Long-lived spin isomerism

Long-Lived (Persistent) Spin isomerism. The singlet of A-tosyl-3,4-dimethylenepyrrole (46f, Scheme 5.10) is a blue, ESR-silent species that is stable over many days in cold matrices. The triplet is also stable for days at low temperatures. This type of long-lived spin isomerism has been attributed... 

This indicates that in these regions large spin differences may occur between levels, and long-lived isomeric states are expected. Indeed, many long-lived excited (isomeric) states were observed in these regions. An extensive list of isomeric states (with T n > 10 ° s) can be found in the... 

For both of the last two classes of isomerism we have detailed—fluxional and spin isomerism—the lifetime of individual isomers may be rather short. Spin isomers, for instance, typically live for about 10 s (but see Section 3.4.1 and the Further Reading at the end of this chapter). Some would argue that classical isomerism refers only to species capable of physical separation, and so of long lifetime. However, with the increasing use of methods which explore short lifetimes—NMR and EPR, particularly, in the present context—it seems sensible to ignore this limitation. 

Cis-trans isomerization of cyclic azo compounds is only possible for six-membered or larger rings. Otherwise only N2 loss is observed, and direct irradiation and triplet sensitization can yield different products, as is the case for the cyclic azo compound 17 (Bartlett and Porter, 1968). This is due to the fact that the biradical R Rformed from the triplet-excited reactant cannot recombine to form a cyclobutane until spin inversion has occurred the triplet biradical can live long enough to be diverted to the rotamer... 

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