Shift reagents lanthanide ions

Figure 2. Dy(P30io)2 is a lanthanide shift reagent commonly used in biological 7Li NMR experiments. The Dy3+ ion has a coordination number of nine with two P3O10 moieties, acting as tetradentate ligands, and one molecule of H2O coordinated in the first coordination sphere up to seven Li+ ions can bind in the second coordination sphere.

The determination of ee with chiral lanthanide shift reagents is normally performed in organic solvents. However, the development of chiral reagents capable of inducing shift nonequivalence in water, which is the appropriate solvent for many natural products, is of great importance. The europium(III) (R)-propylenediaminetetraacetate ion (Table 1) has been proposed as a chiral reagent suitable for use in aqueous solution90. 

Since the unpaired 4f electrons in lanthanide ions occupy orbitals of extremely low radial extension, the Fermi contact term is small for lanthanide shift reagents, and... 

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. 

Other pathways to better resolution and enhanced sensitivity may involve methods used in NMR studies of cellular phosphorus dynamics and humic and fulvic materials of water, soil, and sediments. These techniques include a variety of extractions, ion-pairing reagents, adsorption techniques, and lanthanide-shift reagents. For example, an extraction technique that has greatly enhanced the resolution and sensitivity of 31P FT-NMR spectra of Ehrlich ascites tumor cells (53) and HeLa cells (54) is the addition of 35% perchloric acid and removal of acid-insoluble material, followed by filtration and neutralization with K.2C03 or NaOH. 

The most commonly used shift reagents are tris-chelates of lanthanide ions with the (i-diketones, 2,2,6,6-tetramethylheptane-3,5-dione (dipivaloylmethane, (1)) and l,l,l,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dione (2). Typical reagents are tris-(dipivaloylmethanato) europium and tris-1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dionato europium, the names of which are normally abbreviated to Eu(dpm)3 and Eu(fod)3. 

The assignment of absorptions in the n.m.r. spectrum is greatly assisted by the use of shift reagents. Figure 3.71 shows the proton shifts of the tricyclic dilactam (3) as a function of the concentration of Eu(fod)3. Linear plots with varying slopes are obtained, the largest slope resulting from the four protons at C-5 and C-l 1 which are closest to the lanthanide ion. The protons at C-2 and C-8 which were initially at lower field are shifted least and hence the lines intersect. 

Lanthanides have been used as NMR shift reagents for carbohydrates in solution [106], Carbohydrates and polyols can bind metal ions provided they have three cis hydroxyl groups with axial-equatorial-axial arrangement. A typical binding is illustrated below. 

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