Relaxation ligand centered

More approximate expressions, based on the concepts of ligand-centered dipolar relaxation, were also proposed. The review by Mispelter and co-workers (45) gives the expression ... 

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... 

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. 

The excited state absorption spectra of these luminescent tetranuclear cop-per(I) iodide complexes have also been reported [57]. The absorption spectral data of the cluster-centered triplet states of 4a, 4c, 4f, and are listed in Table 2. The energy of the absorption is in line with the ordering of the n orbital energy of the pyridine ligands. For die piperidine analog 4i, no excited state absorption is observed. It has been suggested that the excited state absorption bands are associated with the transition from a thermally relaxed [cluster-centered] state to Franck-Condon states of the [XLCT] manifold. 

The relaxation of Fe(II) protoporphyrin dimethyl ester in benzene is seen in Figure 1. It can be seen tnat in this complex which has no axial ligands and a high spin state, S=2 (14), there is an excited state absorption band which extends from 520 nm into the blue. The peak is not seen because of experimental cut-off in the spectrum at 450 nm. This difference spectrum also indicates an absorption band extending to the blue from 620 nm and interupted by the strong ground state bleaching at 570 nm. The peak of the excited state absorption band seems to be centered at about 585-590 nm. 

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