Diamagnetic nuclear shielding

Finally, in the third section of this paper, we describe a semiempirical atom dipole model that allows a reliable prediction of molecular electric dipole and quadrupole moments, diamagnetic susceptibilities, and diamagnetic nuclear shieldings. A set of localized bond and atom values are developed for the individual diagonal elements in the total molecular magnetic susceptibility tensor. 

In Table 1-1 we thus report the N nuclear shielding (with its paramagnetic and diamagnetic contributions) and nitrogen lone pair natural population (NP(lp)) for the diazine showing the largest solvent effect, i.e., pyridazine, and its clusters in vacuo (VAC) and in the presence of an external PCM continuum. 

Let us deal with the nuclear shielding tensor. This is a sum of two terms. The diamagnetic term is... 

The spectroscopic determination of the spin-rotation-, spin-spin-, and nuclear-shielding parameters of diamagnetic molecules compiled here requires a resolving power of the order of 1... 10 kHz. This can be achieved with the MWFT method or with molecnlar-beam techniques in current use so that the reader will find several molecules in the tables below where some of the pertinent parameters have been determined. 

Cemusak et al have used the Coupled HF method to study ring currents in six cyclic isomers of (CH)2B2N2. Correlated calculations show that all six have planar structures and Tt-electron diamagnetic ring currents. The nuclear shielding produced by the ring current is compared with that of benzene. 

A nuclear shielding constant consists of diamagnetic and paramagnetic parts. Each of these parts ... 

Recent ab initio type calculations of nuclear shielding of carbon, nitrogen and oxygen in small molecules by Ditchfield et al, [171], essentially show that the diamagnetic contribution to the total shielding for second row nuclei does not vary appreciably from one molecule to another. 

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