Contact contribution

The and operators determine the isotropic and anisotropic parts of the hyperfine coupling constant (eq. (10.11)), respectively. The latter contribution averages out for rapidly tumbling molecules (solution or gas phase), and the (isotropic) hyperfine coupling constant is therefore determined by the Fermi-Contact contribution, i.e. the electron density at the nucleus. 

The concept of spin-polarization has been found to be extremely useful for understanding the magnetic HFCs of organic radicals which are dominated by the Fermi contact contribution. The situation for transition metal complexes is rather different in several respects. The idea of spin-polarization is relatively simple and is best... 

Evaluation of trends in /pp coupling constants in solid-state 31P NMR spectra of P-phospholyl-NHPs allowed one to establish an inverse relation between the magnitude ofM and P-P bond distances [45], The distance dependence of. /pp is in line with the dominance of the Fermi contact contribution, and is presumably also of importance for other diphosphine derivatives. At the same time, large deviations between lJvv in solid-state and solution spectra of individual compounds and a temperature dependence of lJ77 in solution were also detected (Fig. 1) both effects... 

Since 3dyz/4s admixture is symmetry-forbidden for these radicals, the Fermi contact contribution to the isotropic coupling, As, must be entirely from spin polarization,... 

State the Scenario. By scenario is meant a detailed physical description of the sequence of events whereby a potential inadvertent combination of materials may occur. Details such as specific amounts of materials, temperature, confinement (closed or open system) and how long the materials will be in contact contribute to the definition of compatibility. 

A theoretical value for the magnitude of dJ/dEz was obtained using the delocalized molecular orbital approach of Gil and Teixeira-Dias 17> who calculated substituent effects on. The Pople expression for the contact contribution to the coupling constant 1 c-h1 °f a methyl group can be written... 

In the presence of contact contribution to nuclear relaxation, the NMRD profile results as a sum of the dipolar and contact relaxation rates. The profile of contact relaxation as a function of field is characterized by the presence of only one dispersion (Fig. 3), corresponding to the (OsT e dispersion (Eqs.(5) and (6)), in the hypothesis that Xg, = T,e (see Section II.B of... 

The functional form of the nuclear longitudinal relaxation immediately suggests that the contact contribution can provide the values of the contact coupling constant and of 72e = Tso, provided that the lifetime, xm, is longer than T e- No information on the field dependence of electron relaxation can be achieved. On the contrary the functional form of transverse nuclear relaxation contains a non-dispersive term, Tig. The latter, as we have seen for the dipolar contribution, increases with increasing the field (Fig. 3), and therefore the nuclear contact transverse relaxation also increases with increasing the field. Its measurement is thus informative on the t value. 

Ti, = 2-3 X 10 s (providing Xso = 3.5 x 10 s). The measurement of the transverse proton relaxation rate at high fields, in fact, permits to obtain the field dependence of the electron relaxation time from the contact contribution to relaxation. The constant of the contact interaction is calculated to be equal to 0.65 MHz. 

Table II reports the contact coupling constant for different aqua ion systems at room temperature. The contact coupling constant is a measure of the unpaired spin density delocalized at the coordinated protons. The values were calculated from the analysis of the contact contribution to the paramagnetic enhancements of relaxation rates in all cases where the correlation time for dipolar relaxation is dominated by x and Tig > x. In fact, in such cases the dispersion due to contact relaxation occurs earlier in frequency than the dispersion due to dipolar relaxation. In metalloproteins the contact contribution is usually negligible, even for metal ions characterized by a large contact contribution in aqua ion systems. This is due to the fact that the dipolar contribution is much larger because the correlation time increases by orders of magnitude, and x becomes longer than Tig. Under...

At variance with the aqua ion, in most manganese(II) proteins and complexes the contact contribution to relaxation is found negligible. This is clearly the case for MnEDTA (Fig. 33), the relaxivity of which indicates the presence of the dipolar contribution only, and one water molecule bound to the complex 93). Actually the profile is very similar to that of GdDTPA (see Chapter 4), and is provided by the sum of inner-sphere and outer-sphere contributions of the same order. The relaxation rate of MnDTPA is accounted for by outer-sphere relaxation only (see Section II.A.7), no water molecules being coordinated to the complex 94). 

A calculation of the contact contributions to nuclear spin coupling in the H-D molecule has been carried out (114) using the James-Coolidge wave function (55). This calculation gave... 

In other cases, such as in [Au(C6F5)(pzCFI2Fc)] [60], different types of van der Waals contacts contribute to the supramolecular three-dimensional structure. In the lattice, the molecules are associated into pairs via an intermolecular Au- Au interaction, and the pairs form chains held together by —H- Au and —H- F hydrogen bonds. The chains are further linked to form a three-dimensional structure through additional H F interactions, as shown in Figure 5.32. 

Several other studies concerning the proton magnetic resonance spectra of the XFe(R2Dtc)a complexes have been reported (114, 202, 284, 401) and the results have been interpreted in terms of current relaxation theory. In a number of these studies (114, 202, 498) the contact contribution to the isotropic contact shift of the proton resonances was considered to be controlled by the transmission of unpaired spin density through the n framework of the ligand. 

In aliphatic and cycloaliphatic compounds, vicinal carbon-proton coupling constants 3Jch are related to the dihedral angle 0, as known from the Karplus-Conroy relation for iJ[iH. The Fermi-contact contribution to 3JCH as a function of the dihedral angle 0 calculated for propane [131] is displayed in Fig. 3.15, and the Karplus relation given by eq. (3.17) can be derived ... 

I Vs,ms,1,M/)- In the high field approximation (ge/xsBo -4), which always holds for high field NMR experiments, the contact contribution to the chemical shift is (Appendix III) [1]... 

Let us first try to discuss the contact contribution to the shift for protons and deuterons. As we shall see, there are conditions under which the pseudocontact shifts are small and negligible, or can be determined separately and subtracted from the hyperfine shifts. Furthermore, in the case of a monodentate ligand which exchanges rapidly from bound to free, the average pseudocontact shift in the absence of geometric constraints is zero. 

Although the contact contribution can be sizable and sometimes dominant for, 70 and, 4N nuclei directly coordinated to the metal [81], it is often negligible for... 

In this approximation, gj is isotropic. We need to consider (Sz)j and not (Jz) because only the spin is delocalized onto the molecule [81], The contact contribution is therefore... 

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