Biradicals radical pairs

Because orbital momentum, and thus orbital shape and orientation, are involved in intersystem crossing driven by spin-orbit coupling, molecular symmetry and electronic configuration come into play. Consequently, the efficiency of intersystem crossing is usually different for the sublevels of a given multiplicity. This effect is the basis of the so-called triplet mechanism of electron spin polarizations as far as CIDNP is concerned, Hso only plays a role for systems with restricted diffusion (biradicals, radical pairs in micelles). 

FIGURE 48.5 Reaction coordinate (RC) derived from computational studies of the Norrish type I reaction. The RC shows a-cleavage from both Sj and Tj surfaces to yield the acyl-alkyl biradical/radical pair. Subsequent decar-bonylation of the acyl radical generates the alkyl-alkyl biradical/radical pair. 

Wliile the earliest TR-CIDNP work focused on radical pairs, biradicals soon became a focus of study. Biradicals are of interest because the exchange interaction between the unpaired electrons is present tliroiighoiit the biradical lifetime and, consequently, the spin physics and chemical reactivity of biradicals are markedly different from radical pairs. Work by Morozova et al [28] on polymethylene biradicals is a fiirther example of how this method can be used to separate net and multiplet effects based on time scale [28]. Figure Bl.16.11 shows how the cyclic precursor, 2,12-dihydroxy-2,12-dimethylcyclododecanone, cleaves upon 308 mn irradiation to fonn an acyl-ketyl biradical, which will be referred to as the primary biradical since it is fonned directly from the cyclic precursor. The acyl-ketyl primary biradical decarbonylates rapidly k Q > 5 x... 

Avdievich N I and Forbes M D E 1995 Dynamic effects in spin-correlated radical pair theory J modulation and a new look at the phenomenon of alternating line widths in the EPR spectra of flexible biradicals J. Phys. Chem. 99 9660-7... 

Forbes M D E, Avdievich N I, Schulz G R and Ball J D 1996 Chain dynamics cause the disappearance of spin-correlated radical pair polarization in flexible biradicals J. Phys. Chem. 100 13 887-91... 

Two examples from ketone photochemistry that has been recently analyzed within the context of solid-to-solid transformations are the Norrish type and Nor-rish-Yang type Ip44,i45 tactions. In general terms, the type I reaction consists of a homolytic cleavage of bond a-to the carbonyl to generate an acyl-alkyl radical pair (RP-A) or an acyl-alkyl biradical (BR-A) when the ketone is cyclic (Scheme 7.15). 

Time-resolved spectroscopy establishes the formation of an ion-radical pair as the critical reactive intermediate (both from direct excitation of the CT absorption band at 532 nm and from specific excitation of chloranil at 355 nm, see Fig. 3) which undergoes ion-pair collapse to the biradical adduct followed by the ring closure to oxetane, as summarized in Scheme 11. 

The inner-sphere nature of the ion-radical pair may render the biradical (or zwitterionic) intermediate difficult to assess and distinguish from a concerted... 

In photosynthesis radical-ions and triplet states of the pigments, radical-pairs and biradicals involving various chlorophylls and quinones, amino acid radicals, hemes in cytochromes, metal clusters of low and higher nuclearity and even coupled metallo-radical species have been observed. Thus the field of photosyn-... 

Synthetic applications of the reaction are somewhat limited as the highly reactive biradicals and radical pairs tend to undergo reactions that compete with C—C bond formation. As in previous cases, the reaction may have synthetic value for the synthesis of strained structures involving small rings. For example, the preparation of the simplest [2]-ladderane 55 by photodecarbonylation of bicyclo[3.2.0]heptan-3-one 54 gave the bicyclic structure in 5% yield with a ring-opened 1,5-heptadiene being the dominant product (Scheme 2.14) [41]. 

Time-resolved (fs/ps) spectroscopy revealed that the (singlet) ion-radical pair is the primary reaction intermediate and established the electron-transfer pathway for this Paterno-Buchi transformation. The alternative pathway via direct electronic activation of the carbonyl component led to the same oxetane regioisomers in identical ratios. Thus, a common electron-transfer mechanism applies involving quenching of the excited quinone acceptor by the stilbene donor to afford a triplet ion-radical intermediate which appear on the ns/ps time scale. The spin multiplicities of the critical ion-pair intermediates in the two photoactivation paths determine the time scale of the reaction sequences and also the efficiency of the relatively slow ion-pair collapse ( c=108/s) to the 1,4-biradical that ultimately leads to the oxetane product 54. 

The establishment of such an equilibrium should have two effects after a short flash the apparent lifetime of the radical-pair should vary with n (because of the 5ns lifetime, at most, of Chi and of the occurrence of other traps in the antenna), and the amount of radical-pair should decrease while excitation resides in the antenna. To check that hypothesis we have varied n, measuring the amount of biradical state and its lifetime in several PS-II preparations with a different antenna size (n=5 to 200) (Hansson et al., 1987). 

The reaction starts with excitation of the quinone, followed by intersystem crossing and electron transfer from the thiophene to the triplet excited quinone. The ion radical pair collapses to a biradical which loses a chlorine and a hydrogen atom. Yields are high (65-78%) when R1 = halogen and R2 = H, fair (57%) when R1 = R2 = H and poor (2-17%) when R1 = H and R2 = halogen. The regioselectivity has been explained on the basis of calculated electron densities in the cation radicals of thiophenes. 

Afree radical (with just one unpaired electron) is described as an electronic doublet because, in an external magnetic field, the electron can only exist in one of two possible spin states ( up or down ). By contrast, a pair or radicals, or a biradical (a species with two unpaired electrons in the same molecule) can exist in either of two electronic states singlet or triplet. In the singlet state the electrons are paired (opposite spin) a singlet radical pair (or biradical) is thus diamagnetic (W5 = I -1 = 0) and not observable by EPR. The radical pair above is shown in the singlet state. 

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