Transition metals NMR chemical shifts

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

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. 

Different methods of computation have been performed for comprehensive investigations on the thermal effects and vibrational corrections to transition metal NMR chemical shifts. Studies related to TiClxMe4 x (v = 0-3) are included.240... 

Buhl, M. Correlation between Transition-Metal NMR Chemical Shifts and Reactivities , in Modeling NMR Chemical Shifts, Gaining Insights into Structure and Environment (ACS Symposium Series, 732) Facelli, J. C., Ed., ACS Washington DC, 1999, pp 240-250. 

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])...

Juranic N (1989) Ligand field interpretation of metal NMR chemical shifts in octahedral d transition metal complexes. Coordination Chemistry Reviews 96 253-290. 

The methods listed thus far can be used for the reliable prediction of NMR chemical shifts for small organic compounds in the gas phase, which are often reasonably close to the liquid-phase results. Heavy elements, such as transition metals and lanthanides, present a much more dilficult problem. Mass defect and spin-coupling terms have been found to be significant for the description of the NMR shielding tensors for these elements. Since NMR is a nuclear effect, core potentials should not be used. 

The formation of monomer and dimer of (salen)Co AIX3 complex can be confirmed by Al NMR. Monomer complex la show Al NMR chemical shift on 5=43.1 ppm line width =30.2 Hz and dimer complex lb 5=37.7 ppm line width =12.7 Hz. Further instrumental evidence may be viewed by UV-Vis spectrophotometer. The new synthesized complex showed absorption band at 370 nm. The characteristic absorption band of the precatalyst Co(salen) at 420 nm disappeared (Figure 1). It has long been known that oxygen atoms of the metal complexes of the SchifT bases are able to coordinate to the transition and group 13 metals to form bi- and trinuclear complex [9]. On these proofs the possible structure is shown in Scheme 1. 

Table 11-3. Absolute 170 NMR chemical shifts [ppm] in transition metal oxo complexes.

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... 

A different issue associated with NMR chemical shifts for heavy atoms is the influence of relativistic effects. In terms of computing absolute chemical shifts, relativistic effects can be very large in heavy elements. For relative chemical shifts, since relativistic effects are primarily associated with core orbitals, and core orbitals do not change much from one chemical environment to the next, the effect is typically markedly reduced. Nevertheless, accurate calculations involving atoms beyond tlie first row of transition metals are still a particular challenge. 

C NMR Chemical Shifts for Selected Anionic and Neutral Metal Carbonyls of Some Early Transition Elements... 

Chiral Metal Atoms in Optically Active Organo-Transition-Metal Compounds, 18, 151 13C NMR Chemical Shifts and Coupling Constants of Organometallic Compounds, 12, 135 Compounds Derived from Alkynes and Carbonyl Complexes of Cobalt, 12, 323 Conjugate Addition of Grignard Reagents to Aromatic Systems, I, 221 Coordination of Unsaturated Molecules to Transition Metals, 14, 33 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365... 

TABLE 2. Selected 29Si NMR chemical shifts for group-4 transition-metal silyl complexes"... 

TABLE 6. Selected 29 Si NMR chemical shifts for group-6 transition-metal silylene and silyl complexes... 

In the last few years, DFT has also become one of the prime methods for the study of nuclear magnetic resonance (NMR) chemical shifts in transition metal complexes and other large molecules. DFT calculations of NMR chemical shifts have been reviewed (3,4). 

Keywords Density functional calculations Transition metal complexes NMR chemical shifts Nuclear spin-spin coupling constants Relativistic quantum chemistry... 

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