Electric-field effect shielding

Protonation of the amino group usually induces a shielding of all carbon atoms, explained in terms of electric field effects [84] and CH bond polarizations [338], and follows the pattern observed on protonation of carboxylate anions (Section 4.7.4). 

It can be seen from Table 4.78 that the a increments can be reasonably rationalized in terms of inductive effects (Pauling electronegativities), while the shielding of carbons y to the substituent is generally attributed to a steric polarization of the yC — H bond. Inductive and electric field effects contribute to the f) increments. As electric fields can be evaluated only in rare cases, no general trend for the [) effect has been recognized so far. Frequently, the a and increments depend on whether a substituent X is terminal (n) or central (iso). If available, the iso increments are also given in Table 4.78. 

The intermolecular shielding surface and the effects of electric fields on shielding (shielding polarizabilities and hyperpolarizabilities) are useful in the interpretation of long-range intramolecular effects and in intermolecular effects on shielding. 

The effect of nitrogen quaternization on the chemical shifts of several aporphine alkaloids was studied by Marsaioli et al. (39). The conversion of dicentrine (59) to its methiodide salt (60) caused deshielding of C-5 and C-6a whereas C-3a, C-4, C-7, C-7a, and C-llc were all shielded. The shielding of the aromatic carbon atoms may be caused by the electric field effect similar to that observed in nitrogen protonation (8, 12). C-4 and C-7 were most likely experiencing the y steric effect of the new methyl group. 

Nymand et al. ° performed molecular dynamics simulations on liquid water, and they used the electric field effect formalism [Eq. (6)] to explain the gas to liquid shifts of the and O nuclei. For the proton it turned out that the resulting gas to liquid shift of — 3.86 ppm at 300 K compared well with the experimental value of —4.70 ppm, whereas for O the method failed to reproduce the experiment. Even if electric field gradient terms are introduced, requiring additional quadrupolar shielding polarizabilities, no better results could be obtained for the O gas to liquid shifts. Isotropic proton chemical shifts are obviously a special case where many higher order terms cancel, hence it is justified to use the simple electric field equations in these chemical shift calculations. 

Thus, efficient decoupler operation is crucial especially at the higher frequencies where certain samples may absorb more power than at the lower frequencies. Sample heating effects for ionic solutions have been discussed by Led and Petersen (1978) and by Bock, et al. (1980) and a more efficient coil design by Alderman and Grant (1979). A key fact to remember is that ionic heating is an electric field effect like the piezoelectric resonance discussed in VI.B.5. so that some sort of an electrostatic shielding as described there should work here, too. 

Grayson, M., Raynes, W. Electric field effects on the shielding of protons in C-H bonds. Magnetic Resonance Chem. 1995, 33,138M3. 

A (i) hyperfine (electron-nucleus) interaction constant (ii) parameter relating to electric field effects on nuclear shielding... 

Following the literature, we have here mainly discussed the halide shielding in terms of the Kondo - Yamashita overlap mechanism. While this gives a reasonable rationalization of most experimental observations it is important to further consider alternative interpretations, for example to study in more detail electric field effects (cf. Ref. [366]). 

Conformation and Chemical Shifts.—Chemical shifts have been correlated with conformation for H shifts in polystyrene and poly(vinyl chloride) and C shifts in model compounds of polypropylene. In all these papers, the chemical shift is related empirically to the occurrence of three- and four-bond steric interactions, similar to those used in the rotational isomeric state treatment of polymer statistical mechanics, " and the shift is expressed as a sum of compositional and conformational increments. The origin of the shielding contributions (magnetic anisotropy, electric field effects, etc.) is not stated, except for polystyrene in which the magnetic anistropy of the aromatic rings are incorporated... 

Further medium contributions to nuclear magnetic shielding arise from the van der Waals interactions, " electric field effects (if electric moments, polar groups, or... 

As mentioned earlier, eqn [8] enables one to estimate the mean molecular dipole moment, (/r y from dielectric spectra, given that (1) the dipoles do not interact with each other (trae only for very diluted systems), and (2) the local field effects (shielding of the outer electric field) are negligible. Onsager treated the problem of the local field effects for polar molecules by considering the enhancement of the permanent dipole moment of a molecule, //, residing in a spherical cavity of a dielectric continuum, by the polarization of the environment. The theory gives... 

An important characteristic of plasma is that the free charges move in response to an electric field or charge, so as to neutralize or decrease its effect. Reduced to its smaUest components, the plasma electrons shield positive ionic charges from the rest of the plasma. The Debye length, given by the foUowing ... 

This shows that the second and further solvation shells still have a non-negligible effect on NMR chemical shielding constants through the long-range electrical field they create. The approximation of an isolated molecular cluster in vacuo is valid for large clusters only this eventually makes determination of the shieldings of all protons computationally much more expensive than the fully periodic ab initio calculation. 

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