2-dioxetanes. Therefore, it appears that the asynchronous (biradicaloid or biradical-like) concerted mechanism (merged mechanism) is the one consistent with aU the experimental and theoretical data currently available. 

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. 

In fact, the cycloaddition of butadiene to ethylene, as well as cycloadditions of similar non-polar dienes to non-polar alkenes seem experimentally to be cases where concerted and stepwise (biradical or biradicaloid) mechanisms compete. We have recently discussed a number of cases, such as the dimerization of butadiene, piperylene, and chloroprene, the cycloadditions of butadiene or methylated dienes to halogenated alkenes, and others, where non-stereospecificity and competitive formation of [2 + 2] adducts indicate that mechanisms involving diradical intermediates compete with concerted mechanisms10). Alternatively, one could claim, with Firestone, that these reactions, both [4 + 2] and [2 + 2], involve diradical intermediates1 In our opinion, it is possible to believe that a concerted component can coexist with the diradical one , and that both mechanisms can occur in the very same vessel 1 ). Bartlett s experiments on diene-haloalkene cycloadditions have also been interpreted in this way12). 

Biradical (Synonymous with diradical) An even-electron molecular entity with two (possibly delocalized) radical centres which act nearly independently of each other. Species in which the two radical centres interact significantly are often referred to as biradicaloids. If the two radical centres are located on the same atom, they always interact strongly, and such species are called carbenes, nitrenes, etc. The low-est-energy triplet state of a biradical lies below or at most only a little above its lowest singlet state (usually judged relative to kT, the product of the Boltzmann constant k and the absolute temperature T). The states of those biradicals whose radical centres interact particularly weakly are most easily understood in terms of a pair of local doublets. Theoretical descriptions of low-energy states of a biradical display the presence of two unsaturated valences (biradicals contain one fewer... 

First of all, we consider the significance of the presence of C-, +, the coefficient of the in-phase combination of the two hole-pair functions in the So wave function, in Equation 4.12. In the simple model for a perfect biradical, this in-phase combination is exactly equal to the wave function of the Sj state, and it does not enter into those of the So and S, states at all. Thus, in this approximation. So does not spin-orbit couple to the triplet. The same is true in weakly heterosymmetric biradicaloids (0 < 5 < S ), in which the in-phase hole-pair character is shared by S, and Sj, but not Sg, and the former two spin-orbit couple to T, but So does not. In strongly heterosymmetric... 

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