Pseudocontact shifts

Crudely approximated, the magnitude of the pseudocontact shift Afipolai, which predominates in lanthanide shift reagents, decreases with increasing distance between the observed nucleus i and the paramagnetic ion ... 

In accordance with the proportionality (3.8 a), the magnitude of the pseudocontact shift, which predominates for lanthanide shift reagents, decreases with the distance of nucleus i from the paramagnetic ion. Thus, C-9 of camphor is shifted more than C-10 on addition of the europium chelate. Often, the crude approximation (3.8 a) is a valuable assignment aid. 

Only the proton resonances of the heme group and the imidazole ring of the axial histidine (Fig. 14) would be expected to experience sizeable contact shifts, and pseudocontact shifts are probably also largest for these protons. The total intensity of the resolved temperature-... 

A more quantitative estimate of the pseudocontact shifts in DeutCN was obtained from comparison of the methylester resonances in DeutCN and deuteroporphyrin IX zinc(II) (114). Under identical conditions these resonances were both at —3.72 ppm from DSS in the diamagnetic complex, and at —3.78 and —3.82 ppm in DeutCN. The observed shift of ca. —0.08 ppm has to come almost entirely from pseudocontact coupling, since delocalization of unpaired spin density to these positions must be negligibly small, and a small change of the porphyrin ring current field, which might result from the zinc(II) vs. iron(III) substitution, would hardly be noticeable at the methylester positions either. With the assumption of an axially symmetrical g-tensor, and rj -values for the different protons of 10.0, 6.4, 5.7, and 4.5 A (Hoard (41)), one then estimates from the fj 3-dependence (Eq. 10) that the corresponding... 

As mentioned at the beginning of this section the size of the pseudocontact shifts in the NMR spectra could in principle be calculated for all the low spin ferric heme compounds if detailed data on the electronic g-tensors were available (Jesson (47)). Unfortunately the EPR data on the azides can not be used directly, because these complexes are not in a pure low spin state under the conditions of the NMR experiments (see section VI C). For the compounds in Figs. 10 through 20 no. successful single-crystal EPR studies were as yet reported. However only g-values determined in frozen solutions are presently available (Blumberg and Peisach (70) Salmeen and Palmer (95a)), e.g. for dicyanoferri-porphin at 1.4 °Kgi = 3.64, g 2.29, and gs 1.0 were found. 

A more detailed treatment, including the effects of spin delocalization on the pseudocontact shifts, might be warranted once single crystal EPR data will become available for several of the low spin ferric heme compounds. In the hemoproteins it would then be of special interest to investigate pseudocontact shifts for amino acid residues near the heme groups which could yield structural information in a similar way as the ring current shifts. 

In paramagnetic hemoproteins dipole-dipole coupling between the protons of the polypeptide chain and the unpaired electrons of the heme iron may also affect the appearance of the spectral region from —0.5 to — 10 ppm. These interactions will in general produce pseudocontact shifts (Eq. 10), and broadening of the proton resonances (Eq. 5). This... 

This chapter recalls the principles of the hyperfine coupling between electrons and nuclei in terms of energy and deals with its consequences on chemical shift. The equations for contact and pseudocontact shifts are derived and illustrated in a pictorial way. Their physical/chemical backgrounds are discussed as well as their limits of validity. The mechanisms of spin delocalization are illustrated. The perspectives when high field magnets are used are highlighted. 

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. 2.8. Angular dependence of the pseudocontact shift for an axial system, shown as a surface of constant absolute value of S1. In the example S1 is positive along the z axis and negative in the xy plane. The three-dimensional shape of the surface is similar to the representation of a d 2 orbital.

The angular dependence of the pseudocontact shift is shown in Fig. 2.8. For 9 = 54.74°, the pseudocontact shift is zero. If n > then the pseudocontact shift is positive for 9 values less than 54.74°. In the case in which Xxx and Xyy differ, then the angle 2 between the projection of the metal-nucleus vector on the xy plane and the y axis should also be considered (Fig. 2.9). The result is [4]... 

Eq. (2.20), or its simplified version in the axial case, Eq. (2.18), are of general validity. However, the principal directions and components of the molecular X tensor are seldom available. Pseudocontact shifts can be still evaluated by expressing the principal molecular magnetic susceptibility values as a function of the principal g values, in analogy with Eq. (1.38) ... 

As seen in Section 1.6, this approximation holds when the spin multiplet ground state is well isolated from excited electronic states, and ZFS is negligible. If 5 = lh, the g values can easily be measured through EPR spectroscopy and the g directions can be determined by single-crystal EPR measurements. When the latter measurements are not available, sometimes the principal g directions can be guessed from the symmetry of the molecule, and an independent estimate of the pseudocontact shift can still be attempted. 

As in the case of contact shift, the pseudocontact shift does not contain parameters of the resonating nucleus. At variance with contact shifts, the pseudocontact shifts can be evaluated even if more than one S multiplet is populated, provided that Ax values are experimentally available. 

Finally, if the electron is considered to be delocalized on a large part of the molecule as the result of n delocalization, the problem is further complicated. Pseudocontact shift is also provided by spin density in a p orbital which has an orbital contribution to the magnetic moment different from zero. This is very relevant for heteronuclei like 15N or 13C which may bear unpaired spin density on pz orbitals. The picture is of the type of Fig. 2.1 where, besides spin polarization, dipolar interactions occur. For example, fractions of unpaired electrons delocalized onto nitrogen p orbitals in Fe(CN) - are predicted to affect... 

Chemical bonds involving protons and deuterons are very similar to one another. Therefore, since the contact and pseudocontact shifts are independent of the nuclear parameters, the shifts are essentially the same. Both nuclei have the characteristic that only Is orbitals are involved. Therefore, protons and deuterons are discussed together. Sometimes, the use of 2H NMR can be appropriate because it has a smaller linewidth than H (see Section 3.9). 

Let us first try to discuss the contact contribution to the shift for protons and deuterons. As we shall see, there are conditions under which the pseudocontact shifts are small and negligible, or can be determined separately and subtracted from the hyperfine shifts. Furthermore, in the case of a monodentate ligand which exchanges rapidly from bound to free, the average pseudocontact shift in the absence of geometric constraints is zero. 

When is metal-centered pseudocontact shift expected ... 

WHEN IS METAL-CENTERED PSEUDOCONTACT SHIFT EXPECTED ... 

Pseudocontact shift is expected every time there are energy levels close to the ground state. This causes orbital contributions to the ground state, and such contributions are orientation dependent. Therefore, the magnetic susceptibility tensor is anisotropic. Anisotropy of the magnetic susceptibility tensors arises also from sizable ZFS of the S manifold (Section 1.4). 

Pseudocontact shifts are estimated using single crystal magnetic susceptibility data. 

It is foreseeable that pseudocontact shifts occur in low spin iron(III) porphyrins which in octahedral symmetry would have an orbitally triply degenerate ground state. Therefore, g and x values in iron porphyrins are highly anisotropic (see Sections 2.3 and 5.1.2) [75,76]. 

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