Electron shielding field

Shift anisotropy. If the chemical shift depends upon molecular orientation, as it does in an aromatic ring, then this will also contribute to V t) as a result of motionally induced fluctuations in the electron shielding field. The contribution increases with and typically contributes 0.02 Hz to 1/Ti, in high-field spectrometers. Again, if it arises in partially oriented molecules then it affects lineshape, this time asymmetrically. This is discussed further in chapters 5 and 7. 

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 ... 

In the discussion to follow certain aspects of chemical shifts and nuclear spin-spin interactions will be reviewed. However, it is not our intention to give the theories of these effects with any degree of completeness since excellent treatments may be found in reviews and the original literature 108). We only hope here to point out some results and approaches of NMR particularly applicable to inorganic systems. Let it suffice at this point to say that chemical shifts of nuclei arise from shielding effects of nearby electrons. The field Hm seen by the nucleus is not the externally applied field 770, but... 

These solvent effects change the electron shielding around the nucleus in question, hence its response to the applied magnetic and electrical fields. The... 

This local magnetic field is slightly less than the applied magnetic field, B0, due to the effect of the electron cloud (bonding and nonbonding electrons) surrounding the nucleus. This cloud of electrons shields the nucleus from the applied magnetic field by a tiny factor, on the order of parts per million, of the applied field ... 

As discussed previously, the chemical shift of a hydrogen in a molecule is affected by the electrons surrounding it. The moving electrons generate their own small magnetic field that usually opposes the external magnetic field. The electrons shield the hy-... 

Fig. 5.4.1a-c. Basic NMR experiment, a Action of rf field B, (v,) on a nucleus (I = 1/2) preccssing about B0 with the Larmor frequency vn. When Vi = v0 change occurs in the orientation of p (+1/2 to -1/2). b Characterization of the two possible spin states, +1/2 and -1/2, by their energy levels -/i BC and //0B0, their energy difference, AE, and their spin populations, Na and Nfi. c Electronic shielding of the nucleus circulation of electrons under the influence of B , and the resulting secondary magnetic field B,... 

The actual field present at a given nucleus thus depends on the nature of the surrounding electrons. This electronic modulation of the Bq field is termed shielding, which is represented quantitatively by the Greek letter a. The actual field at the nucleus becomes B]ocai and may be expressed as Bq(1 - cr), in which the electronic shielding ct normally is positive. The variation of the resonance frequency with shielding has been termed the chemical shift. 

The small modification due to electronic shielding of the exterior field is negligible in most cases of interest here. As will be demonstrated below, at field strengths close to 30 kG as are generally used in rotational Zeeman effect studies, the magnetic field can uncouple the nuclear spin from the overall rotation. Thus, we will use the uncoupled basis in order to set up the matrix of... 

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