Polymethylene biradicals

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... 

Forbes M D E, Closs G L, Calle P and Gautam P 1993 The temperature dependence of the exchange coupling in polymethylene biradicals. Conclusions regarding the mechanism of the coupling J. Phys. Chem. 97 3384-9... 

Gloss, G. L., Forbes, M. D. E. and Piotrowiak, R, Spin and Reaction Dynamics in Flexible Polymethylene Biradicals As Studied by EPR, NMR and Optical Spectroscopy and Magnetic Field Effects. Measurements and Mechanisms of Scalar Electron Spin-Spin Coupling,/. Am. Chem. Soc., 114, 3285, 1992. 

This scheme is quite similar to that given for radical pairs by reactions (6-21a)-(6-21f), but there is no escape of component radicals in the case of biradicals. For each polymethylene-linked compound with A=pyrene and D=dimethylaniline in acetonitrile at room temperature, the group measured the intensity (Et(B)) of the transition absorption due to A. Their typical results are shown in Fig. 8-4. 

Field-dependent CIDNP experiments in low magnetic fields reveal that CIDNP is of the comparatively rare S-T+i-type. The curves exhibit the bell-shaped behaviour t /pical for intersystem crossing at a level intersection, which has long been known for biradicals with a polymethylene chain. However, the rigidity of the dyads removes a complication present in flexible biradicals, namely, that the CIDNP experiment weighs certain conformations and distances between the radical termini more heavily than others.Hence, in the dyad experiments the value Bq at which the extremum of the field-dependence occurs corresponds directly to the condition y/lBo=2 /. An exponential dependence of / on the spacer length n was found with a damping factor of about 0.75A, which is comparable to that in other biradical systems. 

Azumi et al. carried out variable-field CIDNP experiments on the biradicals resulting from intramolecular hydrogen abstractions in polymethylene-linked xanthone and xanthene moieties [61a, 61b]. They also studied the temperature dependence of the mean exchange interaction for... 

The photochemical behavior of the orr/to-(aminoalkyl)slil-benes 92—94 is also dependent upon the polymethylene linker (Scheme II). Irradiation of 93 and 94 results in formation of the benzazepines 96 and 97. These adducts are presumably formed by regioselective N—H transfer to the proximal end of the stilbene double bond in 93 and the distal end in 94, in both cases resulting in the formation of a 1,7-biradical intermediate. Since intramolecular stilbene-amine addition reactions are non-regioselective, the regioselectivily of N-H transfer must be subject to exciplex conformational control. The biradical inter-... 

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