Orbital paramagnetic term

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. 

A general comment on the use of the empirical correlation between Si and Sn NMR (and likewise on C/ Si or Sn/ Pb NMR) chemical shifts is in order. The basis for this correlation is that the paramagnetic term Op dominates the chemical shift. According to Ramsay s theory, Op is proportional to the reciprocal energy difference h.E between the magnetically active orbitals and proportional to the expectation value for the electron radii (r )np- Thus, a linear correlation between the 5 Si and 8 Sn implies that the ratio of both determining factors of Op is constant for the all compounds of interest. In particular, it is not clear, however, if the ratio for tetravalent silicon and tin compounds is the same as for trivalent silicon and tin compounds. Therefore, the extension of a correlation based exclusively on the... 

Ramsey obtained ay, by first-order and op by second-order perturbation theory (76) variational treatments give similar results (14, 71, 73). The term wp is sometimes called the second-order paramagnetic term and sometimes the high-frequency term (14), because of the dependence of the (temperature-independent) paramagnetism in molecules on the high-frequency matrix elements of the orbital moments (91). 

The most striking feature of Figs. 7 and 8, particularly as compared with the hydride plot, is the plunge downfield of the resultant shielding, following the paramagnetic term, across the row of the central atom for the typical elements, and also for the early transition metals (Fig. 8), with ready circulation of fluorine 2p electrons into empty t g orbitals in the complexes. Fluorine is h hly shielded however in the d molecules and ions, and this was discussed in Section III, B as a possible Cornwell effect (62). 

The expression for the contribution to the spin-orbit induced MCD intensity from perturbation of the ground state is somewhat reminiscent of an expression for the Ag quantity of EPR spectroscopy. The similarity lies in the paramagnetic term, Agp. This term is composed of integrals of a spin-orbit operator over molecular orbitals similar to the expression for the perturbation of the ground state in the presence of spin-orbit coupling (Eqs. 52-56). The paramagnetic contribution to Ag dominates for blue copper proteins and it was suspected that the MCD parameters and Amay have some sort of relationship. It was found that many of the terms that make large contributions to AgP do play a role in the MCD intensity but no simple relationship was found (160). 

Pu being the charge density or occupancy of orbital i of atom A in the molecule. The second term, a%, a paramagnetic term representing a local correction for the molecular environment, involves the mixing of ground and excited electronic states. This term is extremely difficult to calculate and no exact expression has been found using many-electron theory. Karplus and Pople have represented Op as... 

The paramagnetic term depends on the number of electrons in the 2p orbital and multiple bond contributions as well as the average excitation energy A . Therefore, this term will have a significant deshielding effect on n systems. For further discussion the reader is referred to Pople s reference [63]. 

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