Paramagnetic species hyperfine shifts

One of the most commonly studied systems involves the adsorption of polynuclear aromatic compounds on amorphous or certain crystalline silica-alumina catalysts. The aromatic compounds such as anthracene, perylene, and naphthalene are characterized by low ionization potentials, and upon adsorption they form paramagnetic species which are generally attributed to the appropriate cation radical (69, 70). An analysis of the well-resolved spectrum of perylene on silica-alumina shows that the proton hyperfine coupling constants are shifted by about four percent from the corresponding values obtained when the radical cation is prepared in H2SO4 (71). The linewidth and symmetry require that the motion is appreciable and that the correlation times are comparable to those found in solution. 

The carborane analogues of the metallocenes, the so-called metallocarboranes, have been studied by Wiersema and Hawthorne. (193) The paramagnetic species that they chose to study are of the types (C5H5)M(C2B H + 2) and M(C2B H +2)2 where M = Cr(iii), F iii), Ni(iii), and Co(ii), and a = 9,8,7, and 6 (Fig. 4). The B isotropic shifts of the Fe(iii) and Cr(iii) complexes reflect large negative hyperfine coupling constants which are consistent with a 7r-polarization mechanism (194) or parallel spin transfer from ligand to metal (Table VIII). By contrast, the Co(ii) complexes exhibit shifts that reflect... 

Detection and measurement of the two isomeric forms in solution again is most convenient by proton NMR spectroscopy, in which the isotropic proton hyperfine contact shifts in the paramagnetic tetrahedral isomers make recognition easy, and the amount of the shift reflects the proportion of paramagnetic species in solution (107,110-116, 119). Other methods, including UV/vis spectroscopy (106, 112, 114, 116, 118, 119), magnetic moment determination (105, 106, 109, 110, 115,116,118,119), dipole moment measurement (106,109,114), and IR spectroscopy (118), have also been employed. 

Of the several less common spectroscopic methods to combine with electrochemical intermediate generation such as luminescence, Raman, NMR, or X-ray absorption spectroscopy, the EPR method is presented here because of its relative simplicity and pronounced selectivity. Only paramagnetic compounds with a certain, not too rapid relaxation rate from the spin-excited state give detectable signals for EPR spectroscopy, which helps to disregard many simultaneously present species. On the other hand, the rather slow time frame (At 10 s) and the sensitivity of the EPR method to electronic influences from the participating atoms via g-factor shift and hyperfine interaction can render EPR a very valuable method to determine the site of electron transfer (ligand or metal) as well as the spin and thus valence distribution. 

It is apparent from the chemical shifts (g-values), the hyperfine coupling constants (A-values), and the linewidths that the free radicals and vanadyl species are in very similar environments in both samples. It was not possible to obtain meaningful values for the absolute numbers of spins per gram for either species, but estimates of the relative concentrations obtained by measuring peak heights indicate that the vanadyl and free-radical concentrations do not differ significantly between the two asphaltenes. It thus appears that heat treatment of Cold Lake asphaltenes to 320°C does not alter the nature or abundance of paramagnetic centers. 

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