Shift reagent

Lanthanide Shift Reagents.—New theoretical treatments of the origin of the paramagnetic shifts induced in the n.m.r. spectra of substrates bound to 

Petru and A. Mucik, Coll. Czech. Chem. Comm., 1971, 36, 3774. 

Refinements of the use of the experimental data obtained using these reagents, e.g. determination of the agreement factor R in pseudo-contact models and methods of calculating experimental errors, have also been reported. 

An analysis of the shift reagent properties of lanthanide 3-diketonato-complexes, (1 R = R = Bu Ln = Eu, R = CF3, R = Bu ), with respect to mono- and di-functional substrates, has been given. The ability of these complexes to form 1 1 and 1 2 adducts with the substrates seemed to be determined by the nature of the substrate and the relative concentrations of the 

Direct observation of free and complexed substrate in the system [1 Ln = Eu, R = C3F7, R = C(CD3)3]-Me2SO has been reported. Methods of calculating the binding constants and bound chemical shifts of organic substrates associated with shift reagents have been described. 

Lanthanide Shift Reagents.—The solvation numbers and kinetics of substrate exchange in lanthanide shift-reagent systems have been studied.165 At low temperatures, substrate exchange was slow on the XH n.m.r. time-scale, thereby enabling solvation numbers, which were often solvent dependent, to be determined. 

Methods have been devised166 for the separation of pseudo-contact and contact contributions to the shifts of 31P caused by [M(edta)]- (M = Pr- Y) and other complexes bonded to adenosine-5 -monophosphate and cytidine-5 -monophosphate at various pH values. 

From studies of the XH n.m.r. spectra of polyol-lanthanide(m) (La, Pr, Nd, Eu, Tb, Yb) complexes it has been established172 that the contact interaction could be de- 

Thiazoles form173 1 1 adducts with Eu(dpm)3 and are N-bonded. Steric hindrance is low when the 2- or 4-positions are substituted, while the other position adjacent to the N atom is not. 

Chiral complexes have been prepared174 and tested for their utility in inducing chemical shifts between corresponding resonances of enantiomeric species. The complex (10) was the most effective for resolution of enantiotopic resonances, but while (11) and (12) were less effective, they were more easily synthesized. 

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. 

The shifts and line broadenings induced by various lanthanide shift reagents in the proton spectrum of pinacoline have been described. A method of analysis which combined a two-site model for co-ordination of the lanthanide to the carbonyl group with several methods for averaging over internal rotations of the methyl and t-butyl 

Latuszynski, J. Mikulski, I. Penov, A. W. Potempa, A. Zielinski, K. Zuber, and J. Zuber, J. Inorg. Nuclear Chem., 1976, 38, 585. 

Kimizuka, K. Kato, I. Shindo, I. Kawade, and T. Katsura, Acta Cryst., 1976, B32, 1620. 

A100 MHz n.m,r. study of relaxation and chemical shifts induced by interaction 

Interpretation of n.m.r. and H n.m.r. shifts in the spectra of the 3-0-methyl ether and two disaccharide derivatives of methyl 4,6-O-benzylidene-D-glucopyranosides obtained in the presence of Eu(fod)3 has been carried out. The positions of substituents were determined by using a computer search programme on the europium complex with europium in the calculated position predicted from shift values.  

The most commonly used shift reagents for organic compounds employ Eu3+, Pr3+, or Yb3+ as the paramagnetic ion in a chelate of the form VI 

As indicated in Eq. 4.16, the direction of shift depends on the anisotropy in the susceptibility, but it also depends on the angle between the principal axis of susceptibility and the vector R to the nucleus. For Eu3 + the induced shifts are normally to higher frequency and for Pr3+ they are to lower frequency. On occasion some nuclei may lie at an angle 0 54.7°, so that the factor (3 cos2 0—1) changes sign, and shifts occur in the directions opposite those given before. 

FIGURE 4.14 Use of lanthanide shift reagents to alter chemical shifts in alcohols, (a) H NMR spectrum (100 MHz) of n-hexanol in CC14 with the reagent Eu(dpm). (b) H NMR spectrum (100 MHz) of n-pentanol in CC14 with the reagent Pr(dpm)3. From La Mar et al.6  

The configuration of the branch point in branched-chain sugar derivatives (fi.g. methyl 2-benzamido-4,6-0-benzylidene-2-deoxy-3-C-nitromethyl-a-D-allo-pyranoside) has been assigned on the basis of the downfield chemical shifts obtained with Eu(fod)8, as compared with those of model compounds of known configuration. Chemical shifts and J values have been recorded for the 3,6-anhydro-sugar derivatives (507)—(509) in the presence, and in the absence. 

In studies on nucleotides, Williams group has employed edta complexes of lanthanide ions (La +, Pr +, Eu +, and Gd +) to probe the conformations of adenosine and cytidine 5 -phosphates at pH 7.5. The conformation of cytidine 5 -phosphate at pH 2.0 was also investigated using lanthanide ions, and relaxation studies with Gd + cations confirmed the findings derived from the shift data.  

