Paramagnetic systems shift effects

In contrast, soft magnetic solids and paramagnetic systems with weak anisotropy may be completely polarized by an applied field, that is, the effective field at the Mossbauer nucleus is along the direction of the applied field, whereas the EFG is powder-distributed as in the case of crystallites or molecules. In this case, first-order quadrupole shifts cannot be observed in the magnetic Mossbauer spectra because they are symmetrically smeared out around the unperturbed positions of hyperfine fines, as given by the powder average of EQ mj, d, in (4.51). The result is a symmetric broadening of all hyperfine fines (however, distinct asymmetries arise if the first-order condition is violated). 

In paramagnetic systems the Fermi contact term is expected to dominate. However in addition there is also the possibility of a through-space dipolar effect. The shift caused hy the trace of this tensor interaction is often called the pseudocontact effect, which for a lanthanide ion is given by... 

A second clue as to the relaxation mechanism in paramagnetic systems may be obtained from measurements of chemical shift changes. Chemical shift changes observed upon addition of a metal ion for a rapidly exchanging halide nucleus in solution should be due to differences in chemical shifts in the "free" and complexed sites. Chemical shifts in diamagnetic halide complexes are not known. From Table 3.3 it can be estimated that they may at most be of the order of a few hundred ppm. Due to quadrupolar relaxation effects it can be envisaged that it should be very difficult to determine these shifts experimentally. No shift changes have so far been reported for Cl and Br in... 

Carbon-13 shifts of alkynes (Table 4.13) [246-250] are found between 60 and 95 ppm. To conclude, alkyne carbons are shielded relative to olefinic but deshielded relative to alkane carbons, also paralleling the behavior of protons in proton NMR. Shielding relative to alkenes is attributed to the higher electronic excitation energy of alkynes which decreases the paramagnetic term according to eq. (3.4), and to the anisotropic effect of the triple bond. An increment system can be used to predict carbon shieldings in alkynes... 

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