Biradical spin-orbit coupling

However, this model can also be generalized to other cydicalkenes. The ISC reactive conformation of the intermediary triplet biradicals is important for stereoselectivity, as the rate constant for ISC, which is controlled by a spin-orbit-coupling (SOC) mechanism, is heavily dependent on the orientation of the two spin centers. The importance of the ISC process was reasonably proved by the low mdo-sclcctivity in PB reactions with naphthaldehydes or aliphatic aldehydes, in which the excited singlet states may react with alkenes [34]. The stereoselectivity observed in the PB... 

Klessinger M (1998) Triplet Photoreactions. Structural Dependence of Spin-orbit Coupling and Intersystem Crossing in Organic Biradicals. In Parkanyi C (ed) Theoretical Organic Chemistry-Theoretical and Computational Chemistry, Elsevier, Amsterdam, p 581. 

Recently, studies were carried out to explain the exo/endo selectivity the Patemo-Buchi reaction [30]. These studies were carried out mostly achiral or racemic substrates. Excited monocyclic aromatic aldehydes 33 re in their 3n,/rr state with cyclic enol ether derivatives like 2,3-dihydrofuran (Scheme 8) [31]. In these cases, the sterically disfavored endo isomer 35a obtained as major product. This result was explained by the fate of the trip biradical intermediate G. In order to favor cyclization to the oxetanes 35a,b, radical p-orbitals have to approach in a perpendicular fashion to increase spin-orbit coupling needed for the triplet to singlet intersystem crossing [32]. sterically most favored arrangement of this intermediate is depicted as G. encumbering Ar substituent is orientated upside and anti to the trihydrofur moiety. Cyclization from this conformation yields the major isomer 35a. 

The g-value difference and hyperfine coupling mechanism is important in biradicals in which the two radical centers are relatively far apart (1,6-biradicals and longer), and the spin-orbit coupling is weak at almost all biradical geometries, and the T, and Sq levels are almost exactly degenerate (at most a few cm apart). 

In the absence of magnetic field, the three triplet levels are separated in energy by the zero field splitting (roughly 10 -I0" cm" in short biradicals) and are quantized with respect to the molecular axes dictated by the principal directions of the electron spin-spin dipolar coupling tensor primarily responsible for the zero-field splitting. (Spin-orbit coupling affects the... 

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... 

Figure 4.25. Sum-over-atoms factor in the spin-orbit coupling vector // a) in orthogonally twisted ethylene and b) in (0, 90°) twisted trimethylene biradical, using Equation (4.12) and (4.13) most localized orbitals x - Xh and nonvanishing atomic vectorial contributions from Xh (white through-space, black through-bond).

The magnitude of the total spin-orbit coupling strength can change dramatically as a function of biradical conformation (Carlacci et al., 1987). The through-space part can be roughly approximated by... 

The simple theory of spin-orbit coupling in biradicals has been found useful for the interpretation of the lifetimes of triplet biradicals as a function of their structure and conformation (Johnston and Scaiano, 1989 Adam et al., 1990) and of the stereochemistry of their reaction products (Chapter 7). 

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