Spin correlation effect

The discovery that azo compounds undergo singlet sensitized decomposition is particularly relevant to the problem of spin correlation effects in free radical reactions. Any radical pair precursor that gives a difference in products depending upon whether it is produced as a singlet or triplet excited state is said to show a spin correlation effect. 

No spin correlation effect was seen in photolysis of azocumene,ls9b perfluoroazomethane,196 or in azomethane itself.160,193 These results are again explained by the lack of any real triplet sensitized decomposition. 

The behavior of cyclic azo compounds differs considerably from that of the acyclic ones in that spin correlation effects have been seen in several cases. Since singlet decomposition gives a different product distribution than triplet sensitization, these cases provide an elegant test of the sensitization mechanism. 

Another cyclic azo compound which shows a spin correlation effect is the pyrazoline 44201 (Fig. 6). The product distribution under various conditions... 

The concentration dependence of the photocycloaddition of cis- and trans-cyclo-cocetene 136 with aliphatic aldehydes was studied in order to test the spin-selectivity with respect to the cis/trans oxetane ratio and/or the eftdo/exo-selectivity relevant for the cis-photoadduct [156]. The results indicate a moderate but still significant spin correlation effect in the Paterno-Biichi reaction of cyclo-octene with aliphatic aldehydes. The exo-diastereoisomers were formed with similar probability as the e do-diastereo-isomers in the singlet carbonyl manifold, whereas the triplet excited aldehydes preferred the formation of the e Jo-diastereoisomer and trans fused products. 

Under these conditions photochemical decomposition from the singlet or the triplet state of azo compounds should lead to a different product ratio. This phenomenon is called the spin correlation effect. The simple scheme representing the photodecomposition of azo compounds (see above) has to be extended further if the complete mechanism has to be dealt with. 

The results for 46—50 clearly show the spin correlation effect for the different electronic states involved. Shorter wavelengths of irradiation can enhance the spin correlation effect in pyrazolines 67>. 

A clear spin correlation effect is observed for the pyrazolines 4a and 4b 68,69) ... 

For the 6-membered ring azo compound 52 spin correlation effect is operative, too 7°). 

In the singlet reaction exo-86 and endo-56 are produced with predominant retention of configuration. The triplet reaction shows an almost identical product distribution starting from exo-85 or endo-85. One-bond scission and slower rate of ring closure of the triplet biradical explain these facts sufficiently well. A similar spin correlation effect was found for the hetero analog of 85 89>. 

The 5-methoxy derivatives of 3b or 3c show a clear spin correlation effect. The singlet reaction gives different product ratios for exo- and endo-54, whereas the triplet reaction gives the same ratio 79/21 for both stereoisomers. From this result an equilibrated pyramidal or planar diradical was inferred as an intermediate 9°). 

A spin correlation effect is found also for the pentacyclic system 87 93>. 

According to Scheme 5 cis and trans products are formed direct photolysis produces I8a and 18b in a ratio 10 1, whereas triplet sensitization yields a product ratio of 1.4 1, which is nearly equal to what would be expected for an equilibrium distribution between the different rotameric biradicals. A biradical such as this, which gives different products depending upon its multiplicity, is said to exhibit a spin-correlation effect. 

Bergman also investigated the spin state of the diradical species 2 produced during the thermolysis of enediynes. On the basis of chemically induced dynamic nuclear polarization (CIDNP) studies, and trapping experiments, which used the spin correlation effect (SCE), it was deduced that most products (including substituted arenes 3) formed from 2 arise from the singlet state of the diradical species. ... 

Prom the discussion of the results obtained it follows that the spin-correlation effects are described in an essentially different manner in HF and DFT calculations for Ti2 0s crystal and as a result give different descriptions of Ti-Ti interaction in the HF method, this interaction is weak (for energetically more favorable antiferromagnetic ordering) in the DFT method, the ferromagnetic ordering appears to be more favorable and Ti-Ti interaction remains essential. 

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