Shielding definition

Fig. 2. Classification/nomenclature of host—guest type inclusion compounds, definitions and relations (/) coordinative interaction, (2) lattice barrier interaction, (J) monomolecular shielding interaction (I) coordination-type inclusion compound (inclusion complex), (II) lattice-type inclusion compound (multimolecular/extramolecular inclusion compound, clathrate), (III) cavitate-type inclusion compound (monomolecular/intramolecular inclusion...

NMR data for 4-methyloxazole have been compared with those of 4-methylthiazole the data clearly show that the ring protons in each are shielded. In a comprehensive study of a range of oxazoles. Brown and Ghosh also reported NMR data but based a discussion of resonance stabilization on pK and UV spectral data (69JCS(B)270). The weak basicity of oxazole (pX a 0.8) relative to 1-methylimidazole (pK 7.44) and thiazole (pK 2.44) demonstrates that delocalization of the oxygen lone pair, which would have a base-strengthening effect on the nitrogen atom, is not extensive. It must be concluded that not only the experimental measurement but also the very definition of aromaticity in the azole series is as yet poorly quantified. Nevertheless, its importance in the interpretation of reactivity is enormous. 

Fundamental constants (Cx), spatial tensors in the principal axis frame ((fi3 m,)F), and spin tensors (Tjm) for chemical shielding (a), J coupling (J), dipole-dipole (IS), and quadrupolar coupling (Q) nuclear spin interactions (for more detailed definition of symbols refer to [50])... 

Typical sections through vent panels evaluated in the suppressive shields program are shown in Fig. 36, together with definitions of vent area ratios which were found to correlate with attenuation of transmitted blast waves. 

Figure 36. Definition of Vent Area Ratios for Various Suppressive Shield Structural Configurations. (Ref. 38)...

In order to extract some more information from the csa contribution to relaxation times, the next step is to switch to a molecular frame (x,y,z) where the shielding tensor is diagonal (x, y, z is called the Principal Axis System i.e., PAS). Owing to the properties reported in (44), the relevant calculations include the transformation of gzz into g x, yy, and g z involving, for the calculation of spectral densities, the correlation function of squares of trigonometric functions such as cos20(t)cos20(O) (see the previous section and more importantly Eq. (29) for the definition of the normalized spectral density J((d)). They yield for an isotropic reorientation (the molecule is supposed to behave as a sphere)... 

The shielded chemical shift is in turn due to the strong magnetic anisotropy of the sp -hybridized carbons. All the above dications are definitively classical, nonbridged cations, as also shown by the chemical shift additivity criterion of Schleyer and coworkers. 

I and the shielded nuclide 136Cs correlations have definite curvature. 

The conclusion appears to be that furan, if regarded as an aromatic compound, probably has less than about 20% of the aromaticity of benzene, and perhaps has none. Only a definition of aromaticity that identifies it with NMR shielding by ring currents, and little else, clearly suggests an aromaticity of 60% or more. 

We shall briefly review some definitions to compute molecular magnetic properties in the presence of a static electric field, L e., hypermagnetizabilities and shielding polarizabilities, Zafr and a1. ... 

Table IV. Shielding increments (Ac, in ppm) of selected protons over a carbon-carbon double bond on structures in Figure 2 calculated by GIAO, predicted by equation 2, and experimentally observed. See text for definition of Ac.

Now, by its very definition, the global hardness rj is a measure of the HOMO-LUMO gap of a compound. Consequently, it seems reasonable to assume that the AEE AE should scale with the global hardness. The <(ag/r)3> term describes the p-orbital expansion or contraction as electrons are added or removed from the shielded nucleus. Using effective atomic numbers Znp and Slater rules ... 

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