Carbon spin-orbit relaxation

The first criterion really related to the content of this book is the analysis of T and 72. As the dominant contribution to nuclear relaxation is dipolar in nature, Tfl and linewidths will decrease as we move farther from the paramagnetic center. Even the contact contribution to relaxation often decreases with the number of chemical bonds from the paramagnetic center. A caveat, however, should be given. Spin density transfer causes ligand-centered relaxation. Significant spin density on ax orbital of an sp2 carbon may relax an attached proton more than the paramagnetic center itself, owing to the different distances and to the sixth power dependence on distance. 

In contrast to pyridine adducts, those of pyridine-iV-oxide produce isotropic shifts which support a dominant -delocalization mechanism. Spin density distributions over the aromatic carbons have been determined from H and spectra of several Ni(acac)2(py-NO)2 complexes. (79) relaxation measurements indicate that Tj values arise mainly from hyperfine dipolar interaction induced by spin density localized on Ni(ii) and on the C-centred 2p orbital. Adducts of Ni(acac)2 with aniline, (80) fluoroanilines, (80,81) alkylanilines, (81-83) aniline derivatives, (541, 542) and nitrogen heterocycles (543) have been extensively studied. The results are consistent with a dominant n-spin delocalization mechanism. 

By the same experimental technique, the temperature dependence of the nuclear spin relaxation rates was investigated for the radical cations of dimethoxy- and trimethoxybenzenes [89], The rates of these processes do not appear to be accessible by other methods. As was shown, l/Tfd of an aromatic proton in these radicals is proportional to the square of its hyperfine coupling constant. This result could be explained qualitatively by a simple MO model. Relaxation predominantly occurs by the dipolar interaction between the proton and the unpaired spin density in the pz orbital of the carbon atom the proton is attached to. Calculations on the basis of this model were performed with the density matrix formalism of MO theory and gave an agreement of experimental and predicted relaxation rates within a factor of 2. 

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