Structure lanthanide-induced shifts

SN NMR spectroscopy is still underexploited in structural analysis of nitrogen-containing heterocycles. It can however be a powerful tool, for instance in tautomerism studies <2002JP2126>. Some 1SN NMR investigations of lanthanide induced shift have been performed on bicycles 50-52, showing that the complexing site of these 4,5-dihydro-l/f-l,2,3-triazoles is located on N-3. 

The angular and distance information provided by the lanthanide induced shift has found widespread application from the determination of solution structures of Ln chelates [18,19] to gaining structural information on proteins, nucleotides and amino acids [19], More recently anion binding to coordinatively unsaturated lanthanide complexes has been effectively signalled as the observed lanthanide induced shift has been directly correlated to the nature of the donor atom in the axial position [8,20,21], It is the polarisability of the axial donor that ranks the second order crystal field coefficient, B02, and hence determines the magnitude of the observed shift. Values of the mean shift of the four most-shifted axial protons of the 12-Nq ring for [Yb.la]3+ are collated in Table 2. 

PARAMAGNETIC NMR LANTHANIDE INDUCED SHIFTS FOR EXTRACTING SOLUTION STRUCTURES... 

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... 

Paramagnetic NMR Lanthanide Induced Shift for Extracting Solution Structure by Claude Piguet, University of Geneva, Switzerland,... 

Paramagnetic NMR lanthanide induced shifts for extracting solution structures 353... 

Claude Piguet and Carlos F. G. C. Geraldes, Paramagnetic NMR lanthanide induced shifts for extracting solution structures 353... 

It follows from the above discussion that the systems of 1 2 stoichiometry are complex and that 1 1 systems are preferable for lanthanide induced shifts in the determination of structures of organic molecules. The 1 1 stoichiometry is encountered in the case of Ln(dpm)3 in organic solvents [33], Ln(fod)3 in CHCI3 [32], [Ln(EDTA)]- in aqueous solution [34]. 

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 chemical shifts and lanthanide-induced shifts of the methyl resonances of a series of stictane and related flavicane triterpenoids have been examinedand provide supporting evidence for previous structural assignments (see Vol. 4, p. 219). 

Only the pseudocontact shifts give exact information on the location of the cation, and, therefore, on the structure of the complex. According to an equation developed by McConnell and Robertson,the lanthanide-induced shift, Av, is... 

Lanthanide Shift Reagents.—The effects of random co-ordinate error in analysis of lanthanide-induced axial pseudocontact shifts have been discussed, " and the contributions of contact and pseudocontact shifts in the n.m.r. spectra of isoquinoline and of endo-norbornenol have been evaluated. An experimental and computational approach to the use of lanthanide-induced shifts as a rigorous method for structure determination has been described. The method was used to predict the lanthanide-induced shift behaviour of a substrate. The recording of experimental data in excellent agreement with the molecular structure was reported. Contact shift contributions to lanthanide isotropic shifts have been found to be important for organic compounds even where the carbon atom is five bonds away from the lanthanide. 

New piperidine amide 154 was isolated from Achillea falcata, along with several known amides. Its structure was established using MS, UV, IR and and 13C NMR [419]. 13C NMR spectra were obtained, and H NMR-lanthanide induced shifts were determined for the amides [419],... 

A new piperideide (155) was isolated from Achillea millefolium, and its structure was determined using MS, UV, and H NMR [420]. Several previously known piperidine amides were also isolated from this plant, including the dominant 156. The stereochemistry of amides 157 and 158 was determined using the lanthanide induced shift method [420],... 

Physical Properties of 1,2,3-Thiadiazoles. Lanthanide-induced shifts have been used for elucidation of the structure of thermally generated monoxides of 1,2,3-thiadiazoles ( H and C). A calculation has been developed for the evaluation of lanthanide-induced shifts according to the McConnell-Robertson equation. The o and Or parameters of triazoles were also determined by n.m.r. spectroscopy. ... 

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... 

Structural investigations by use of lanthanide-induced shifts are usually complicated by uncertainties regarding the exact location of the lanthanide atom. A computer study of time-averaged geometries of complexes of some amines, alcohols, ketones, and other polar compounds with Eu(dpm)3 and Eu(fod)3 has indicated that Eu---X distances (where X is the co-ordinated atom) vary little with the nature of X but are sensitive to steric factors.Steroid chemists... 

As lanthanide-induced shifts often have values of up to 20 ppm, the increased dispersion, and hence interpretability, of NMR spectra afforded by this method is enormous. Moreover, pseudo-contact shifts diminish regularly with the distance from the paramagnetic center, r in Figure 12.14, and since this can be approximately located (the lanthanide complex is attached to the lone pair), their magnitudes give valuable structural information. 

R 214 C. Piguet and C.F.G.C. Geraldes, Paramagnetic NMR Lanthanide Induced Shifts for Extracting Solution Structures , p. 353... 

Determination of Absolute Configuration and Solution Conformation. Abraham et have employed H and C lanthanide induced shifts produced by Yb(fod)3 as an experimental probe of the solution conformations of cyclopen-tanol and cis- and trans-cyclopentane-l,2-diol. Analysis of the LIS was undertaken in conjunction with molecular mechanics and ab initio calculations of structure. 

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