Unpaired electron density

The EPR spectrum of the ethyl radical presented in Fig. 12.2b is readily interpreted, and the results are relevant to the distribution of unpaired electron density in the molecule. The 12-line spectrum is a triplet of quartets resulting from unequal coupling of the electron spin to the a and P protons. The two coupling constants are = 22.38 G and Op — 26.87 G and imply extensive delocalization of spin density through the a bonds Note that EPR spectra, unlike NMR and IR spectra, are displayed as the derivative of absorption rather than as absorption. 

Esr spectroscopy has proved useful for characterizing the reduced species, enabling assessment of whether the unpaired electron density is localized on the metal or delocalized onto the ligand. A typical study is the reported electrochemical reduction of the Ni(n) complex of (289) (Bailey, Bereman, Rillema Nowak, 1984) a reversible one-electron reduction occurs to yield a product whose esr spectrum shows two g values which are characteristic of a Ni(i) derivative. In contrast, the reduction product formally represented by (290) has been shown to have extensive delocalization onto the ligand it is probably best described as involving coordination of afree radical to a central Ni(n) (Lovecchio, Gore Busch, 1974). 

Electron paramagnetic resonance (EPR) yields the location of unpaired electron density from hyperfine splitting by metals or atoms with nuclear spin.21 The S = 0 Fe(III)—O 2 state of oxy-Mb or Hb would be indicated by the absence of an EPR signal, although other results such as the IR or resonance Raman absorption of the O2 moiety would be needed for positive confirmation. 

EPR. The EPR of the Cu complex 62c has been reported as a 1% powder sample at 77 K and is given in Table XI (Section IV.A), which compares the EPR data for 62c to Cu[pz(SMe)8] (48) (Scheme 9), Cu(TPP) and Cu(pc). The spectrum is typical of a monomeric square-planar copper with axial symmetry. The EPR spectrum for 62c closely matches that of 48 implying that although the peripheral tridentate coordination geometry has an effect on the ir-clcctronic structure of the pz ring it does not effect the electronic properties of the central Cu2+ ion, whose unpaired electron density is in a a orbital. 

Table 1.1 shows that the nature of the a-substituent in the radical centre enormously influences the Si hfs constants. These constants, which can be used as a guide to the distribution of unpaired electron density, were initially correlated to changes in geometry at the radical centre by analogy with C hfs constants of a-substituted alkyl radicals. Indeed, it was suggested that by... 

Low-lying vacant orbitals of alkali metal cations can, consequently, accept an unpaired electron density even if it is delocalized over an extended n system of carbon chains. The anion-radical of 1,4-diphenylbutadiene can exist in i-trans and in -cis forms. The relative amounts of these geometrical isomers appear to depend highly on the counterion/solvent system. Li and K+ were studied as counterions THF, 2-MeTHF, and DME were employed as solvents (Schenk et al. 1991). Interaction between the anion-radical and the cation contributes to a stabilization of... 

To disrupt the carbon-chloride bond at position 5 of the substrate anion-radical, population of this bond with an unpaired electron should be increased. However, if a spin density at carboncarrying chlorine is too great, the initial chlorine-containing anion-radicals dimerize instead of cleaving the chloride ion. Thus, in the isomeric 6-chloro-27/,3//-benzo[b]thiophenedione-2,3 anion-radical, unpaired electron density at C-6 is five times greater than at C-5, and the dimerization proceeds much more rapidly than the cleavage of carbon-chlorine bond (Alberti et al. 1981). 

To elevate p-selectivity in nitration of toluene is another important task. Commercial production of p-nitrotoluene up to now leads with twofold amount to the unwanted o-isomer. This stems from the statistical percentage of o m p nitration (63 3 34). Delaude et al. (1993) enumerate such a relative distribution of the unpaired electron densities in the toluene cation-radical—ipso 1/3, ortho 1/12, meta 1/12, and para 1/3. As seen, the para position is the one favored for nitration by the attack of NO (or NO2 ) radical. A procednre was described (Delande et al. 1993) that used montmorillonite clay supported copper (cupric) nitrate (claycop) in the presence of acetic anhydride (to remove excess humidity) and with carbon tetrachloride as a medinm, at room temperature. Nitrotoluene was isolated almost quantitatively with 23 1 76 ratio of ortho/meta/para mononitrotoluene. 

As a rule, if the unpaired electron density in the anion-radical is redistributed, the rotation barrier decreases. Thus, the barrier of the phenyl rotation in the benzaldehyde anion-radical is equal to 92 kJ mol", whereas in the 4-nitrobenzaldehyde anion-radical, the barrier decreases to 35 kJ mor (Branca and Gamba 1983). Ion-pair formation enforces the reflux of the unpaired electron from the carbonyl center to the nitro group. Being enriched with spin density, the nitro group coordinates the alkali metal cation and fixes the unpaired electron to a greater degree. The electron moves away from the rotation center. The rotation barrier decreases. The effect was revealed for the anion-radical of 4-nitrobenzophenone and its ionic pairs with lithium, sodium, potassium, and cesium (Branca and Gamba 1983 Scheme 6.19). 

Calculations indicate that the unpaired electron density in the cation-radical of tetrathiafulvalene resides principally on sulfur, hut with the internal carbon being the site of second highest density. The product of coupling of an a-carbonyl radical to sulfur, an a-carbonyl-sulfonium salt would be destabilized by the adjacent dipoles. The transition state would be expected to mirror this, thus slowing down the C-S coupling and permitting the observed coupling to the carbon of tetrathiafulvalene. 

The isotropic shift. The isotropic shift is the sum of two contributions the contact and the dipolar contributions. The former is due to the presence of unpaired electron density on the resonating nucleus. The latter arises from the anisotropy of the magnetic susceptibility tensor, modulated by the distance between the unpaired electron and the resonating nucleus, and is also dependent on the orientation of the metal nucleus vector with respect to the principal axes of the magnetic susceptibility tensor. Some problems arise when the spin delocalization is taken into account in calculating the dipolar coupling, but we will not address this problem except when strictly necessary. 

There is a relationship (the McConnell equation) between the unpaired electron density on a given atom and the polarization interaction with each attached proton ... 

The introduction of modest rhombic terms (E < < D) mixes the doublets because of the off-diagonal matrix elements in E. This causes g and gy to separate second-order effects slightly reduce the average value of g, and gy and cause g to fall below 2. Unlike the S = f system discussed earlier, S = states are not orbital singlets, and there is the possibility of angular momentum contributions from admixture of d orbitals. This effect will be stronger if the separation between orbitals is small and if the unpaired electron density is localized on the metal... 

The radical cation of 9,10-diphenylanthracene is much more stable than that of anthracene, because, with 9,10-diphenylanthracene, the 9- and 10-positions, which are reactive because of the high unpaired-electron densities, are masked by phenyl groups and the unpaired electrons are delocalized. The stabilization of the radical cation of anthracene also occurs by introducing other substituents like -NH2 and -OCH3 in the 9,10 positions. 

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