Metal NMR

Buhl, M., 1997, Density Functional Calculations of Transition Metal NMR Chemical Shifts Dramatic Effects of Hartree-Fock Exchange , Chem. Phys. Lett., 267, 251. 

Despite the difficulties in explaining the metal NMR shifts, it is still worthwhile measuring them because the huge chemical shift range makes it easy to observe the different species present, for example diastereoisomers [18]. 

Transition metal NMR studies of organometallic and inorganic coordination compounds... 

Transition metal NMR spectroscopy is not only a valuable source of structural information, but metal chemical shifts may also permit predictions of the reactivity and possibly even catalytic activity of a complex.51 The key to such analyses is frequently the observation of correlations of metal chemical shifts... 

Compound Calculated Metal Shielding (ppm) Metal NMR Chemical Shiftb (ppm) Calculated ... 

Correlations between Transition-Metal NMR Chemical Shifts and Reactivities... 

Modem methods based on density-functional theory (DFT) can describe relative activation barriers of organometallic reactions, i.e. relative reactivities, as well as the transition-metal NMR chemical shifts of the reactant complexes involved. It is thus possible to reproduce or rationalize observed correlations between these properties or to predict new ones. NMR/reactivity correlations that could be reproduced theoretically ("intrinsic correlations") are summarized. Newly predicted NMR/ reactivity correlations are discussed for the ethylene polymerization with V(=0-X)R3 or V(=Y)R3 catalysts. When X or Y are varied (X = A1H3, Li+, SbF5, H+ Y = NH, O, S, Se), both... 

In the preceding chapter it has been shown that the DFT methods currently available can be used to reproduce relative trends in both reactivities and transition-metal NMR chemical shifts. Thus, NMR/reactivity correlations can be modeled theoretically, at least when relative reactivities are reflected in relative energies on the potential energy surfaces (activation barriers, BDEs). It should in principle also be possible to predict new such correlations. This is done in the following, with the emphasis on olefin polymerization with vanadium-based catalysts. 

Whereas some interesting 15N-FTNMR complexation studies have been carried out by Roberts (52), more promising results have been obtained from alkali metal NMR (53-57). The nuclear and spectral properties of the alkali and alkaline earth elements are shown in Table... 

As with any electrolyte, various aggregate species are expected to form as the concentration of solute increases. In particular, both the electrical conductivity and (metal) NMR data (Fig. 4) signal the appearance of neutral species at metal concentrations in excess of 103 MPM (37), the conductivity via a Morse-like behavior in the equivalent conductance, the magnetic resonance via a finite Knight (contact) shift... 

The application of DFT methods to the computation of transition-metal NMR has been reviewed in the past [1-4]. A short overview was recently prepared by Buhl [5], NMR calculations on heavier transition-metal complexes have further been discussed in reviews devoted to relativistic NMR methodology [6-9], Thus, the present overview does not attempt to give a full coverage of the available literature, but to present a number of illustrative examples, the present status of such computations and their accuracy and limitations, along with a description of the underlying methodology. Because of the high importance of relativistic effects on NMR parameters, which is clearly represented in the available literature on DFT NMR computations of transition-metal complexes, the reader will find that a substantial portion of this paper is devoted to this topic. 

Fig. 5 DFT computed metal NMR chemical shifts (B3LYP hybrid functional) for a large number of inorganic and organometallic transition-metal compounds. The dashed line is not a fit but indicates where dcaic = <5expt.- (Graphics courtesy of Btlhl [5])...

The chemistry of the group 13 metals makes binding studies difficult, as careful control of pH and bicarbonate concentration is necessary to prevent formation of species such as Al(OH)4 and Ga(OH)4". Nevertheless, UV difference spectra have shown that Al3+, Ga3 +, In3+, and Tl3 all form transferrin complexes with two metal ions per molecule (126, 144-146). Ae values imply the ionization of two tyrosines per bound metal ion, as for other specifically bound metals. NMR studies using 13C-enriched bicarbonate show virtually identical spectra for Al3+ and Ga3+, implying equivalent metal-anion environments (99). 

Of special interest is the first recording of a cyclopropene C(metal) NMR signal which appeared at d 126.2 ppm (t, /c-p 23.0 Hz), considerably lower than the (5108.7 ppm observed for cyclopropene. ... 

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