Lanthanide-Induced Shifts LIS

A study195 based on the NMR lanthanide-induced shifts (LIS) for a series of cis- and trans-3-substituted, and 3,3-disubstituted thietane oxides concluded that all cis-substituted oxides (5c R = CH3, t-Bu and aryl) exist exclusively in the diequatorial conformation. The trans-3-substituted isomers (185) prefer the equatorial oxygen conformation (R = CH3,86% t-Bu, 65-75% aryl, 75%), which means an axial preference for the substituents (e.g. 185d), at least when they are bound to a shift reagent (equation 75). 

The recently introduced lanthanide-induced shift (LIS) technique has also found useful application in configurational studies of sulfur compounds. For example, the absolute configurations of a new type of bridged steroidal sulfoxide 203, which is epimeric at sulfur, were... 

Figure 15. Plots81 of the lanthanide-induced shift (LIS) for the methoxy group hydrogens in the two diastereomeric Mosher esters of 1-phenylethanol versus the molar ratio of Eu(fod)3.

This technique (see Section 4.1.1.4.) is well suited to the exploration of double-bond configurations. This is because these compounds arc often rigid and therefore there are no conformational rearrangements to complicate lanthanide induced shift (LIS) interpretations. However, if present, mobile groups within a molecule must be taken into consideration. 

The first applications of lanthanide induced shifts (LIS) in 13C NMR appeared in 1971. (7-10) Shortly thereafter it was shown (11) that Eu(dpm)3 induced chemical shift changes in cholesterol which extended from the coordination site at C-3 clear across the ring system, the assignments used being those of Roberts group. (2) Although the LIS... 

The simultaneous least-squares fits of lanthanide induced relaxation (LIR, 7j) according to eq. (18) and lanthanide induced shifts (LIS) according to eq. (47) with the crystal structures of [R(L6)(N03)](N03)2 as starting molecular models display only minor shift of Will) from its original location. For the lighter lanthanides (R = Ce-Eu), the best fit shows a slight shift... 

The solution structure of the EGTA lanthanide complexes was investigated using the H and NMR spectra of the various lanthanide complexes and the Lanthanide Induced Shift (LIS) for one of the acetate resonances for a series of Ln-EGTA complexes. The spectra of the lighter lanthanides up to Sm show five resonances in the H spectrum and seven in the spectrum at room tempera-... 

The lanthanide induced shifts (LIS) arise from the interaction of the magnetic nucleus with the magnetic field of the paramagnetic lanthanide ion. This could arise via two mechanisms either a pseudocontact (dipolar) through-space fashion involving a dipole dipole interaction, or by a contact mechanism with delocahzation of unpaired spin density from the lanthanide f-orbitals via the lanthanide substrate bond on to the organic molecule the former is more important. 

Quantitative separation of the n contact and pseudocontact contributions to the lanthanide induced shifts (LIS) in aniline and m-and p-toluidines has been reported. (395, 396) The contact shift patterns are estimated from the rr-spin density distribution of the appropriate cation radical or from the Ni(acac)2 induced shifts. The separation of the shifts was checked by comparing the relative contact shift contribution with the <5 ) value of Golding and Halton (389) and the remaining pseudocontact contribution with the calculated values of Bleaney s theory. (380)... 

Solution structure determination essentially relies on NMR data, via either a classical analysis of the number of signals and of their chemical shifts with the help of two-dimensional COSY, NOESY, and NOEDIF measurements, or the more sophisticated investigation of both lanthanide-induced shifts (LIS) and relaxation times (LIR). When the major species in solution is the heterobimetallic helicate, analysis of the... 

Shift Reagents. In 1970, Williams, elaborating on the preliminary work of Hinckley, discovered that adding trw-jS-diketonate lanthanide complexes to solutions of substances with lone pairs of electrons available for coordination (such as oxygen- and nitrogen-containing organic compounds) resulted in vastly more dispersed NMR spectra. This phenomenon, named lanthanide-induced shift (LIS), arises from the... 

Figure 12.14. The magnitude of the lanthanide-induced shift (LIS) as a function of molecular geometry. H is a proton on the molecule whose resonance is being shifted by the presence of the lanthanide.

More recently, Abraham and coworkers [18] reexamined the conformational behaviour of thiane-l-oxide (3) by analysis of lanthanide-induced shifts (LIS) in the appropriate NMR spectra. Their results compare reasonably well with those obtained by Barbarella et al. [17], suggesting the applicability of the LIS method to sulfoxide conformational analysis. 

Paramagnetic complexes, mainly the lanthanide shift reagents (LSR) bringing about the lanthanide induced shifts (LIS) [1, 2, 7]. 

The hydration number q-. this parameter influences strongly the IS contribution (equations (3) and (4)). If this number increases from 1 to 2, the relaxivity increases by about 30%, but most of the Gd-complexes have a q number equal to one. This number can be measured by fluorescence study of europium or terbium complexes or by NMR measuring the lanthanide induced shift (LIS). 

The H n.m.r. spectra of the two diastereoisomeric trans-alcohols (18), and of (19) and (-)-myrtenol (20) have been assigned in Eu-induced shift studies, and pseudo-contact effects analysed. In a lanthanide-induced shift (LIS) study of various 7,7-disubstituted-endo- and -exo-norbornene anhydrides [21 X = Y = H X, Y = (CH2)2 X, Y = (CH2)4] the results suggest that the bridge substituents do not strongly perturb the anhydride-Eu(fod)3 interaction, and that complexation occurs on the... 

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