Paramagnetic screening constant

Figure 2. Direct dependence of the one atom paramagnetic screening constant, on the orbital electron density, q, for carbon-13. Modified from Pugmire et aL (1968).

The diamagnetic screening constant describes the circulation of electrons around the nucleus, whereas the paramagnetic screening constant is related to bond orders, intemuclear distances, and the energy difference between filled and empty orbitals. For H, ad dominates, but for heavier atoms, ap becomes the dominant factor. Thus, H has a small chemical shift range relative to heavier atoms, but H shifts are straightforward to interpret in terms of chemical environment. 

The diamagnetic screening constant, involves the rotation of electrons around the nucleus and is important for proton NMR. These electrons may be immediately associated with the atom in question, or with circulating electrons associated with proximate functionalities, i.e., anisotropic effects. For the paramagnetic screening constant, cr (which makes the major contribution to the nuclei Fe, Rh, Sn, Pt,. .. etc.), the average energy approximation, for an... 

Because of the difficulty in evaluating the paramagnetic contribution, ab initio calculations of NMR shielding constants are few in number and limited to rather small molecules. Jaszunski and Sadlej used a variational perturbation approach with an ab initio SCF wave function of H20 and found aH = 28.3 ppm. [M. Jaszunski and A. J. Sadlej, Theor. Chim. Acta, 27, 135 (1972).] The known proton screening constant in H2 is 26.6 ppm,6 and gaseous H20 shows a proton chemical shift of 3.6 ppm relative to gaseous H2 hence the experimental H20 proton shielding constant is 30.2 ppm. 

As for all nuclei, the 33S nuclear screening constant a can be approximated as the sum of two independent terms of opposite sign, the diamagnetic and the paramagnetic contribution 32... 

The most significant contribution to the overall screening constant is due to the diamagnetic term, which arises from the electrons in the filled shells surrounding the nucleus. It depends on the electronic ground state of the molecule and has a shielding effect. The variations in chemical shift can be ascribed mainly to variations in the paramagnetic term. This has been demonstrated experimentally for 33S in 3- and 4-substituted benzenesulpho-nates.33... 

The screening constant a is the trace of a second-rank tensor, the components of which can be observed in solid-state NMR. In high-resolution solution NMR, however, an average value, a, is measured as a result of rapid reorientation of the molecule in the magnetic field. The screening constant has two components, diamagnetic (ad) and paramagnetic (ap) ... 

NMR chemical shifts are perhaps best understood in terms of an additive dependency of the paramagnetic, OoP, and diamagnetic, screening constants Equation (1) [8]. 

The relative nitrogen screening constants of pyridine, pyrazine, pyridazine, pyrimidine, 1,3,5-triazine and 1,2,4,5-tetrazine show a very good linear correlation with theoretical values of the local paramagnetic term calculated by the AEE approximation. (47) A similarly satisfactory... 

---