Anion spin densities

The EPR spectra of semidione radical anions can provide information on the spin density at the individual atoms. "The semidione derived from butane-2,3-dione, for example, has a spin density of 0.22 at each oxygen and 0.23 at each carbonyl carbon. The small amount of remaining spin density is associated with the methyl groups. This extensive delocalization is consistent with the resonance picture of the semidione radical anion. 

Analyze the hyperfine coupling in the spectrum of the butadiene radical anion given in Fig. 12.PI I. What is the spin density at each carbon atom according to the McConnell equation ... 

Repeat your analysis for localized and delocalized allyl radical and allyl anion. Focus on location of the spin density in the former and on the negative charge in the latter. 

Repeat your analysis fox phenoxy radical. Instead of charge, focus on the spin density. Calculate the delocalization energy using phenoxy radical at phenol geometry. Is it of the same order of magnitude as that for phenoxy anion Explain. 

Spin density surface for trans radical anion shows location of unpaired electron. 

Examine the electrostatic potential map and spin density surface of Q radical anion (Q ). Draw all of the resonance contributors needed to account for these data. Examine the CO bond distances and spin density surface of QH radical (QH ). Draw all of the resonance contributors needed to account for these data. 

Tile low-temperature ESR spectrum of the anion radical of purine disclosed that about 45% of the spin density is localized at position 6 (80BCJ1252), although a single very broad signal for N(7) and N(9) did not allow discussion of the tautomerism. 

Tile ESR spectra of the radical anions, generated by one-electron reduction of the a-oxothioketone 173 and the dithiete 172, were determined, and spin densities were calculated from the coupling constants and, especially, from the anisotropic values (87CB575). 

Irradiation of the molecular radical anion of DESO, which has a yellow color, with light of X = 350-400 nm partially restores the red color and the ESR spectrum of the radical-anion pair. Similarly to the case of DMSO-d6 a comparison of the energetics of the photodissociation of the radical anion and dissociative capture of an electron by a DESO molecule permits an estimation of the energy of the hot electrons which form the radical-anion pair of DESO. This energy is equal to 2eV, similarly to DMSO-d6. The spin density on the ethyl radical in the radical-anion pair of DESO can be estimated from the decrease in hfs in comparison with the free radical to be 0.81, smaller than DMSO-d6. 

Next, we discuss the symmetries, bond lengths and spin densities for the anion and cation radicals of fulvalene (XXI) and heptafulvalene (XXIII) using the dynamic theory. We use the semiempirical open-shell SCF MO formalism in conjunction with the variable bond-length technique. 

The calculated bond lengths for the 2 structure of the anion radical of heptafulvalene shown in Fig. 7 indicate that in one of the ring there exists a significant bond fixation to the same extent as that in the neutral heptafulvalene, while in the other ring bond lengths are nearly equalized. The calculated spin densities, presented in Table 3, indicate that the unpaired spin is localized essentially on the latter ring. 

E.s.r. showed that, X. ray irradiation of tetraalkyldiphosphine diphosphides gave phosphoranyl radicals with t.b.p. structures (39).114 A structure has been assigned to phosphiny1hydrazy1s (40). The dimethy1 ami no radical was particularly persistent.115 The e.s.r. parameters of the electrogenerated pyrazine radical cations (41) have been recorded.116 The spectra of a stable furanyl phosphate radical adduct117 and a phenalene radical anion which involves injection of spin density into half an attached cyclophosphazene ring,11 are reported. 

Such 7r-bonding is evident, however, in the ground state of triarylborane radical-anions. The Na+ or K+ salt of [Ph3B] (43) has the free spin densities shown in 43a. Since fully 75% of the free spin is delocalized onto the phenyl groups, B—C ir-overlap as expressed by structure 43b is undoubtedly significant.40... 

Table 5-7. Fe—X distances (A) and Fe spin densities for different states and anions of 11 V S (SCI F >41...

MO) with the protons in the nodal plane. The mechanism of coupling (discussed below) requires contact between the unpaired electron and the proton, an apparent impossibility for n electrons that have a nodal plane at the position of an attached proton. A third, pleasant, surprise was the ratio of the magnitudes of the two couplings, 5.01 G/1.79 G = 2.80. This ratio is remarkably close to the ratio of spin densities at the a and (3 positions, 2.62, predicted by simple Hiickel MO theory for an electron placed in the lowest unoccupied MO (LUMO) of naphthalene (see Table 2.1). This result led to Hiickel MO theory being used extensively in the semi-quantitative interpretation of ESR spectra of aromatic hydrocarbon anion and cation radicals. 

Table 2.1 Hyperfine parameters and spin densities for aromatic radical anions. (Data from ref. 11.)...

When ESR spectra were obtained for the benzene anion radical, [C6II6] and the methyl radical, CH3, the proton hyperfine coupling constants were found to be 3.75 and 23.0 G, respectively, i.e. they differ by about a factor of 6. Since the carbon atom of CH3 has a spin density corresponding to one unpaired electron and the benzene anion carries an electron spin density of 1/6, the two results suggest that the proton coupling to an electron in a n-orbital is proportional to the spin density on the adjacent carbon atom ... 

For aromatic hydrocarbon radical anions, this approach works pretty well. Figure 2.7 shows a correlation plot of observed hyperfine splitting versus the spin density calculated from Hiickel MO theory. It also correctly predicts the negative sign of aH for protons attached to n systems. 

Table 4.5 E Electron spin densities in cobalt(o) radical anions...

---