Hyperfine contact

THE INDUCED MAGNETIC MOMENT PER METAL ION IN POLYMETALLIC SYSTEMS, THE HYPERFINE CONTACT SHIFT, AND THE NUCLEAR RELAXATION RATES... 

Two different nuclei in TDAE-C60 have been investigated so far by NMR protons of methyl groups of the TDAE molecule and 13C nuclei of the Qq ion. The main difference between these two nuclei (in addition to their relative sensitivity) is that methyl protons experience mostly the dipolar fields of the Qq magnetic moments. On the other hand 13C nuclei on each Cgo ion will, in addition to dipolar fields, also feel the hyperfine contact field of the unpaired electron spin. Details of the 13C NMR results will be given in the next section. Here we... 

The temperature dependence of the NMR relaxation rate Tf1 for the Au compound (Fig. 9) exhibits a typical behavior of one-dimensional conductors with deviations to the Korringa law (Tf1 T) shown by the upward curvature at high temperatures similarly to (TMTTF)2PF6 [41] and TTF[Ni(dmit)2] [42]. Since there are no localized spins on the dithiolate chain, the relaxation comes from the hyperfine contact and dipolar interactions, 7 1 + r j, produced by the spins of the itinerant electrons along the perylene stacks. The enhancement of the relaxation is, however, less important than that shown by the Bechgaard salts [45]. 

These two coupling mechanisms have effects both on the chemical shifts and on the relaxation rates. The contact contribution to the shift is proportional to the electron spin multiplicity and to the hyperfine contact coupling constant (McConnell and Chesnut, 1958),... 

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. 

The coupling of the unpaired electrons with the nucleus being observed generally results in a shift in resonance frequency that is referred to as a hyperfine isotropic or simply isotropic shift. This shift is usually dissected into two principal components. One, the hyperfine contact, Fermi contact or contact shift derives from a transfer of spin density from the unpaired electrons to the nucleus being observed. The other, the dipolar or pseudocontact shift, derives from a classical dipole-dipole interaction between the electron magnetic moment and the nuclear magnetic moment and is geometry dependent. 

The Fermi contact, or isotropic contact or isotropic hyperfine contact mechanism applies when there is finite unpaired electron density at the nucleus. This either adds to, or subtracts from the external field, depending upon the sign and magnitude of (the hyperfine coupling constant). 

The conditions necessary for observation of proton magnetic resonance spectra in paramagnetic systems are well established (1). Either the electronic spin-lattice relaxation time, T, or a characteristic electronic exchange time, Te, must be short compared with the isotropic hyperfine contact interaction constant, in order for resonances to be observed. Proton resonances in paramagnetic systems are often shifted hundreds of cps from their values in the diamagnetic substances. These isotropic resonance shifts may arise from two causes, the hyperfine contact and pseudocontact interactions. The contact shift arises from the existence of unpaired spin-density at the resonating nucleus and is described by 1 (2) for systems obeying the Curie law. 

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