Isotropic shielding

The NMR magnetic shielding for atoms like carbon is affected greatly by what it is bonded to and the type of bond to its neighbor. Use the inner carbon atoms of normal butane as the reference atom and calculate the shift in C isotropic shielding for 2-butene and 2-butyne. Can you explain these shifts as a function of the changing molecular environments ... 

Isotropic shielding constants for the heavy elements are listed in Table 1 and show a consistent picture. In aU cases we find that relativity reinforces the shielding. 

Table 1. Isotropic shielding constants (ppm) for F, I and Xe calculated with different response formalisms (see text)...

The chemical shift of a given proton is thus determined by a combination of isotropic shielding by die electron cloud surrounding die proton and by... 

A case study of the basis set dependence of the anisotropy and the isotropic shielding has been presented by Enevoldsen and Oddershede (55) for 19F in the CH3F molecule. The RPA (SCF level) results are shown in Tables IV and V. 

Table I. Major LMO contributions to 13Ca isotropic shielding in V-formyl-glycine amide, V-formyl-alanine amide, V-formyl-valine amide, and V-formy 1-phenylalanine amide (deMon program, PW91, uniform IGLOII basis set)... 

For Cp in alanine, there are four main LMO contributions to shielding, from Ca and from the three Hp atoms. The three H atoms contribute differently to Cp shielding, as shown in Figure 3, but when these contributions to C13 shielding are combined with those of Cp-Ca, we find good accord with the total overall isotropic shielding surface, shown in Figure 4. 

Figure 3. Deviations of the theoretical 19F isotropic shielding values for C6H5F (relative to CFC13) from the corresponding experimental chemical shift of 113.12 ppm (23).

In-plane components (a22 and a33). The feature of conformation dependence of the g22 and g33 components is better understood by taking p-IONONE as an example. Here, the conformation-dependent shifts of C5 and C8 are examined. As can be seen from Figure 6, the isotropic shieldings of both carbons exhibit clear periodic dependence on the twist angle about the C6-C7 bond,cp6.7. For example, the a-cp6.7 curve of C5 appears to be approximated by a sine wave whose period is 180°. The curve for C8 shifts in phase by a half period relative to that for C5. These curves are expressed by using the Fourier series of expansion as follows ... 

Figure 6. Conformation dependence of the isotropic shieldings of C5 (left) and C8 (right) of (3-IONONE.

The effect of basis set superposition error (BSSE) was estimated for this series using the counterpoise method of Boys and Bemardi (16), The isotropic shielding value of methane by itself was calculated to be 31.8276 ppm. Isotropic shielding values corrected for BSSE for the proximal proton of methane were calculated for each distance from ethene (Table I). The difference between the isotropic shielding value calculated for methane alone and the isotropic shielding values at each geometry for the proximal proton of methane obtained for the methane-ethene pair in a counterpoise calculation including the basis functions of ethene (but no electrons) is... 

Table I. The isotropic shielding values (in ppm) calculated by GIAO for the proximal proton of methane over ethene as shown in Figure 3 with correction for basis set superposition error (BSSE).

Distance above Ethene, A Isotropic shielding with BSSE correction Isotropic shielding in counterpoise calculation Estimated BSSE... 

Table VII. Prediction of 170 NMR chemical shifts assuming AE = T for the decavanate ion [V10O28]6 Experimental values and attributions are from ref (74). Atomic parametrization was r t(V) = 135 pm and ex°(V) = 3.633 eV leading to e2t = 3.9 eV. The computed diamagnetic isotropic shielding constant was found to be ad = 418.1 0.1 ppm for all oxygen sites.

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