Contact shifts

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

NMR is the only technique capable of assigning the cysteines ligating specific Fe atoms, since the asymmetric coupling of the three Fe + ions results in different temperature dependence for the contact shifts... 

Four strongly downshifted signals in each spectrum, between 50 and 110 ppm, were assigned to the four CB protons of the cysteines coordinating the Fe ". The contact shifts of the protons reflect the coordination of cysteine to the Fe " of the antiferromagnetically coupled Fe "-Fe" pair as the cysteine protons sense the spin down orientation of the Fe " (S = ) site. This is supported by the observation that the temperature dependence of the cysteine H" protons (measured between 276 and 308 K) follows Curie behavior (decreasing contact shift with increasing temperature). 

With these assignments at hand the analysis of the hyperfine shifts became possible. An Fe(III) in tetrahedral structures of iron-sulfur proteins has a high-spin electronic structure, with negligible magnetic anisotropy. The hyperfine shifts of the protons influenced by the Fe(III) are essentially Fermi contact in origin 21, 22). An Fe(II), on the other hand, has four unpaired electrons and there may be some magnetic anisotropy, giving rise to pseudo-contact shifts. In addition, there is a quintet state at a few hundred cm which may complicate the analysis of hyperfine shifts, but the main contribution to hyperfine shifts is still from the contact shifts 21, 22). 

Contact shifts give information on the electronic structure of the iron atoms, particularly on the valence distribution and on the magnetic coupling within polymetallic systems. The magnetic coupling scheme, which is considered later, fully accounts for the variety of observed hyperfine shifts and the temperature dependence. Thus, through the analysis of the hyperfine shifts, NMR provides detailed information on the metal site(s) of iron-sulfur proteins, and, thanks to the progress in NMR spectroscopy, also the solution structure 23, 24 ). 

By assuming that the hyperfine shifts are contact shifts in origin, it is possible to evaluate the hyperfine coupling constant from the following equation (50) ... 

Instead of measuring only the time-dependent dipolar interaction via NOE, it is also possible to determine dipolar couplings directly if the solute molecule is partially aligned in so-called alignment media. The most important resulting anisotropic parameters are RDCs, but residual quadrupolar couplings (RQCs), residual chemical shift anisotropy (RCSA) and pseudo-contact shifts (PCSs) can also be used for structure determination if applicable. 

This peak is broadened and contact shifted down field by the unpaired electron (Fig. 24). A spectrum of a mixture of methylcobinamide and free nitroxide shows broadening of the methyl resonance but no shift in resonance position. Thus the nitroxide must remain attached to the cobalt atom in solution. 

The observed ya-SCS(X) values in 7-exo-substituted norcaranes 97 (225) and those of M(CH3)3 (M = Si, Ge, Sn, or Pb) in cyclohexyl and bicy-clo[2.2. l]heptane derivatives (133) were later interpreted on the same basis. The back-lobe-overlap treatment was further supported by interpretations of H and 13C contact shifts of aliphatic amine signals in the presence of nickel acetyl-acetonate and by INDO calculations (226,227). Additional support came from extensive investigations of the structure dependence of three-bond coupling constants 3JCX (X = H, C, or F) (228,229), although the interpretation of these data has been subjected to criticism (230). 

On the other hand, if the chemical entity under consideration has a contribution of triplet or higher-spin configurations, its nmr signals are expected to suffer from contact shifts and broaden readily due to paramagnetic relaxation of the observing nuclear spins. It could be extremely difficult to observe nmr signals for such species under the above experimental conditions. 

No significant pseudo-contact shifts could be induced in the spectra of dibenzothiophene, its sulfoxide, or its sulfone, with Eu(dpm)3 although a marked shielding of the protons of one ring was observed in the spectrum of the chromium tricarbonyl complex of dibenzothiophene,presumably due to a contact shift mechanism. 

Comparative studies of the effects of (1 Ln = La Gd, Dy -> Yb R = R = Bu ) on the n.m.r. spectra of pyridine revealed that the Dy complex was the best high-field shift reagent whereas the Yb complex was the most effective low-field shift reagent. contact shifts were also observed in the spectra of [Bu4N]3[LnClg] (Ln = La Sm, Er, Tm, or Yb). The dpm complexes were also effective in causing shifts of the spectrum of 2,4,6-tri-fluoroaniline. °... 

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