Chemical Shifts and Shielding Effects

Chemical Shifts and Shielding Effects.—Phosphorus-31. The positive shifts (<5p) which are reported in this chapter are upfield from 85 % phosphoric acid. 

Applications including Chemical Shifts and Shielding Effects... 

Chemical Shifts and Shielding Effects.—Phosphorus-31. The sign convention used for expressing shifts in this Report is not the same as was used in earlier volumes. Positive chemical shifts are now downfield from 85 % phosphoric acid, and are given without the appellation p.p.m. Since both conventions are in use, it remains necessary to state the sign convention used in each paper published. 

Chemical Shifts and Shielding Effects.—Chemical shifts are usually given without the appellation p.p.m. The scope of n.m.r. pulsed methods has been reviewed. ... 

Applications including chemical shifts and shielding effects... 

Chemical Shifts and Shielding ElSects.—Phosphorus-31. In this section, positive P chemical shifts (dp) are upheld from 85% phosphoric acid. A linear relationship between dp and vapour pressure for elemental phosphorus in the gas phase has been reported. Solvent effects have also been examined. dp compounds. The n.m.r. parameters of various protic and deuteriated... 

Chemical Shifts. The shielding effect of spherically symmetrical s electrons is discussed in Section 18.3.2.1. This diamagnetic upfield shift affects all nuclei since every molecule has s electrons. For electrons in /t-orbitals there is no spherical symmetry and the phenomenon of diamagnetic anisotropy is used to explain some otherwise... 

Vicinal Fluorine Substituents. The examples provided in Scheme 3.15 exemplify the specific effect of vicinal fluorine substituents on both chemical shift and upon spin-spin coupling constants. As indicated in Section 3.3.1 (Scheme 3.12) each of the fluorines is somewhat shielded by the presence of the other. 

The deuterium isotope effects on chemical shift consists of intrinsic isotope effect (direct perturbation of the shielding of X atom) and equilibrium isotope effect (perturbation of the equilibrium caused by the isotopic substitution). The values of deuterium isotope effects are to some extent independent of chemical shifts and allow determination of the mole fraction of the forms in equilibrium. 

Results obtained from the alkali iodides on the isomer shift, the NMR chemical shift and its pressure dependence, and dynamic quadrupole coupling are compared. These results are discussed in terms of shielding by the 5p electrons and of Lbwdins technique of symmetrical orthogonalization which takes into account the distortion of the free ion functions by overlap. The recoilless fractions for all the alkali iodides are approximately constant at 80°K. Recent results include hybridization effects inferred from the isomer shifts of the iodates and the periodates, magnetic and electric quadrupole hyperfine splittings, and results obtained from molecular iodine and other iodine compounds. The properties of the 57.6-k.e.v. transition of 1 and the 27.7-k.e.v. transition of 1 are compared. 

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... 

Figure 9.11—Relationship between chemical shifts, magnetic field, and shielding effects (proton NMR).

Rapid molecular motions in solutions average to zero the dipolar and quadrupolar Hamiltonian terms. Hence, weak interactions (chemical shift and electron-coupled spin-spin couplings) are the main contributions to the Zeeman term. The chemical shift term (Hs) arises from the shielding effect of the fields produced by surrounding electrons on the nucleus ... 

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