Fermi resonances contact shift

The HNMR spectra of [Os(bipy)3]w and of [Os(phen)3]3+ have been measured and contact, pseudo-contact and Fermi-contact shifts studied 118149,153 the ESR spectra were also measured.149 The l3C resonance spectra of [Os(bipy)3]2+, [Os(4,4-Cl2bipy)3]2+, [Os(4,4-(OEt)2bipy)3]2+, [Os(4-OMebipy)3]2+ and [Os(4-NMe2bipy)3]2+ have been recorded.153 The resonance Raman spectrum of [Os(bipy)3j2+ has been measured.118... 

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. 

Figure 8 shows that in Ln2Sn20y compounds (Ln = Eu, Tm, Yb, Sm, La, Pr, Nd), the experimental isotropic chemical shifts for the " Sn resonance correlate nicely with theoretical values calculated from Eq. (5) by assuming no variation of a with the rare earth cation considered. This result suggests that the Fermi contact shift contribution dominates in such cases. By contrast, the resonance shifts measured in lanthanide-substituted compounds Y2 -, LniSn207 and... 

The portion of the isotropic nuclear resonance shift arising from the Fermi contact hyperfine interaction is called the contact contribution (as discussed in Chapter 2 Section 8.3). Contact shifts of a nucleus in a molecule with electron spin S and an axially symmetric g tensor are given by... 

The isotropic shifts in the proton resonances observed in the presence of a paramagnetic lanthanide ion could arise both from a Fermi contact term and the geometric dipolar term (as an example see Quereshi and Walker (1974)). In rare earth complexes, particularly in aqueous solution where there are uncertainties regarding the geometry and composition of the coordination sphere and where there may be substantial interactions between the cation and solvent molecules, the division of the observed shift into its components is not an easy problem. Treatment of data in terms of the psuedo-contact shift only must be made with some caution even in what would appear to be the most favorable cases (Moeller, 1972). (A complete discussion of problems of these sorts will be found in chs. 18, 38, 39 and in Fischer (1973)). 

The so-called heavy-atom chemical shift of light nuclei in nuclear magnetic resonance (NMR) had been identified as a spin-orbit effect early on by Nomura etal. (1969). The theory had been formulated by Pyykktt (1983) and Pyper (1983), and was previously treated in the framework of semi-empirical MO studies (PyykktJ et al. 1987). The basis for the interpretation of these spin-orbit effects in analogy to the Fermi contact mechanism of spin-spin coupling has been discussed by Kaupp et al. (1998b). 

Nuclear magnetic resonance spectra of these compounds are highly informative in this regard. The lineshapes of the Sn and resonances are dominated by the coupling of the nuclei with the unpaired electron spins. This interaction results in large resonance shifts arising from both the Fermi contact and the pseudo-contact (dipolar) interaction. 

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