Biradicals spin-separated

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... 

One can note some interesting features from these trajectories. For example, the Mulliken population on the participating atoms in Figure 1 show that the departing deuterium canies a full electron. Also, the deuterium transferred to the NHj undergoes an initial substantial bond stretch with the up spin and down spin populations separating so that the system temporarily looks like a biradical before it settles into a normal closed-shell behavior. 

As seen in the previous section, one characteristic of the triplet state is its paramagnetism. This alone would of course not suffice as a definition of the triplet since there are many odd-electron species that also exhibit paramagnetism but do not exist as triplets. Thus we might state that a triplet is a paramagnetic even-electron species. This still does not constitute a limiting definition since compounds containing even numbers of electrons may exhibit two, three, or even five distinct electronic levels. For example, when in a biradical the radical centers are separated by several carbon atoms as below, no interaction between the electron spins occurs and the radicals appear as two doublet states ... 

A number of diradicals (also called biradicals) are known.186 When the unpaired electrons of a diradical are widely separated, e.g., as in CHjCHjCHjC, the species behaves spectrally like two doublets. When they are close enough for interaction or can interact through an unsaturated system (as in trimethylenemethane,187 they can have total spin numbers of + 1, 0, or -1, since each electron could be either + or -i. Spectroscopically... 

Second, there is usually no escape channel because the two radical termini cannot separate completely. This usually causes a fundamental difference between CIDNP from radical pairs and CIDNP from biradicals Being nuclear spin sorting as described above, intersystem crossing between IS) and ITq) crucially relies on both exit channels leading to different products, whereas intersystem crossing between IS) and IT i) occurs by simultaneous electron-nuclear spin flips, and so creates net nuclear polarizations without the need of different exit channels. 

Of particular relevance to the present discussion is the observation that the CSS, which is a biradical cation, is formed with essentially pure triplet spin correlation. For energetic reasons, this triplet radical pair cannot recombine to form the MLCT state and can only form the singlet ground state. Therefore, direct recombination is spin forbidden. Moreover, because the radical pair which constitute the CSS product can separate only to a limited distance, essentially every CSS recombination event is between the same geminate radical pair—in other words, every reduced acceptor is ultimately oxidized by the donor radical cation that was formed from the same initial photochemical event. The spin behavior of the DC A triad CSS can be effectively explained by application of the relaxation mechanism of Hayashi and Nagakura. ... 

A number of diradicals (also called biradicals) are known,and the thermodynamic stability of diradicals has been examined. Orbital phase theory has been applied to the development of a theoretical model of localized 1,3-diradicals, and used to predict the substitution effects on the spin preference and S-T gaps, and to design stable localized carbon-centered 1,3-diradicals. When the unpaired electrons of a diradical are widely separated, for example, as in CH2CH2CH2CH2, ... 

In the absence of magnetic field, the three triplet levels are separated in energy by the zero field splitting (roughly 10 -I0" cm" in short biradicals) and are quantized with respect to the molecular axes dictated by the principal directions of the electron spin-spin dipolar coupling tensor primarily responsible for the zero-field splitting. (Spin-orbit coupling affects the... 

The nature of the e.s.r. spectrum of a species which contains two unpaired electrons depends upon the extent to which these electrons interact. If they are considerably separated and behave independently, the spectrum is effectively a superposition of the spectra of the individual radicals the species is termed a biradical. If, however, there is a significant interelectronic interaction, the species is termed a triplet and gives a markedly different type of spectrum whose structure can be understood from the following considerations. The two unpaired electrons correspond to an effective total spin of imity and therefore to the three spin-states, m, = 4-1, 0, —1. Whereas for a single unpaired electron the energies of the two available spin-states are the same in the absence of a... 

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