Diradical pair

For conjugated carbonyl compounds, such as a,) -enones, the orbital diagram would be similar, except for the recognition that the HOMO of the ground state is ij/2 of the enone system, rather than the oxygen lone-pair orbital. The excited states can sometimes be usefully represented as dipolar or diradical intermediates ... 

The thermal reversal of the photochemical a-cleavage, i.e., the direct recombination of the resulting radical pair or diradical, can be recognized as such only when at least one of the a-atoms is chiral and is epimerized in the process. In fact, the frequently rather low quantum yields observed in the phototransformations of nonconjugated steroidal ketones may be largely due to the reversal of a-cleavage. 

As can be seen from Fig. 5, both the Tg and T i states can mix with the S state but at different inter-radical distances X, At the point Jee cjj, X is small. The radical pair will not spend much time in this region either it will collapse to give a c-product or the two radicals will diffuse apart to the region where Jee and Tg-S mixing becomes possible. T i-S mixing is thus rather uncommon except where the radical separation is restricted by comjilexing or in a diradical (see Section IV). 

The diradical mechanism b is most prominent in the reactions involving fluorinated alkenes. These reactions are generally not stereospecificand are insensitive to solvent effects. Further evidence that a diion is not involved is that head-to-head eoupling is found when an unsymmetrical molecule is dimerized. Thus dimerization of F2C=CFC1 gives 106, not 107. If one pair of electrons moved before the other, the positive end of one molecule would be expeeted to attack the negative end of the other. 

The frontier orbital interaction can be free from the symmetry restriction. A pair of the reaction sites is close to each other while the other pair of the sites is far from each other (Scheme 25b). This is the geometry of the transition state leading to diradical intermediates. 

At the equilibrium inter-atomic distance R, two paired electrons of occupy the bonding orbital with a closed-shell low-spin singlet (S = 0). When the bond length is further increased, the chemical bond becomes weaker. The dissociation limit of corresponds to a diradical with two unpaired electrons localized at each atom (Fig. 1). In this case, the singlet (S spin-antiparaUel) and triplet (T spin-parallel) states are nearly degenerate. Different from such a pure diradical with... 

It is well known that Hund s rule is applicable to atoms, but hardly so to the exchange coupling between two singly occupied molecular orbitals (SOMOs) of a diradical with small overlap integrals. Several MO-based approaches were then developed. Diradicals were featured by a pair of non-bonding molecular orbitals (NBMOs), which are occupied by two electrons [65-67]. Within the framework of Hiickel MO approximation, the relationship between the number of NBMOs,... 

To explain the stereochemistry of the photoaddition, Buchi proposed that the reaction of electron-rich olefins and excited ketone involves an interaction of the electron-deficient carbonyl lone-pair orbital with the electron-rich 7r-olefin orbitals to form a diradical intermediate which could subsequently close to give the observed products. Indeed, reaction to yield the most stable diradical intermediate usually does nicely rationalize the observed product distribution. Examples of this are as follows11005 ... 

This chapter has to do with reactions wherein the photochemical event is the breaking of a bond in a molecule. For a single bond this results in the formation of a pair of radicals or a diradical. For a double bond as in diazo compounds or in azides a carbene or a nitrene and nitrogen are formed. All these intermediates will then undergo further mono- or bi-molecular dark reactions or eventually recombine to ground state starting materials. 

The initial step in a photochemical extrusion reaction is again cleavage of a bond to give a pair of radicals or a diradical, but now a second bond is broken immediately after the first one, thus leading to the extrusion of a small molecule as CO, C02, N2 or S02. As in a recent review on photoextrusion of small molecules 248) only such reactions will be outlined wherein the small molecule that is to be expelled is initially part of a ring, and where the product formed is again a cyclic system. 

When the central pair of benzene rings in the para-connected diradical is replaced by three or more benzene rings, the substance is markedly paramagnetic in spite of the fact that a coplanar structure is sterically possible. 

In a similar fashion the bonding in H2 might be formally regarded as a complementary pair of one-electron donor-acceptor interactions, one in the ot (spin up ) and the other in the 3 (spin down ) spin set.8 In the long-range diradical or spin-polarized portion of the potential-energy curve, the electrons of ot and (3 spin are localized on opposite atoms (say, at on HA and 3 on HB), in accordance with the asymptotic dissociation into neutral atoms. However as R diminishes, the ot electron begins to delocalize into the vacant lsB(a) spin-orbital on HB, while (3 simultaneously delocalizes into Isa on HA, until the ot and (3 occupancies on each atom become equalized near R = 1.4 A, as shown in Fig. 3.3. These one-electron delocalizations are formally very similar to the two-electron ( dative ) delocalizations discussed in Chapter 2, and they culminate as before (cf. Fig. 2.9) in an ionic-covalent transition to a completely delocalized two-center spin distribution at... 

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