NMR contact shift

When Con,-EDDA-complexes are synthesized under conditions favoring equilibrium state, the cis-a-compound is essentially obtained72. Cis-(3-complexes can also be obtained, most favorably as mixed ligand complexes with a bidentate ligand, e.g. carbonate, oxalate or 1,2-diamines73-75. But in all cases known, it is the cis-a-isomer which is formed in a greater amount. Reilley and co-workers conclude from the simplicity of the NMR-contact shift spectra of [Ni(EDDA)(H20)2] that the compound is present in only one isomeric form, and assume that this is the ezs-a-form28. ... 

Owen and Thomley (547) have reviewed covalency in transition metal ions and, in particular, in nickel complexes. The NMR contact shift method has been used by Eaton et al. (195) to determine spin densities on organic ligands of paramagnetic molecules. In particular (194), a series of nickel aminotroponeiminates have been studied whereby conjugative and hyperconjugative effects within the molecule may be monitored. Similar studies of contact shifts have been carried out on Co and Ni pyrromethenes and porphyrins (196) and on many transition metal acetylacetonates (193) by Eaton et al. 

Isotropic coupling constants are in agreement with those obtained from the NMR contact shifts within experimental error except for three coupling constants, for which the corresponding values from the NMR contact shifts are given in parentheses for comparison. 

Morishima et al. (171) measured NMR contact shifts for Ni (2-I-) (acac)2 complexes of piperidine(I), 4-methylpiperidine (II), iV-methyl-piperi-dine(III), 1,4-dimethylpiperidine(IV), and quinuclidene(V). The P carbon atom of the ligand in complexes (III) and (IV) show an attenuation of the contact shift relative to this same carbon in (I) and (II). The authors rationalized this discrepency on the basis of the orientation of the lone pair on nitrogen. For complexes (I) and (II) the lone pair prefers an equitorial position, whereas, in (III) and (IV) it prefers an axial position. For (I) and (II) a zig-zag path of distribution is necessary. Apparently (see data in Table LVII) the zig-zag path is more favorable. For quinuclidene (V) the y carbon shows a downfield shift (positive spin density) (opposite to I-IV). The downfield shift may be accounted for by spin delocalization through space involving the lone-pair electrons. 

Anderson and Matwiyoff(4) have obtained NMR contact shifts for the l,l -dimethylmetallocenes of vanadium, chrominium, cobalt, and nickel. V and Ni dimethylmetallocenes possess A g ground states and onpiopeotd should be negligible as a result. To determine the relative importance of and the value of was determined and compared with the theoretical... 

The NMR contact shifts of chromocene and vanadocene indicate the presence of negative spin density (upheld shift) again probably due to the Levy-Orgel mechanism (32). Burkert et al. (32) also measured contact shifts for cobaltocene and nickelocene both of which possess positive spin density (downfield shift)—see Table LIX. 

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... 

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 ). 

As already mentioned, chiral cations are involved in many areas of chemistry and, unfortunately, only few simple methods are available to determine their optical purity with precision. In the last decades, NMR has evolved as one of the methods of choice for the measurement of the enantiomeric purity of chiral species [ 110,111 ]. Anionic substances have an advantage over neutral reagents to behave as NMR chiral shift agents for chiral cations. They can form dia-stereomeric contact pairs directly and the short-range interactions that result can lead to clear differences in the NMR spectra of the diastereomeric salts. 

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. 

A systematic carbocation concentration dependency study on NMR chemical shifts was performed for the C-l-protonated 477-cyclopenta[fi e/ phenanthrenium cation 7H+ and the C-l-protonated pyrenium cation 2H+ (Fig. 11). Shielding of the PAH arenium ion protons and carbons was observed with decreasing FSO3H PAH ratios without noticeable line-broadening. This was attributed to cation-anion interactions in the low FSO3H PAH domain and possible formation of contact ion... 

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