Shift dipolar

Combines sensitivity of EPR and high resolution of NMR to probe ligand superhyperfine interactions For paramagnetic proteins enhanced chemical shift resolution, contact and dipolar shifts, spin delocalization, magnetic coupling from temperature dependence of shifts Identification of ligands coordinated to a metal centre... 

Fig. 3. Experimental dipolar shift ratios [for Pr(III)] of lysozyme nuclei plotted against values calculated from the X-ray structure, for a defined direction of the magnetic symmetry axis (see Ref. 5).

From the dipolar interaction energy, the dipolar shift can be obtained by evaluating from Eq. (2.16) A d,p between two states differing by A A// = 1 and dividing it by the nuclear Zeeman energy hyiBo (Appendix IV) ... 

This contribution to the shift is isotropic because it is already averaged out over all the orientations. Then it is similar to the contact shift and is called pseudocontact shift 5. In the literature it is also referred to as dipolar shift or isotropic dipolar shift. 

Note that, when x is isotropic, the pseudocontact shift is zero (Eq. (2.18)), but a dipolar shift is observed in the solid state (Eq. (2.17) and Eq. (IV.9)). That the dipolar interaction energy averages zero for isotropic x can be easily verified by averaging Eq. (2.15) over all orientations, as... 

Fig. 2.21. Calculated patterns of dipolar shifts ( ) [79] and —(5.) ( ) [81] values (proportional to the contact shifts) induced by lanthanide ions.

Eq. (IV.8) gives the dipolar shift in the solid state. Note that when x is isotropic it reduces to... 

Eq. (2.20), written in a form containing direction cosines (Eq. (2.21)), can be also intuitively derived from Eq. (2.15), i.e. from the shift experienced by the nucleus when the magnetic field is along the Xzz direction. According to Eq. (2.15), the dipolar shift in this situation is given by ... 

Chapter 1 describes the interactions between a spin, electronic or nuclear, and a magnetic field just some basic physics which cannot be avoided. Chapter 2 deals with contact and dipolar shifts. The aim here is to be clearer and more rigorous... 

We proceeded to verify if the approximation of neglecting the contact contribution to the LIS values is warranted. Plots of fa/ Sz)j vs Cj/(Sz j (eq. (66)) and Spara/C vs (Sz)j/Cj (eq. (67)) for the XH NMR data of [7 2(L15-2H)3] (R = Ce, Pr, Nd, Eu) do not give very good linear correlations (fig. 61a). The irregularities are possibly due to deviations of the relative dipolar shift values from relative Bleaney s constants Cy, in particular for the Ce and Pr ions and inner-ring protons, as observed before (Buchler et al., 1992). The contact T) and pseudocontact Bq(G + G ) terms, obtained according to least-squares fits of eq. (65), indicate that some of the protons do have non-negligible contact shifts (table 26). 

It is usually the case that there is a considerable contact shift on the resonances of nuclei of atoms in the first coordination sphere of Ln(III) ions. The contact shifts can be separated from the dipolar shifts by quite simple procedures15,16) involving the study of complexes of the ligand with different Ln(III) assuming that the dipole shifts have axial symmetry, Fig. 2. The deduced contact shifts are quite useful in that they show which atoms are directly bound to Ln(III) in the complexes. We shall not concern ourselves further with contact terms in this article as no detailed structural information is available from them. The coordination sphere is usually generated by the searches using only dipolar shift and relaxation data. 

Fig. 2. Plot used to eliminate contact contribution from shift of 31P in AMP (Fig. 3). <50bs is the observed shift and is plotted as a ratio to the shift on H5. (no contact) relative to the expected contact term (theoretical) as a ratio to <5H5.. The points are for different Ln(III) shift reagents and the intercept gives the dipolar shift...

Fig. 3. Example spectra from the one-dimensional dipolar-shift experiment taken from reference 7. (a) (Top) Experimental l3C chemical shift anisotropy powder pattern for Ru(C5H5)2 and (below) for comparison, the dipolar shift l3C spectrum for the same compound, (b) Calculated dipolar-shift lineshapes for different angles (indicated) between the lH-13C dipolar and chemical shift anisotropy tensor principal z-axes.

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