Quadrupolar metallic nuclei

The second Group Vb transition metal nucleus, Nb, has also been fairly extensively studied. Nb has spin / = (100%), a magnetic moment of 6144/In, and a quadrupolar moment of —0-22 barn. 

In some cases, when a spin-1/2 nucleus such as P, or is coupled to a quadrupolar nucleus, the simulation by a computer program of the unusual lineshape of the spin-1/2 may provide information about the chemical shift anisotropy, the quadrupole coupling constant and the indirect scalar spin-spin coupling constant involving the metal atom. This method has been applied to the P spectra of phosphines bound to cobalt in heteronuclear clusters [17]. 

In addition to mechanistic studies, Co is the transition metal that has found most widespread use in thermal measurements. Since it is a quadrupolar nucleus, a symmetric chemical environment around the central cobalt nucleus is necessary to observe narrow lineshapes for precise temperature measurements. Co... 

As mentioned above, the alkali metal chlorides have cubic symmetry about the chlorine nucleus which requires that the EFG to be essentially zero in a perfect crystal. This results in CT NMR spectra with narrow lines that are free of quad-rupolar effects. As the environment aroimd the chlorine nucleus is transformed to lower symmetry, second-order quadrupolar effects begin to be observed, leading to broadened CT lines with quadrupolar line shapes. The broadening of the signals in chlorine CT NMR spectra as the quadrupolar effects become more significant is the most serious limitation to the types of materials which can be studied with typical SSNMR methods. 

The spectroscopy of multinuclear magnetic resonance in solution is one of the most important analytical methods in structural studies of transition metal hydride complexes. Among different nuclei, the proton plays the main role because H NMR provides a direct information about spectral properties of the hydride ligands. In addition the proton, being a non-quadrupolar and long-relaxing nucleus, gives rise to well-resolved NMR spectra which are very... 

NMR resonance of solid platinum metal. The chemical shift of 42.S MHz lies between those of platinum metal (41.5 MHz) and HaPtQt (43.0 MHz), and is interpreted as being a consequence of the submetallic nature of the 1.6 nm platinum particles. Thus the size issue for the transition from metallic properties to molecular properties is introduced. The symmetry of the band is surpriang since more than 50 % of the platinum atoms are surface atoms at this particle size and might be expected to be resolved from the resonances for the internal platinum atoms. [234, 236] The surface atoms can, in fact, be resolved after the addition of CO to the colloid, which results in the appearance of a low intensity resonance at 42.46 MHz (Fig. 6>29b), that is, up field (lower frequency) of the main band. This signal is assigned to the surface platinum atoms as the adsorption sites for CO. Displacement of CO by iodide broadens this resonance through the interaction with the quadrupolar iodide nucleus. 

A second clue as to the relaxation mechanism in paramagnetic systems may be obtained from measurements of chemical shift changes. Chemical shift changes observed upon addition of a metal ion for a rapidly exchanging halide nucleus in solution should be due to differences in chemical shifts in the "free" and complexed sites. Chemical shifts in diamagnetic halide complexes are not known. From Table 3.3 it can be estimated that they may at most be of the order of a few hundred ppm. Due to quadrupolar relaxation effects it can be envisaged that it should be very difficult to determine these shifts experimentally. No shift changes have so far been reported for Cl and Br in... 

In cases where molecular motion is restricted [wr is not 1) the situation is more complex. Such cases arise when the alkali metal is bound to the surface of a large molecule such as a protein or membrane surface and thereby has its motion restricted. The quadrupolar interaction with the nucleus shifts the energies of the Zeeman levels according to the square of the quantum number to a first approximation. Thus, the energy level splittings for a nucleus with 1 = (e.g. Li, Na, and Tlb) become as illustrated in Figure 1. With rapid isotropic motion, as described above, the multiple line pattern will... 

One can get an idea of the magnitude of the quadrupolar coupling constant x from solid state studies (Table 18). The very small values of the quadrupolar coupling constant x for l Cs is in line with the tiny quadrupole moment of this nucleus. For the other nuclei listed in Table 18, values of the quadrupolar coupling constants in a pure solvent are probably somewhat smaller than the solid state values, because symmetrical solvation of the alkali metal cation should reduce the magnitude of the efg q. Hence, the values in Table 18 should be taken as upper limits. For instance, solution studies imply quadrupolar coupling constants in the range 0.2-2 MHz (72). 

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