Regularities in Nonbonded Distances

In some molecular geometries, the so-called intramolecular 1,3 separations are remarkably constant. The 1,3 label refers to the interactions between two atoms in the molecule which are separated by a third atom. The 

Element Covalent radius s (A) 1,3 Intramolecular nonbonded radius (A) van der Waals radius (A) 

Of course, the 1,3 intramolecular nonbonded radii (Table 3-5) are purely empirical, but so are the other kinds of radii. Thus, the 1,3 nonbonded radii may be updated from time to time (see, e.g.. Ref. [3-60]). 

The alkali sulfate molecules used to appear in old textbooks with the following structural formula  

In some molecular geometries, the so-called intramolecular 1,3 separations are remarkably constant. The 1,3 label refers to the interactions between two atoms in the molecule which are separated by a third atom. The near equality of the nonbonded distances in the ONF3 molecule is a special case. What is more commonly observed is the constancy of a certain 1,3 nonbonded distance throughout a series of related molecules. Significantly, this constancy of 1,3-distances may be accompanied by considerable changes in the bond lengths 

Numerous examples of molecular structures have been introduced in the preceding sections. They are all confirmed by modem experiments and/or calculations. We would like to know, however, not only the structure of a molecule and its symmetry, but also, why a certain structure with a certain symmetry is realized. 

The structure of a series of the simplest AX type molecules will be examined in terms of one of these useful and successful qualitative models. A is the central atom, the Xs are the ligands, and not necessarily all n ligands are the same. 

Qualitative models simplify. They usually consider only a few, if not just one, of the many effects that are present and are interacting in a most complex way. The measure of the success of a qualitative model is in its ability to create consistent patterns for interpreting individual structures and structural variations in a series of molecules and, above all, in its ability to correctly predict the structures of molecules, not yet studied or not even yet prepared. 

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