NMR Paramagnetic Shifts

The existence of a linear correlation between 15N shifts of piperidine derivatives with predominantly axial N-H bonds and those with predominantly equatorial N-U bonds and the 13C shifts of hydrocarbon analogs showed that, whereas 15N chemical shifts of N-methylpiperidines depend on the configuration of the V-substituent, those of W-H-substituted piperidines 

Morishima, K. Okada, M. Ohashi, and T. Yonezawa, Chem. Comimm33 (1971). 

The nature of the ring fusion in quinolizidine derivatives may be determined from the 15N chemical shifts, which are to lower field in the transfused derivatives. This is shown for the indolo[a]quinolizidines 73 and 74 (chemical shifts to low field of external anhydrous liquid ammonia)115 and by the perhydropyrido[l,2-c][l,3]thiazines 8 66.9 trans conformer 312, 8 43.9 S-inside cis conformer 313.ll5a 

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A chiral dichlororuthenium(IV) complex of a Z)4-symmetric porphyrin, [Ru (Z)4-por )(Cl)2], has been prepared by heating [Ru (Z>4-por )(CO)(MeOH)] in CCI4. The complex is characterized by NMR (paramagnetically shifted pyrrolic protons at = 52.3 ppm), FAB-MS, and magnetic susceptibility measurement (/.teff= 3.1/.tB). It is a very active catalyst for enantioselective alkene epoxidations using 2,6-dichloropyridine A-oxide as the terminal oxidant, with a turnover number of up to 2000 the ee of the epoxides is 50-80%. The complex can be incorporated into sol-gel and turnovers of over 10" can be achieved." ... 

The tetrakis-dithiophosphinate complex [PPI14][Pr(S2PMe2)4], whose crystal structure has also been determined, has a distorted tetragonal antiprismatic geometry with Pr—S = 2.888-3.0150.400 The complex ions [M S2P(OEt)2 4]", have been the subject of an NMR paramagnetic shift study (see Section 39.2.9.4). 

The complexes [Rh°(TMP)], [Rh°(TTEPP)], [Rh (TTiPP)], [Ir°(TTEPP)], and [Ir°(TTiPP)] are sufficiently bulky to completely prevent dimerization (TMP = tetramesitylporphyrinato TTEPP = tetra(2,4,6-triethylphenyl)porphyr-inato TTiPP = tetra(2,4,6-triisopropyl- phenyl)porphyrinato). The EPR spectroscopy, NMR paramagnetic shifts and line-broadening studies have proven useful to smdy the structme and reactivity of these paramagnetic complexes (and their adducts with ethene and CO, see below) (133-135). The EPR parameters of the rhodium complexes [Rh (TMP)J and [Rh TTiPP)] are... 

Any variation of the slope for linear plots of Y / Sz)j versus 5ip/ Sz)j along a series of lanthanide complexes j implies that the ratio of the geometrical factor Gi/G changes, hence the structure of the complex does too. The systematic application of nuclear relaxation measurements (Eqs. (28) and (29)) for obtaining R Fj-iuclei distances, combined with the detailed analysis of NMR paramagnetic shifts with the help of the two nuclei method (Eq. (37), detection of isostructural series) as well... 

Mouesca JM, Rius G, Lamotte B. 1993. Single-crystal proton ENDOR studies of the [Fc4S4] cluster determination of the spin population-distribution and proposal of a model to interpret the H-NMR paramagnetic shifts in high-potential ferredoxins. J Am Chem Soc 115 4714 731. 

Actinides. The NMR paramagnetic shift temperature dependence in actinide methyl borohydrides has been calculated. The H, B, and C NMR spectra of [U N(SiMe3)2 2(N(SiMe3) SiMe2CH2B(C6F5)3 )] have been reported. ... 

McGarvey BR, Nagy S (1987) Anisotropy in the proton NMR paramagnetic shift of cyclooctatetraene and cyclopentadiene anions coordinated to U(IV). Inorg Chim Acta 139 319-321... 

Figure 3. Monitoring the uptake of Li+ into human erythrocytes after incubation in media containing 2 mM Li+ using 7Li NMR spectroscopy. The signals corresponding to the intra-, and extracellular Li+ are separated by the presence of the paramagnetic shift reagent, Dy(P30io)2, the extracellular medium [34].

This indicates that the deviations are due to systematic errors, for example deficiencies of the applied methods and basis sets. DFT-based methods, such as GIAO/DFT calculations are known to overestimate paramagnetic contributions to the chemical shielding. This results, for critical cases with small HOMO/LUMO separations, in overly deshielded competed chemical shifts. Notorious examples for these deficiencies are 29Si or 13C NMR chemical shift computations of silylenes, silylium ions or dienyl cation .(5/-54) Taking into account the deficiencies of the applied method, and bearing in mind very reasonable correlations shown in Figures 4 and 5, the computational results do support the structural characterization of the synthesized vinyl cations. 

In the cyclic 1 1 complex, glucose is held close to the anthracene aromatic face, as represented in Scheme B10.4.1. In fact, the 1 H-NMR spectrum exhibits a very large paramagnetic shift for the H3 proton (—0.3 ppm), which thus points towards the re-electrons of the anthracene moietya,b). 

NMR has also been used to characterize binding [22, 23]. When cyt c binds to ccp the paramagnetically shifted heme methyl groups of cyt c shift further downfield by 1-2 ppm. Normally, binding occurs in the fast exchange regime, so that the fraction bound can be assessed from the frequency shift (Fig. 10). Two... 

Magnetic susceptibilities of solutions — These are useful parameters for determining equilibrium constants for reactions involving spin changes. The Evans nmr method utilizes the observed shift in the resonance line (say of a proton of t-BuOH or hexamethyldisilox-ane) in solution when a paramagnetic substance is added. The paramagnetic shift A/ is related to the magnetic moment (jj, of the solution at TK by the approximate expression... 

A general comment on the use of the empirical correlation between Si and Sn NMR (and likewise on C/ Si or Sn/ Pb NMR) chemical shifts is in order. The basis for this correlation is that the paramagnetic term Op dominates the chemical shift. According to Ramsay s theory, Op is proportional to the reciprocal energy difference h.E between the magnetically active orbitals and proportional to the expectation value for the electron radii (r )np- Thus, a linear correlation between the 5 Si and 8 Sn implies that the ratio of both determining factors of Op is constant for the all compounds of interest. In particular, it is not clear, however, if the ratio for tetravalent silicon and tin compounds is the same as for trivalent silicon and tin compounds. Therefore, the extension of a correlation based exclusively on the... 

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