Shielding field magnetic

Shielding effects (Section 14.3 A) An effect in NMR caused by small induced magnetic fields of electrons in the opposite direction to the applied magnetic field. Shielding decreases the strength of the magnetic field felt by the nucleus and shifts an absorption upheld. 

Table 3-19. Magnetic-Field Shielding Effectiveness of Three Typical Conductive Coatings...

Figure 1.18. The circulation of 7r-electrons around a benzene ring produces a magnetic field shielding (A) and deshielding (B) zones around benzene ring.

Relaxation by the spin-rotation mechanism can lead to 5/Ti correlations, as observed in Sn resonance for methyltin halides, since is directly related to the spin-rotation tensor, and the spin-rotation relaxation rate to its square. With the use of higher magnetic fields, shielding anisotropy relaxation is more often observed AajTi, T2 relationships are reported for Pt(IV) complexes with Pt anisotropies of 10 ppm. ... 

A SQUID [2] provides two basic advantages for measuring small variations in the magnetic field caused by cracks [3-7]. First, its unsurpassed field sensitivity is independent of frequency and thus dc and ac fields can be measured with an resolution of better than IpT/VHz. Secondly, the operation of the SQUID in a flux locked loop can provide a more than sufficient dynamic range of up to 160 dB/VHz in a shielded environment, and about 140 dB/>/Hz in unshielded environment [8]. 

The shielding at a given nucleus arises from the virtually instantaneous response of the nearby electrons to the magnetic field. It therefore fluctuates rapidly as the molecule rotates, vibrates and interacts with solvent molecules. The changes of shift widi rotation can be large, particularly when double bonds are present. For... 

The two primary causes of shielding by electrons are diamagnetism and temperature-independent paramagnetism (TIP). Diamagnetism arises from the slight unpairing of electron orbits under the influence of the magnetic field. This always occurs so as to oppose the field and was first analysed by Lamb [7]. A simplified version of his fomuila. 

Chemical shielding G So 102-10 Alteration of the magnetic field by the electrons... 

Acetylenic hydrogens are unusual in that they are more shielded than we would expect for protons bonded to sp hybridized carbon This is because the rr electrons circulate around the triple bond not along it (Figure 13 9a) Therefore the induced magnetic field is parallel to the long axis of the triple bond and shields the acetylenic proton (Figure 13 9b) Acetylenic protons typically have chemical shifts near 8 2 5... 

FIGURE 13 9 (a) The TT elec trons of acetylene circulate in a region surrounding the long axis of the molecule (b) The induced magnetic field associated with the tt electrons opposes the applied field and shields the protons... 

FIGURE 13 10 More shielded (red) and less shielded (blue) protons in (a) [18]annulene and (b) [16]annulene The induced magnetic field associated with the aromatic ring current in [18]annulene shields the inside protons and deshields the out side protons The opposite occurs in [16]annulene which is antiaromatic... 

Section 13 4 The energy required to flip the spin of a proton from the lower energy spin state to the higher state depends on the extent to which a nucleus is shielded from the external magnetic field by the molecule s electrons... 

Shielding (Section 13 4) Effect of a molecule s electrons that decreases the strength of an external magnetic field felt by a proton or another nucleus... 

---