BONDING AND ELECTRON DISTRIBUTION

Within the last two decades, significant progress has been achieved in the theoretical computation of NMR spectroscopic parameters,110 particularly of nuclear screening constants. Theoretical calculations of 33S NMR parameters have received a certain attention for two main reasons (i) they can help in identifying and elucidating the structural properties of sulphur-containing molecules, and (ii) they can provide important information on the electron distribution and bonding situation around the sulphur atom. 

It turns out, in fact, that the electron distribution and bonding in ethylene can be equally well described by assuming no hybridization at all. The "bent bond" model depicted at the right requires only that the directions of some of the atomic-p orbitals be distorted sufficiently to provide the overlap needed for bonding. So one could well argue that hybrid orbitals are not real they do turn out to be convenient for understanding the bonding of simple molecules at the elementary level, and this is why we use them. 

Fig. 2. ir-Electron distribution and bond orders in 1,3,4-thiadiazole from R. Zahradnik and J. Kouteoky Collection Czech. Chem. Commun. 26, 156 (1901)1 (a cs = 0.8). 

It has been possible to correlate molecular structure and stereochemistry, including parameters such as electron distributions and bond angles, to observed coupling constants. However, as is the case with chemical shifts, a firm theoretical framework for calculation of coupling constants is absent, and those semi-theoretical treatments that have been put forward must be applied with care. We will consider other aspects of spin-spin coupling in Section 4(cXii). 

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