Studies of complexes formed between [Eu(dpm)3] or [Eu(fod)3] and diacetoneglucose, aniline, and compound (6) have revealed that, whereas the 

Chemically induced dynamic polarization has been observed in the presence of [Eu(fod)3] and [Pr(fod)3]. 

Gas chromatographic studies of ethers, ketones, alcohols, esters, olefins, and alkanes, using columns with liquid phases containing P-diketonate complexes in squalane, revealed that the more nucleophilic organic substrates reacted more strongly with the P-diketonates than the less basic ones. Er complexes of fluorinated P-diketones (especially 3-trifluoroacetyl-d-camphor-ate, facam) reacted more strongly with these nucleophiles than did similar non-fluorinated Er complexes. The retention time of THE increased exponentially with the inverse of metal ionic radius in the facam chelates of a variety 

With [Eu(dpm)3], 3,3-dimethylthietane 1 -oxide and DMSO gave 1 1 adducts, which had near perfect wedged octahedral structures, the Lewis bases occupying one of four equivalent positions of lowest symmetry. The species [M(dpm)3] (M = La, Pr, Eu, Er, Ho, or Lu) were monomeric in dry CCI4, and 1 1 adducts were formed with pyridine, borneol, and neopentanol. However, lanthanide shift reagents (L) could react with substrates (S) by a two-step mechanism, viz. 

The pseudo-contact shifts recordedfor the 1 1 adduct of [M(dpm)3] (M = Eu, Pr, or Yb) with o-phenanthroline or 2,2 -bipyridyl were not in accord with the Robertson-McConnell expression, which assumed axial symmetry about the metal Lewis-base bond. The spectra of the phen complexes were relatively insensitive to variations in temperature, whereas the temperature variations of the spectrum of [M(dpm)3(bipy)] could be explained by biphenyl-type rotational isomerism of unidentate bipy. Further co-ordination by substrates added to [M(dpm)3(phen)] and its bipy analogues was not observed. In addition to 1 1 and 1 2 adducts, the system [M(fod)3]- 7t-C5H5)Fe(CO)2-CN] (M = Pr, Eu, Ho, or Yb) also contained 1 3 adducts. However, the relative successive formation constants for the 1 2 adducts decreased in the order Pr Eu Ho Yb, indicating greater steric crowding with the heavier metal complexes. 

Complexes of Eu(fod)j with permethylated aldohexosylaldohexoses consisting of D-gluco- and D-galacto-pyranosyl residues in (1 - 2)- or (1 4)- or (1 6)- 

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Chemicals from brine Chemical shift reagents Chemical shifts Chemicals in war Chemical treatments... 

Chiral liquid crystals Chiral recognition Chiral separation Chiral separations Chiral shift reagents... 

Contact lens fluid Contact nucleation Contact poisons Contacts, electrical Contact shift reagents... 

Europium (III) acetate (2H2O) [62667-64-5] M 383.1, pKj 8.31 (for aquo Eu " ). Recrystd several times from water [Ganapathy et al. J Am Chem Soc 108 3159 1986]. For europium shift reagents see lanthanide shift reagents in Chapter 4. 

Figure 2.24, Determination of the enantiomeric excess of 1-phenylethanol [30, 0.1 mmol in 0.3 ml CDCI3, 25 °C] by addition of the chiral praseodymium chelate 29b (0.1 mmol), (a, b) H NMR spectra (400 MHz), (a) without and (b) with the shift reagent 29b. (c, d) C NMR spectra (100 MHz), (c) without and (d) with the shift reagent 29b. In the C NMR spectrum (d) only the C-a atoms of enantiomers 30R and 30S are resolved. The H and C signals of the phenyl residues are not shifted these are not shown for reasons of space. The evaluation of the integrals gives 73 % R and 27 % S, i.e. an enantiomeric excess (ee) of 46 %...

If the amount of the sample is sufficient, then the carbon skeleton is best traced out from the two-dimensional INADEQUATE experiment. If the absolute configuration of particular C atoms is needed, the empirical applications of diastereotopism and chiral shift reagents are useful (Section 2.4). Anisotropic and ring current effects supply information about conformation and aromaticity (Section 2.5), and pH effects can indicate the site of protonation (problem 24). Temperature-dependent NMR spectra and C spin-lattice relaxation times (Section 2.6) provide insight into molecular dynamics (problems 13 and 14). 

Fig. 2.7. NMR spectrum of 1-phenyIethyIamine in the presence of a chiral shift reagent, showing differential chemical shift of methine and methyl signals and indicating ratio of R- to iS-enantio-mers. [Reproduced from J. Am. Chem. Soc. 93 5914 (1971) by permission of the American Chemical Society.]...

Clobazam (54, X = Cl) and related compounds (X = H, X = CF3) exist in the dioxo tautomeric form [80JHC551,87JCS(P2)1071], as do the analogous pyrazolo[3,4-h][l,4]diazepinediones (89JHC949). Tliese conclusions were mainly based on careful NMR studies including the use of lanthanide shift reagents (LSR). 

The enantiomeric excess was determined by HNMR with ( + )-(/ )-binaphthol as a chiral shift reagent. The absolute configuration of the adducts was not determined. 

Nuclear Magnetic Resonance Shift Reagents (see Table 19.12)... 

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