Paramagnetic shielding contribution

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) ... 

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. 

According to Equation 124 the diamagnetic shielding terms for the C atoms differ only in the expression Zr IRr q for the monosubstituted allenes, where R denotes the atom of the substituent R which is bonded to Cr. Then, one can introduce reduced substituent constants (125) which give only the paramagnetic shielding contribution to the C / resonance that is, for the allenes we have... 

For a nonspherical charge distribution about the phosphorus nucleus, the shielding constant largely results from a local diamagnetic shielding contribution ffj and a local paramagnetic shielding contribution Op (Pople et al., 1959) ... 

Shabab (45) attributed the a-SCS to a subtle balance of both the paramagnetic and diamagnetic (40-42) shielding contributions in which the latter increases more rapidly than the former with increasing atomic number of X. Thus, the net observed effect becomes less deshielding as the atomic number of X increases (45). 

Only for basis set ZZZ, a a shielding polarizabilities are very close to the cr 7 values for both nuclei. The paramagnetic apIofly contributions, are approximately of the same quality for basis set JJand UJ. Our results are compared with those of Bishop and Cybulski (7,9), and with those of Coriani et al (18), both for the SCF level, and the agreement is very good, even if, those authors have used larger basis sets. 

The NMR properties of B in three and four coordination with oxygen have also been studied (Tossell, 1986 Tossell and Lazzeretti, 1988c). For three-coordinate and four-coordinate B, the B 2/2-0 2p o-bonding orbitals make the dominant contribution to the paramagnetic shielding, o-p. Large-basis-set calculations on indicate that it has a less anisotropic... 

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