Direct delocalization

The spin densities deduced from the formula aftft, = ( A1 + 2 A )/3 are presumed to be positive because of direct delocalization of the unpaired electron from the (a3da + b4s) hybrid orbital. It should be noted, however, that Aft1 and A , measured for rotating rings at T = 150 K, are not principal values of the proton hfs tensor, i.e. a , determined in this way does not exactly correspond to the isotropic coupling obtained by the common formula aftft, = (A" + A" + A )/3. 

The different shift mechanisms may be understood in more detail by considering the effect of the magnetic field on the populations and energies of the different crystal orbitals (Figure 7a). Transfer of electron density via the 90° interaction arises due to a direct delocalization of spin density due to overlap between the half-filled tzg. oxygen jt, and empty Li 2s atomic orbitals (the delocalization mechanism. Figure 7b).This overlap is responsible for the formation of the tzg (antibonding) molecular orbital in a molecule or the tzg crystal orbital (or band) in a solid. No shift occurs for the 180° interaction from this mechanism as the eg orbitals are empty. 

For 19F Fermi contact shift of fluorine bound to sp2 carbon atoms, spin density on the nucleus arises from spin polarization by as for analogous CC moieties, and from spin polarization by /Op, which occurs via direct delocalization through C—F ji bonding (Fig. 2.18). The hyperfine coupling is therefore... 

Fig. 2.18. Spin polarization (through a bonding) and direct delocalization (through jt bonding) mechanisms for unpaired spin density transfer from an sp2 carbon to a fluorine nucleus.

The direct (delocalization) contribution, which is always positive and derives from the spin density at the nucleus due to the orbital nominally containing the unpaired electron. 

Aromatic substituents noticeably affect reaction thermochemistry only when such groups either directly delocalize the odd-electron or lead to a difference in strain energy between reactants and products. For example, meta- or para-alkyl groups, ether linkages, hydroxyl groups, etc. will not noticeably influence reaction thermochemistry (117). 

Low-spin Fe(iii) porphyrins have been the subject of a number of studies. (638-650) The favourably short electronic spin-lattice relaxation time and appreciable anisotropic magnetic properties of low-spin Fe(iii) make it highly suited for NMR studies. Horrocks and Greenberg (638) have shown that both contact and dipolar shifts vary linearly with inverse temperature and have assessed the importance of second-order Zeeman (SOZ) effects and thermal population of excited states when evaluating the dipolar shifts in such systems. Estimation of dipolar shifts directly from g-tensor anisotropy without allowing for SOZ effects can lead to errors of up to 30% in either direction. Appreciable population of the excited orbital state(s) produces temperature dependent hyperfine splitting parameters. Such an explanation has been used to explain deviations between the measured and calculated shifts in bis-(l-methylimidazole) (641) and pyridine complexes (642) of ferriporphyrins. In the former complexes the contact shifts are considered to involve directly delocalized 7r-spin density... 

As a last open shell system, we have chosen the H2C0 radical, which has been well characterized both at the theoretical and experimental level [77-80]. In this radical the symmetry of the singly occupied molecular orbital (a rt-in plane orbital located mainly on the oxygen center) [79] determines that only spin polarization effects contribute to isotropic hcc s of C and O, whereas spin densities at hydrogens have also a direct delocalization contribution. The results of table 5 show that, as expected, isotropic hcc s at the H atoms are well reproduced by aU the functionals, whereas the results for heavy atoms are much more scattered. In particular, the hyperfine constant of the carbon atom is -35 G at the PBEO level, in better agreement with the experimental value (-39 G)[80] than the B3LYP prediction (-34 G). It is noteworthy that the PBEO functional provides values for the H and C atoms which are sufficiently accurate. Unfortunately, no experimental value is available for the oxygen atom. 

Figure 183. Carboxylatc ion. Overlap of p orbitals in both directions delocalization of tt electrons, and dispersal of charge.

Metal orbital containing electron Direct delocalization (contact interaction) Spin polarization (configuration interaction) ... 

If via M-L CT bond, the value would be very small compared with direct delocalization. 

Janzen and Gerlock found that the epr spectrum of 2-trifluoromethylni-trobenzene anion radical exhibits a distinct alternation in line width (118). They related this alternation to the restrietion of rotation about the carbon-carbon bond, LII. They also pointed out that there may be an important interaction between the oxygen and fluorine atoms which leads to the direct delocalization of spin density to the trifluoromethyl group.8... 

All the results are consistent with the effectively greater electropositivity of deuterium. It is reasonable that the effect should be least for benzoic acid, where direct delocalization of charge onto the ring cannot occur. The effect on the ionization of phenol appears significantly larger than on that of aniline, but it is probably best to reserve further comment on the relative magnitude of these effects until additional measurements have been made. 

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