Diamagnetic shielding contribution

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) ... 

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. 

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). 

It follows from Table V that spin-orbit effects are relevant for the heavy metal shieldings and, since the spin-orbit contribution does not always have the same sign, for the relative chemical shifts. In this connection, it is interesting to note that the ZORA spin-orbit numbers are shifted as compared to their Pauli spin-orbit counterparts. This effect can be attributed, at least partly, to core contributions at the metal while scalar contributions of the core orbitals are approximately accounted for by the frozen core approximation (6,7), spin-orbit contributions of the core orbitals are neglected. Hence, the more positive (diamagnetic) shieldings from the ZORA method are due to spin-orbit/Fermi contact contributions of the s orbitals in the uranium core. 

This nuclear Diamagnetic Shielding operator contributes to the NMR shielding tensor. 

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... 

The relative contributions of the paramagnetic shielding and the diamagnetic shielding constants within this increment system have been discussed Empirical systems of this kind allow an estimation of chemical shifts with a mean deviation of less than one ppm. Modern data bank systems, e.g. SPECINFO or CSEARCH, are able to reproduce the chemical shifts of any given alkane even more accurately within a few seconds. In a slightly different approach, a substituent constant for 34 different alkyl groups was defined, and it was shown that the chemical shift of a nucleus X in a compound XR4 obeys equation 1, where m and b are constants. 

The early calculations by Dickinson [168] of the diamagnetic shielding in atoms using Hartree or Hartree-Fock SCF atomic orbitals made it clear that the major contribution to is due to inner shell electrons and that the relative contribution of the outer electrons decreases markedly with atomic number Z. As a consequence of this are the values for atoms rather insensitive to the effective atomic... 

On summing paramagnetic p and diamagnetic A contributions in Eqs. (77) and (103), full cancellation of equal terms linear in ds takes place on the rJi.s., then the total CTOCD-DZ shieldings are origin... 

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