The Direct Field Effect

The influence of the direct field effect on the hyperfine structure is considered in more detail in the following section. Some technical properties are summarized in Appendix A4.1. 

B- are the effective fields acting on the nucleus when tUg = +V2 and rus = -Vi, respectively 

This type of spectra are frequently observed at X-band for n-electron radicals of the common type H—Ca. The appearance of the spectra is different for weakly coupled nuclei, in which case so-called spin flip lines appear. 

Nuclear spin flip (Am/ = 1) lines occur in systems with anisotropic hyperflne couplings of smaller magnitude than the nuclear Zeeman energy. In this case satellite lines appear on each side of an often unresolved doublet of central lines. At X-band the satellites are separated approximately by Bn = 0.5 mT from the 

The intensity ratio between the main and satellite lines has been used to estimate the distance between a paramagnetic centre and protons at surrounding molecules. The method is based on the assumptions that the point dipole approximation applies for the anisotropic hfc, and that this coupling is much smaller than the nuclear Zeeman energy of the proton. It was found advisable to measure this ratio at as high microwave frequency as possible to achieve the latter condition, i.e. Ba Bn in terms of the direct field model. The procedure was adequate at Q-band but not at X-band to obtain the distance between a proton and the P03 radical in a single crystal of Na2HP03-51120, see [37] for details. 

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Three aspects of the inductive effect have to be considered the inductive effect, the inducto-electromeric or rr-inductive effect, and the direct field effect. The first of these is the one most frequently... 

Excluding the phenomenon of hyperconjugation, the only other means by which electronic effects can be transmitted within saturated molecules, or exerted by inductive substituents in aromatic molecules, is by direct electrostatic interaction, the direct field effect. In early discussions of substitution this was usually neglected for qualitative purposes since it would operate in the same direction (though it would be expected to diminish in the order ortho > meta > para) as the cr-inductive effect and assessment of the relative importance of each is difficult however, the field effect was recognised as having quantitative significance. ... 

Earlier in this book ( 7.2.1) the direct field effect was mentioned in rather general terms. The foregoing discussion brings out the fact that there are three aspects of the effect which have to be considered ... 

Ingold introduces the terms substrate field effect and reagent field effect to describe those aspects of the direct field effect numbered (z) and (3) in 9.1.2. His description of the substituent effect of the trimethylammonio group is thus given substantially in terms of the substrate field effect and the TT-inductive effect, i.e. it is an isolated molecule description. The reagent field effect is seen to be significant in nitration and to produce qualitatively the same 226... 

The greater sensitivity of the alkaline reaction to changes in substituent is readily explained in this way —the comparison of cis and trims compounds isolates not only the steric effect but also the direct field effect —but the correlation is poor with steric effects in this system. 

E4.7 Suggest a reason why the direct field effect causes complications in ESR but not in ENDOR. 

Pyroelectrics. Pyroelectric ceramics are materials that possess a uoique polar axis and are spontaneously polarized ia the abseace of an electric field. Pyroelectrics are also a subset of piezoelectric materials. Ten of the 20 crystal classes of materials that display the piezoelectric effect also possess a unique polar axis, and thus exhibit pyroelectricity. In addition to the iaduced charge resultiag from the direct pyroelectric effect, a change ia temperature also iaduces a surface charge (polarizatioa) from the piezoelectric aature of the material, and the strain resultiag from thermal expansioa. 

Meyers212 collected together several examples of the anomalous behavior of sulfones in which the group was close to the reaction center and interpreted these in terms of the negative direct field effect of the S02 oxygen atoms (cf. Meyers and colleagues141). 

Direct conversion of the 6-sulfonates of D-galactopyranosides into their 6-deoxy-6-fluoro derivatives is usually hindered, because of the polar field-effect exerted by the lone electron pairs of the axial 0-4 (Ref. 158). Thus, the 6-0-mesylgalactoside 294 gave ° the 3,6-anhydro derivative 297 (23%) as the only isolated product on reaction with CsF in boiling ethanediol. How-... 

In short fiber composites, energy absorption mechanisms, such as interfacial debonding and matrix cracking, most often occur at the fiber ends (Curtis et al., 1978). The damage model proposed by Bader et al. (1979) assumes that short fiber composites fail over a critical cross-section which has been weakened by the accumulation of cracks, since the short fibers bridging this critical zone are unable to carry the load. In fatigue loading, sudden fracture takes place as a direct result from the far-field effect of the composite, rather than due to the near field of the crack tip... 

An appreciation of the crystal field effect on the vibrations of the Bravais cell which is repeated to build the crystal is extremely important when interpreting the vibrational spectra of many substances, since in the presence of a crystal field influence the number of observed bands in the spectrum cannot be directly determined from the formula unit which goes to make up the unit cell. In other words, there is almost always a larger number of bands to account for when investigating solid state samples. The solid state effects often cause degenerate bands to split in the same degree as symmetric and antisymmetric stretching modes split. 

Piezoelectric materials are materials that exhibit a linear relationship between electric and mechanical variables. The direct piezoelectric effect can be described as the ability of materials to convert mechanical stress into an electric field and the reverse, to convert an electric field into a mechanical stress. The use of the piezoelectric effect in sensors is based upon the latter property. 

Piezoelectric materials are materials that exhibit a linear relationship between electric and mechanical variables. Electric polarization is proportional to mechanical stress. The direct piezoelectric effect can be described as the ability of materials to convert mechanical stress into an electric field, and the reverse, to convert an electric field into a mechanical stress. The use of the piezoelectric effect in sensors is based on the latter property. For materials to exhibit the piezoelectric effect, the materials must be anisotropic and electrically poled ie, there must be a spontaneous electric field maintained in a particular direction throughout the material. A key feature of a piezoelectric material involves this spontaneous electric field and its disappearance above the Curie point. Only solids without a center of symmetry show this piezoelectric effect, a third-rank tensor property (14,15). 

The term piezoelectric nonlinearity is used here to describe relationship between mechanical and electrical fields (charge density D vs. stress a, strain x vs. electric field E) in which the proportionality constant d, is dependent on the driving field, Figure 13.1. Thus, for the direct piezoelectric effect one may write D = d(a)a and for the converse effect x = d(E)E. Similar relationships may be defined for other piezoelectric coefficients (g, h, and e) and combination of electro-mechanical variables. The piezoelectric nonlinearity is usually accompanied by the electro-mechanical (D vs. a or x vs. E) hysteresis, as shown in Figure 13.2. By hysteresis we shall simply mean, in the first approximation, that there is a phase lag between the driving field and the response. This phase lag may be accompanied by complex nonlinear processes leading to a more general definition of the hysteresis [2],... 

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