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Shift isotropic

In addition to line broadening due to accelerated relaxation, paramagnetic salts and chelates give rise to isotropic shifts when added to samples containing molecules with groups susceptible to coordination with metal ions, e.g. —OH, —NH2, — SH, — COOH, -C = 0. The isotropic shift A of a nucleus i in a sample S is the difference between chemical shifts measured before and after addition of the paramagnetic shift reagent SR [103]  [Pg.123]

The pseudocontact term Afipo,ar is due to the valence electrons of the shift reagent SR they cause an additional intramolecular field in the adduct S + SR, shielding or deshielding the nucleus i in the molecule S. The Fermi contact term Afon ac accounts for interaction between the nucleus i and the field of the unpaired electrons of the paramagnetic additive SR which may be delocalized within the adduct S + SR [103]. [Pg.124]

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 [Pg.124]

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  [Pg.124]

For expanding NMR spectra of aqueous sample solutions, lanthanide salts such as the chlorides, nitrates, or perchlorates of europium and praseodymium are used [103]. In organic solvents, tris-(/ -diketonato)-europium(III) chelates (1) are usually applied [103]. Chiral europium(III) chelates such as tris-(3-tm-butylhydroxymethylene)-D-camphora-to-europium(III) (2) separate the signals of enantiomers in the NMR spectra of racemic solutions [103], [Pg.124]


Physical background. MAS will narrow the inliomogeneously broadened satellite transitions to give a series of sharp sidebands whose intensity envelopes closely follow the static powder pattern so that the quadnipole interaction can be deduced. The work of Samoson [25] gave real impetus to satellite transition spectroscopy by showing that both the second-order quadnipolar linewidths and isotropic shifts are fiinctions of / and Some combinations of / and produce smaller second-order quadnipolar effects on the satellite lines than... [Pg.1485]

The isotropic shift and the quadnipole-induced shift (QIS) can easily be obtained from the data. The QIS is different m both dimensions, however, and is given by... [Pg.1489]

Flence the isotropic shift can be easily retrieved using... [Pg.1489]

Application of isotropic shifts to the investigation of structures and structural equilibria of metal complexes. R. H. Holm, Acc. Chem. Res., 1969, 2, 307-316 (38). [Pg.52]

The divalent Co(salen) complex (69a) is one of the most versatile and well-studied Co coordination compounds. It has a long and well-documented history and we shall not restate this here. Recent applications of (69a) as both a synthetic oxygen carrier and as a catalyst for organic transformations are described in Sections 6.1.3.1.2 and 6.1.4.1 respectively. Isotropic shifts in the HNMR spectrum of low-spin Co(salphn) (69b) were investigated in deuterated chloroform, DMF, DMSO, and pyridine.319 Solvent-dependent isotropic shifts indicate that the single unpaired electron, delocalized over the tetradentate 7r-electron system in CHCI3, is an intrinsic property of the planar four-coordinate complex. The high-spin/low-spin equilibrium of the... [Pg.34]

Fig. 54 Central part of the 74.63 MHz spectrum of octamethyl-l,2-3,5-tetrastannacyclohexane. The isotropic shift is -78 ppm, very close to the solution value of -78.5 ppm. Sets of lines marked with an asterisk are satellites due to one-bond tin-tin coupling. The linewidths of lines 1 to 4 are 10 Hz... Fig. 54 Central part of the 74.63 MHz spectrum of octamethyl-l,2-3,5-tetrastannacyclohexane. The isotropic shift is -78 ppm, very close to the solution value of -78.5 ppm. Sets of lines marked with an asterisk are satellites due to one-bond tin-tin coupling. The linewidths of lines 1 to 4 are 10 Hz...
Rearrangement of Equation 9.23 separates isotropic shift from anisotropic shift ... [Pg.164]

In this study, the distribution of Al atoms over the individual T sites of the ZSM-5 framework is examined by a combined experimental and computational approach. Eleven differently prepared samples of ZSM-5 were studied by 27Al 3Q MAS NMR spectroscopy. To interpret the observed NMR spectra, we calculated the isotropic shift of Al occupying all the distinguishable T sites of ZSM-5 and used the calculated isotropic shifts to assign the observed resonances to individual distinguishable T sites. [Pg.30]

