Nonbonding Contacts

Steric effects resulting from nonbonding contacts are well-known in organic chemistry where they are invoked to explain why crowded molecules are difficult to prepare, but this kind of strain is also found in inorganic compounds, most notably in the hydrogen bond where it is the origin of the O-H... O asymmetry discussed in 

When hydrogen bonds to an atom such as oxygen, the whole hydrogen atom, both nucleus and its valence electron, is drawn into the oxygen valence shell. Without a lone pair to block the formation of a second (acceptor) bond, the hydrogen nucleus can attract another anion (oxygen for example) to form a second bond. Following the principle of maximum symmetry (1), we would expect the two 

More symmetric hydrogen bonds are kept linear by the strong repulsion between the oxygen atoms. Longer hydrogen bonds are usually bent since the increased 

Vr is the residual valence associated with the Lewis function 

Acid strengths are calculated by assuming the hydrogen bonds have a valence of 0.2 vu and the base strengths by assuming the anions have a coordination number of 4 

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The contact energy E of a given conformation is typically calculated by summing the values of energies over all nonbonded contacts in the lattice. 

R. S. Rowland, R. Taylor, Intermolecular nonbonded contact distances in organic crystal structures. J. Chem. Phys. 100 (1996) 7384. 

Napper AM, Head NJ, Oliver AM et al (2002) Use of U-shaped donor-bridge-acceptor molecules to study electron tunneling through nonbonded contacts. J Am Chem Soc 124 10171-10181... 

Structural analyses of the triiodide ion, in crystals of this ion with various counterions, show that the I3 unit is always linear, or nearly so, and that there is considerable variety in the I I distances, which range from 2.67 A (covalent bond length) to 4.30 A (nonbonded contact distance). Further, there is strong correlation between the two I I distances in the I I I ion. Very similar correlations were obtained for the S-S S grouping in the thia-thiophthenes 37, for the O-H O groupings in a number of hydrogen-bonded... 

The 20th century brought two important advances. With the development of diffraction techniques, the arrangement of atoms could be determined on a metric basis. Depending on die size of the molecule, the quality of the crystal, and the accuracy of the experiment, well-defined bond lengths, valence and torsional angles, as well as nonbonded contacts, can be determined. 

The earliest attempts at model analysis of polysaccharides -typified by the x-ray crystal structure analysis of amylose triacetate - were usually conducted in three steps ( L). In the first step, a model of the chain was established which was in agreement with the fiber repeat and the lattice geometry, as obtained from diffraction data. In the second step, the invariant chain model was packed into the unit cell, subject to constraints imposed by nonbonded contacts. This was followed, in the third step, by efforts to reconcile calculated and observed structure factor amplitudes. It was quickly realized that helical models of polysaccharide chains could be easily generated and varied using the virtual bond method. Figure 1 illustrates the generation of a two-fold helical model of a (l- U)-linked polysaccharide chain. 

Despite the nonbonding contacts between the copper centers of 29 and 30, both compounds are reported to be luminescent, displaying broad structureless bands with maxima at 533 and 543 nm, respectively. The strong emissions of the free ligands L29 and L30 are observed blue shifted at 477 and 467 nm, respectively. The authors tentatively attribute the emission of the polymers to MC d—>s excited states or MLCT states. 

The most striking detail of this structure is the transannular bond Se-3—Se-7 whose length of 284 pm is significantly less than the van der Waals distance (400 pm) and even less than the nonbonding contacts Se-2-Se-8 of 336 pm and Se-4-Se-6 of 330 pm. The other bond lengths vary between 229 and 236 pm and average to 231.8 pm (80). 

Gd2Cl (15, 16), Sc5Cl (]J), and Y r (18). It is not by accident that the halide examples are found among the metals with fewer valence electrons and larger metallic radii. In these structures the M-M and X-X distances must be equal along the chain, consequently the larger spacing required by Cl-Cl or Br-Br nonbonded contacts dictate lower M-M bond+orders. 

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