Radical cations spin populations

Methyl tricyclo[4.1.0.0 ]heptane-l-carboxylate gives a cation-radical in which the spin density is almost completely localized on C-1 while the positive charge is on C-7. The revealed structural feature of the intermediate cation-radical fairly explains the regioselectivity of N,N-dichlorobenzenesulfonamide addition to the molecular precursor of this cation-radical. In the reaction mentioned, the nucleophilic nitrogen atom of the reactant adds to electrophilic C-7, and the chlorine radical attacks C-1 whose spin population is maximal (Zverev and Vasin 1998, 2000). 

Both theory and experiment point to an almost perpendicular orientation of the two butadiene H2C=C(t-Bu) moieties (see Scheme 3.53). On passing from the neutral molecule to its anion-radical, this orthogonal orientation should flatten because the LUMO of 1,3-butadiene is bonding between C-2 and C-3. Therefore, C2-C3 bond should be considerably strengthened after the anion-radical formation. The anion-radical will acquire the cisoidal conformation. This conformation places two bulky tert-butyl substituents on one side of the molecule, so that the alkali metal counterion (M+) can approach the anion-radical from the other side. In this case, the cation will detain spin density in the localized part of the molecular skeleton. A direct transfer of the spin population from the SOMO of the anion-radical into the alkali cation has been proven (Gerson et al. 1998). 

A simple model which correlates the HOMO and LUMO of cycl-[3,3,3]azine with the degenerate nonbonding orbitals of [12]annulene accounts for the ESR spectra of the radical cations and anions of 2a and some derivatives. This model also predicts the almost identical rc-spin population observed for the 2a-cation and the isoelectronic phenalenyl... 

The electron spin resonance (E.S.R.) spectra of a paramagnetic organic molecule, e.g. free radical, radical cation or radical anion, is directly related to its unpaired electron distribution (spin density). In the region of a magnetic nucleus the hyperfine interaction between the magnetic moments of the nucleus and the electron is a function of the spin density. It has been shown that, for an atom N, a direct correlation exists between its observed hyperfine coupling constant, and [pa—pP), the unpaired electron population of its atomic orbitals 1). 

However, QMe (or B) may vary with type of radical, especially charge type, and also with the nature of the a-atom. Thus, methyl hyperfine splittings in alternant hydrocarbon anion- and cation-radicals, for which Hiickel MO theory predicts identical spin populations at the methylated positions, typically differ by a factor of at least two.15... 

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... 

From the ESR spectrum, the spin density q for the radical cation (76) is obtained as 0.25, which is very close to the q value (0.28) of the hexamethyl (3) radialene radical anion. Although the effects of bonding and charge must be considered, the n spin population of the two radical ions is not expected to differ greatly in view of the similarity of the HMO energy diagrams of both radical ions.i >... 

The structures of compounds 1 and 2 as well as their radical cations were geometry-optimized at the PM3 level. The spin density distribution in the radical cations was shown to be delocalized over the entire molecules. For 2" this result is in agreement with ESR measurements, while in contrast, electron spin resonance (ESR) measurements showed the spin population to be higher on the sulfur atoms in 1... 

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. 

Cation-radicals of selenanthrene (196) are the only radicals in this category to have been observed. The ESR spectrum of the cation-radical of 196 itself, in concentrated sulfuric acid, shows no resolved proton hyperfine structure, which betokens a high spin population on Se. ° Various MO calculations bear this... 

Hanson and co-workers have studied substituent effects on the distribution of spin in cation, neutral, and nitroxyl radicals of phenothiazine. The substituent effects were described quantitatively using Taft s (Tj and various ctr values, and are understandable in terms of a redistribution of spin population principally between N and S in the cation-radicals and phenothiazinyls, and between N and O in the nitroxyls. 

Figure 3.10. Change in spin population with time of cure of the TGDDM epoxy resin MY720 cured with 26.5% DDS at 160 °C. Brief UV irradiation is used to create the radical cation species analysed. Reproduced with permission from St John (1993).

The McConnell equation (Equation 1.18) expresses the relationship of the observed coupling constant (an) to the unpaired spin population on the adjacent carbon atom (pc) The constant Q is different from one situation to another, but when an electron in a pz orbital on a trigonal carbon atom couples to an adjacent hydrogen, it is about -24 G. Applied to aromatic hydrocarbons, where it is particularly easy to generate radical cations and anions, there proves to be a good correlation between coupling constants and the calculated coefficients in the HOMO and LUMO, respectively.62... 

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