Delocalized density

The increase of the exocyclic C—C bond stretching frequency from 1208 cm in toluene to 1264 cm in the benzyl radical and the simultaneous decrease of the C—C ring bond stretching frequencies (from 1494 and 1460cm to 1469 and 1446cm , respectively) result from electron density delocalization in the benzyl system. Furthermore, the force constant value for the C—C bond in the C6H5CH2 radical (5.5 X 10 N m ) is between the values for the ordinary C—C bond (4.5 x 10 N m ) and the double C=C bond (9.0 X 10 N m ) and is close to the corresponding force constant in the allyl radical (5.8 x 10 N m ). 

Contrary to naive expectation, an extended ir-electron system snch as that in the original silvery polyacetylene film does not imply perfect bond conjngation or perfectly like bonds, or conduction along the chain It only implies a degree of charge-density delocalization. Such a material has the electronic structure of a... 

It is well known that delocalized stable radicals may have potential for the construction of solid state conducting materials. The phenylalenyl radical 40 has been considered a good candidate with its spin density delocalized over 13 carbons in its jt-conjugated system. Unfortunately, 40 exists in equilibrium with its dimer and it decomposes at modest temperatures. To overcome the dimerization problem, Goto et al. and Koutentis et al. synthesized substituted radicals 41 and 42. 

We are therefore faced here with radical complexes which easily distort depending on the structural arrangement and whose SOMO is different for every crystal structure associated with a given counter-ion, a very original feature in these series. The unfolded d1 complexes can be described as Mo(IV) complexes with a spin density essentially localized on the dithiolene ligand while the more folded complexes have a stronger metal character. This variable spin density delocalization is expected to influence strongly the amplitude and dimensionality of intermolecular interactions between radical species in the solid state, as detailed below in Sect. 3. 

From a comparative analysis of H NMR spectra of structurally similar pairs of nitroxyl radicals of 3-imidazoline and 3-imidazoline-3-oxide, it was concluded that the nitrone group contributes to a more efficient long-range spin density delocalization in the conjugated n -system of functional groups bonded with atom C-4 (404). 

Table II reports the contact coupling constant for different aqua ion systems at room temperature. The contact coupling constant is a measure of the unpaired spin density delocalized at the coordinated protons. The values were calculated from the analysis of the contact contribution to the paramagnetic enhancements of relaxation rates in all cases where the correlation time for dipolar relaxation is dominated by x and Tig > x. In fact, in such cases the dispersion due to contact relaxation occurs earlier in frequency than the dispersion due to dipolar relaxation. In metalloproteins the contact contribution is usually negligible, even for metal ions characterized by a large contact contribution in aqua ion systems. This is due to the fact that the dipolar contribution is much larger because the correlation time increases by orders of magnitude, and x becomes longer than Tig. Under...

It is also often assumed that A is constant along a series of lanthanide complexes with the same ligand. The mechanism providing spin density delocalization from the metal to the ligands is due to a weak covalent bonding involving the 6s metal orbital, which, in turn, can transfer unpaired spin density on nearby nuclei through spin polarization from 4f orbitals [82]. 

As anticipated in Sections 2.2.2 and 3.1, the unpaired electrons should not be considered as point-dipoles centered on the metal ion. They are at the least delocalized over the atomic orbitals of the metal ion itself. The effect of the deviation from the point-dipole approximation under these conditions is estimated to be negligible for nuclei already 3-4 A away [31]. Electron delocalization onto the ligands, however, may heavily affect the overall relaxation phenomena. In this case the experimental Rm may be higher than expected, and the ratios between the Rim values of different nuclei does not follow the sixth power of the ratios between metal to nucleus distances. In the case of hexaaqua metal complexes the point-dipole approximation provides shorter distances than observed in the solid state (Table 3.2) for both H and 170. This implies spin density delocalization on the oxygen atom. Ab initio calculations of R m have been performed for both H and 170 nuclei in a series of hexaaqua complexes (Table 3.2). The calculated metal nucleus distances in the assumption of a purely metal-centered dipolar relaxation mechanism are sizably smaller than the crystallographic values for 170, and the difference dramatically increases from 3d5 to 3d9 metal ions [32]. The differences for protons are quite smaller [32]. 

R m calculations in the presence of ligand-centered contributions are possible for metal complexes with ligands having dominant ti spin density delocalization mechanisms. With certain approximations, the relaxation rates of protons and carbon atoms in sp2 CH moieties can be expressed [33-35] as the sum of a... 

Figure II 2 9a-s. The valence electron iso-density lines in the plane of B atoms (a-b plane) for equilibrium (a) and distorted structures (b-e). The electron density is localized at B atom positions for equilibrium structure (a). The B atoms displacements ( Af = 0.005) induce the alternating interatomic charge density delocalization, different for the particular types of the distortion (b-d). Nuclear microcirculation enables then effective charge transfer over the lattice in an external electric potential. The Fig (e) corresponds to the case of the distortion (d) over the larger lattice segment...

The X-ray and neutron diffraction data mentioned previously have been used in conjunction with the technique of polarized neutron diffraction (at 4.2 K) to deduce spin-density distributions in [MnPc].519,520 In further investigations514 it proved possible to determine individual 3d and 4s orbital populations on the manganese ion together with an estimate of 24% for the d-orbital electron density delocalized in the macrocyclic ring. From these studies it appears that the charge on the manganese is approximately +1 this charge appears to be achieved primarily by the loss of a 3d electron rather than a 4s electron. 

As the axial ligand is weakly bound in BCP (Randall et al., 2000), the spin density delocalized on it is small. Indeed, in azurin the resonances of the axial methionine protons do not experience a significant hyperfine shift contribution. Electron delocalization onto a Hy of the axial Met has been detected in plastocyanin (signal F in Fig. 3B), suggesting some covalency for the Cu-S(Met) bond. The absence of spin density on the axial Gin ligand in stellacyanin has been attributed to the fact that the y-CH2 Gin geminal couple is four bonds away from the metal ion, whereas the equivalent protons in a bound Met residue (such as in plastocyanin) are only three bonds away (Bertini et al., 2000). 

Reduction by electron transfer yields superoxide ion (02 -)> which has its negative charge and electronic spin density delocalized between the two oxygens. As such, it has limited radical character (H-OO bond energy AGbf, 72kcalmoU ) and is a weak Bronsted base in water... 

According to the BSR theory polyconjugatcd systems are characterized by the degree of spin density delocalization, which can be estimated on the basis of the ESR line width [6], The parameter n corresponding to the number of equivalent protons interacting with an unpaired electron, can be estimated from the following expression ... 

Native lignin arises via an oxidative coupling of the aforementioned alcohols with each other and (more important) with a growing polymer end unit. The oxidation produces a phenolic radical with unpaired electron density delocalized to positions... 

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