Assignment of the isotropically shifted signals observed for the CuNiSOD example discussed in the previous paragraph has been achieved by means of anion titrations (not discussed here) and nuclear Overhauser enhancement spectroscopy (NOESY), to be discussed next. In Figure 3.24B the CuNiSOD active site is depicted with histidine nitrogens and protons identified for the discussion of the NOESY results. The copper(II) ion is coordinated to the N ligand atoms of his46... [Pg.112]

C NMR chemical shifts of a series of higher substituted a-vinyl substituted vinyl cations 24-27 were calculated to explore the sensitivity of the predicted isotropic shifts to electron correlation, basis set and geometry effects in differently substituted l,3-dienyl-2-cations.51... [Pg.136]

Carbocations have been generated in zeolites and were characterized by CPMAS 13C NMR spectroscopy and accompanying quantum chemical calculations.110 13C NMR isotropic shifts calculated at the GIAO-MP2/TZP/DZ level for the 1, 3-dimethylcyclopentenyl (98) and 1,2,3-trimethylcyclopentenyl cations (99)111 are in agreement with the observed experimental spectra of the carbocations in the zeolite.112... [Pg.155]

The ratios of the isotropic shifts at different proton sites of one complex are not the same in another complex in the same series. [Pg.185]

The unpaired electron(s) on the metal, Fe(III) in the case of the nitrophorins, act as beacons that illuminate the protons in the vicinity of the metal, by causing shifts (isotropic shifts) of the resonances from those observed in a diamagnetic protein, for example, the NO-bound forms of the nitrophorins. These shifts allow much to be learned about the intimate details of the electron configuration at the iron center. The two contributions to the isotropic shifts are the contact (through bonds) and dipolar or pseudocontact (through space) contributions, and these are discussed in considerable detail elsewhere 92-94). For the purposes of this chapter, a detailed physical treatment of these two contributions is not necessary or even desirable, but rather, the differences in the proton NMR spectra of the heme substituents, and what they tell us about the electronic environment at the iron(III) center, the effect of... [Pg.314]

The isotropic shift. The isotropic shift is the sum of two contributions the contact and the dipolar contributions. The former is due to the presence of unpaired electron density on the resonating nucleus. The latter arises from the anisotropy of the magnetic susceptibility tensor, modulated by the distance between the unpaired electron and the resonating nucleus, and is also dependent on the orientation of the metal nucleus vector with respect to the principal axes of the magnetic susceptibility tensor. Some problems arise when the spin delocalization is taken into account in calculating the dipolar coupling, but we will not address this problem except when strictly necessary. [Pg.71]

The aims of this study were to observe isotropically shifted signals for metal ions like copper(II), which usually give signals broadened beyond detection, and to relate the observed shifts and relaxation times to those of the uncoupled ions in order to understand the phenomena in theoretical terms. This approach allows the power of the NMR technique to fully exploit paramagnetic species and obtain information on spin delocalization, chemical bonding and so on. It is likely that the theory also applies to coupled metal ion-H adical systems like those proposed for derivatives of peroxidases (compound I), which contain iron(IV) and a heme radical (44). [Pg.80]

Estimated fiom temperature dependence of NMR isotropic shifts... [Pg.154]


See other pages where Shift isotropic is mentioned: [Pg.1493]    [Pg.1493]    [Pg.257]    [Pg.61]    [Pg.62]    [Pg.69]    [Pg.71]    [Pg.574]    [Pg.577]    [Pg.578]    [Pg.224]    [Pg.30]    [Pg.31]    [Pg.32]    [Pg.32]    [Pg.33]    [Pg.154]    [Pg.164]    [Pg.249]    [Pg.112]    [Pg.424]    [Pg.217]    [Pg.184]    [Pg.185]    [Pg.185]    [Pg.187]    [Pg.188]    [Pg.205]    [Pg.256]    [Pg.317]    [Pg.71]    [Pg.74]    [Pg.77]    [Pg.155]    [Pg.157]   
See also in sourсe #XX -- [ Pg.123 ]




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