Bond order dependent valence terms

For these complexes, the isotropic and 15N chemical shifts and the 15N chemical shift tensor elements were measured as a function of the hydrogen bond geometry. Lineshape simulations of the static powder 15N NMR spectra revealed the dipolar 2H-15N couplings and hence the corresponding distances. The results revealed several correlations between hydrogen bond geometry and NMR parameters which were analysed in terms of the valence bond order model. It was shown that the isotropic 15N chemical shifts of collidine and other pyridines depend in a characteristic way on the N-H distance. A correlation of the and 15N... 

Making several of the energy terms depend critically on calculations of bond orders for each atom is of central importance to describe variable atomic bonding. It is assumed that the bond order of any given atom can be calculated by evaluation of interatomic distances to neighbouring atoms and that all such contributions add up to a net bond order for each atom. In particular, this prevents otherwise pathological tendencies of the atoms to make bonds beyond their natural valency capabilities by inclusion of an overcoordination penalty. 

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