Cation spin distribution

The obtained data clearly show that the g-anisotropy of the triplet states is larger than that of the respective cation-radical. A similar effect has been observed for the triplet states of the primary donors in PS II231 and in the bacterial RC.111112114 This can be explained by the fact that the triplet electrons probe the spin distribution in two different orbitals (HOMO and LUMO), and the latter has a rather large spin density at the nitrogens and the central magnesium (cf. references 216, 218), by which the spin-orbit coupling and the g-anisotropy is increased. 

The stable chloride salt of 92 (R1 = R2 = H) was unexpectedly obtained on treatment of phenazine with the ion UOC12" in moist nitromethane. Similarly prepared were cation-radical salts of benzophenazines and 2,3-bis(2-pyridyl)quinoxaline.320,321 A kinetic study of the relative reactivity of 92(R = R2 = H) and the fully reduced 9,10-dihydro phenazine in electron transfer to an azo compound showed the cation-radical is the less reactive.322 Soviet workers have examined the consequence of ring substitution for the spin distribution in a series of 9,10-dihydrophenaziniumyl radicals.323... 

Anodic oxidation of 4-dimethylaminoantipyrene in acetonitrile has yielded a cation-radical (165), the ESR spectrum of which indicates the spin population is mainly exocyclic on the tertiary amino group. The second-order rate constant for the decay of 165 was determined HMO calculation of the spin distribution indicates twist in the C—NMe2 bond.551... 

Further investigation by the Adam group [140] showed electronic substituent effects on the diyl sites profoundly influences the regioselectivities of the 1,2 shift. The data obtained illustrate the distinct electronic character of the cationic intermediates involved in the electron transfer oxidation vs acid catalysis of the housanes. The radical anions and cations of dipleiadiene 64 and its 12b,12c-homo derivatives 65 are characterized by EPR and ENDOR spectroscopy [141]. The 7i-spin distribution over the perimeter is similar in the radical cations 64 +, 65, and an analogous statement holds for the corresponding radical anions. 

These radicals are unusual in possessing no protons the only hyperfine structure, therefore, in their ESR spectra is due to the low-abundance isotopes C, and As expected intuitively, the spin population is largely localized in the dithiin moiety in the cation-radical and in the thio-phenedione moieties in the anion-radical. The authors were able to make a detailed analysis of the ESR spectra including the signs of hyperfine splittings and to show the spin distributions to accord with the predictions of a simple MO model. 

The same group has investigated 10-phenylphenoxazine (263) and -phenothiazine (264) cation-radicals and the influence of substituents in the phenyl group upon the spin distribution. The interest here lay in the effect of the conformation on electronic effects. Earlier studies on similarly shaped radicals had indicated that a twist of about 65° must exist in the V-phenyl bond (cf. phenylxanthenyl and isologs Sections II,B,3,a III,C,1 IV,B,1). This value may be calculated trigonometrically for a model of such radicals which has planar aromatic components, each C—C and C—H bond equal to those in benzene, and in which the phenyl ortho and heterocyclic 1- and 9-protons are at van der Waals separation. A rather similar angle has to be allowed in MO calculations in order correctly to reproduce experimental hyperfine splittings. 

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