Shielding paramagnetic

As indicated, the possibility of orbital decomposition is retained in the diamagnetic and paramagnetic terms in eqs.(29,30). The value for the paramagnetic shielding contribution extracted via eq.(30) does not, of course, correspond to a well-defined basis set, but is at the same level of numerical quality as the LORG calculation used for the total shielding. 

TABLE 6.1. Paramagnetic Shielding Contributed by Neighboring Atoms (ppm) ... 

These results were deduced from those indicated in Ref. 115 and represent crP(KK) + (7P(MK), as defined in this reference. The local paramagnetic shielding discussed in the text is cr (M) (Eq. 9 of Ref 115). Source. Ref 115. 

So far we have learned that, in certain series of related compounds, it is the local paramagnetic shielding that governs the changes in total shielding... 

Apart from pyrrole, the chemical shifts for the [3-protons increase with decreasing electronegativity of the heteroatom. In contrast, the chemical shifts of the a-protons do not display any obvious regularity, probably due to paramagnetic shielding contributions which will become more important with increasing availability of 7-orbitals. The chemical shift of the pyrrole N-H is solvent dependent. 

For nuclei other than hydrogen, such as 13C, the paramagnetic shielding term [Pg.110]

Upheld shifts may also arise from substitution of an atom (e.g. H) by its heavier isotope (,e.g. 2H). The upheld shift follows from a lower potential energy of the ground state, hence an increased AE in eq. (3.4), and a reduction of bond distance. The influence of both effects will be to decrease the paramagnetic shielding term. Deuterium isotope shifts depend on the degree of deuteration and reach values of up to 1.5 ppm (Table 3.1). 

Parallel-spin pairing at a carbon atom causes a decrease in the paramagnetic shielding term and, consequently, the carbon resonates at higher field. 

Bulky //-substituents force carbons to resonate at lower field due to orbital contraction, which causes an increase in the paramagnetic shielding term. 

The large carbon-13 shifts of the central sp carbons in allenes (200-210 ppm) relative to the terminal ones (70-90 ppm, Table 4.15) are attributed to an increased paramagnetic shielding due to two localized n bonds originating from the central carbon. The / effect of methyl substitution to the central carbon is considerably smaller than that found for alkenes and alkynes. 

EXAMPLE 6.2 Does paramagnetic shielding cause a nucleus to precess at higher or lower frequency ... 

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