Biradical perturbed

Applications of electron propagator methods with a single-determinant reference state seldom have been attempted for biradicals such as ozone, for operator space partitionings and perturbative corrections therein assume the dominance of a lone configuration in the reference state. Assignments of the three lowest cationic states were inferred from asymmetry parameters measured with Ne I, He I and He II radiation sources [43]. 

If one examines the minimal sequences of reaction steps for [2+2] cycloadditions, Eqs. 12—18, 32—35, one concludes that stereochemistry of addition, and perhaps relative reactivities might be calculable at several points. Oriented complexes could control regiospecificity, or the transition state leading to a biradical could be the important stage. Relative rates of product formation would be derived from relative perturbation stabilization energies for different configurations of the two reactants. 

The trans compound cannot be formed by a concerted reaction from the 33t-7t state because the predominant level perturbation is the almost degenerate interaction of K(jr) and 0(jr), that would give cis geometry of product. Since the trans adduct must be formed via biradical intermediate (the only other possibility), it has usually been convenient to suppose that the cis adduct is also formed from the same biradical intermediate. Another choice of mechanism is that cis compound is formed via a concerted reaction, and the trans compound arises from a biradical pathway. In this case, the spin prohibition could be outweighed by two factors, the favorable geometry and the stabilizing first-order perturbation. 

From examination of Fig. 11, it is inferred that the zn-n state is less reactive, and a biradical mechanism should be the major reaction pathway. The degenerate stabilizing perturbation of the bonding levels is missing, and concerted pathways are not likely if stabilized only by much smaller secondary interactions. If the hi-n singlet state could be intercepted in some way, the all-suprafacial concerted mechanism would be favored [K(ji ) -0(jr )] relative to the suprafacial-antarafacial mechanism [O(tt) - -K( i )]. 

Fig. 7. Possible potential curves for the reaction (10-1). (The splitting of the perturbed curves I and II may be exaggerated in the region of the localized biradical intermediate.)...

Qualitative studies, like the perturbational approach by Dougherty [27] and the correlation to biradical structures by Michl [28], were of limited impact. Only the work of Salem gave, for the first time, a general view of the most important photoreactions of ketones. 

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