Atomic charges and NMR shifts

TABLE 6.3. Atomic Charges (me) and NMR Shifts" of Olefinic Carbon Atoms... 

Kraka and Cremer have calculated the and C NMR chemical shifts and magnetic susceptibility as a function of interaction distance of both the cycloheptatriene and norcaradiene systems They point out that both the magnetic susceptibility and the shift difference between the endo and exo protons at C(7) are at a maximum at the transition state for the valence tautomeric rearrangement between the two systems. The transition state is characterized by a C(l)-C(6) distance of 1.864 A and an almost complete equalization of C—C bond lengths, bond orders, atomic charges and shifts of C(2)—C(5). 

In some cases selected atoms in molecules could be marked with special labels, indicating their particular role in a modeled property. Some examples are (i) local properties, such as atomic charges or NMR chemical shifts, which should always be attributed to a given atom(s), (ii) anchor atoms in the given scaffold to which substituents are attached (Figure 1.13), (iii) atoms forming a main chain in polymers and (iv) reaction centers in a set of reactions. Zefirov et al. have applied labeling in QSPR studies of chemical NMR shifts and... 

One of the most important features of the bicyclic bishomoaromatic germylium ion 6 is the considerably shielded NMR chemical shifts of the endocyclic atoms Cl and C2, relative to those of the neutral precursor 5. Despite the partial delocalization of the positive charge over Ge3, Cl and C2, significant high-field shifts... 

Admittedly, things have not been easy. But now we can benefit from the beauty of simplicity and learn about one unique kind of atomic charge rooted in quantum theory the only ones that satisfy highly accurate correlations with experimental NMR shift results and that are at the same time directly applicable to bond energy calculations. 

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