Nonbonded distances

Example For two atoms having point charges of 0.616 and -0.504 e and a constant dielectric function, the energy curve shows a switching function turned on (Ron) at a nonbonded distance of 10. A and off (R(,rr) al a distance of 14 A. Compare the switched poieniial with the abruptly inincaied poteiiiial. 

In recent years the number of relatively simple sulfonyl molecules for which geometry has been determined in the vapor phase has increased considerably. The additional data are listed in Table 3. At this point there are a total of 40 geometries for sulfonyl compounds that have been determined in the vapour phase all of them are indicated in the r sin a versus r cos plot in Figure 19. While this number corresponds to a much larger sample of data than that considered previously, unfortunately the accuracy and the experimental sources are much more variable than for the smaller sample therefore, no rigorous conclusions should be drawn on the basis of these data. It is remarkable, however, that the mean O O nonbonded distance of the 40 geometries is the same, 2.484 A, as in the smaller... 

FIGURE 19. Molecular configuration of XSOzY sulfonyl derivatives and the r sin a versus r cos a plot characterizing the variations of O O nonbonded distances with changing S=0 length (r) and 0=S=0 angle (2a). 

For electron diffraction, the nonbonded distances of the tetrahedral sulfur bond configurations in sulfonyl molecules are closely spaced and cause strong correlation among the relevant parameters. Assuming one of them (O O) to be known can greatly facilitate the determination of the rest of the parameters (see more details in Reference 5). 

Scheme 12 Short nonbonded distances between metal atoms...

There are many four-membered metallacycles containing short metal—metal nonbonded distances. Cyclodisilazanes (Scheme 12a) isoelectronic to 1,3-cyclodis-iloxanes also have short Si—Si distances [136, 137]. 

Close intramolecular contacts (clashes). A rough measure has been proposed and implemented in the program CONCORD [4,11] - the close contact ratio (CCR). The CCR of a 3D stracture is defined as the ratio of the smallest nonbonded distance to the smallest acceptable value for this distance. Normally, structures with CCR>0.8 are acceptable. Some programs as CORINA [5] or CONCORD [4, 11] have fallback procedures for attempting to relax close contacts in structures with unacceptably low CCR. 

Table 7.1. Comparison of nonbonded distances in the HF/6-311G and MP2/6-311G optimized structures of various conformations of n-hexaneoa...

Fig. 7.5 Illustration of how dispersion forces affect gauche (G) conformations. Compared to structures with gauche forms devoid of dispersion forces (i.e., HF-optimized), structures with gauche forms subject to dispersion forces (MP2 optimized) contract in such a way that the 1. ..5 nonbonded interactions in an attractive part of the van der Waals potential are shortened. Thus, in GG-pentane (shown above), MP2-optimized torsional angles are contracted by several degrees compared to the HF-optimized geometry, causing a reduction in the 1...5 nonbonded distances by several tenths of an A. For additional details and the numerical values see R. F. Frey, M. Cao, S. Q. Newton, and L. Schafer, J. Mol. Struct. 285 (1993) 99.

An even stronger case of intramolecular RAHB coupling is provided by the maleate ion (HOOCCH=CHCOO-), whose H-bonded and open conformers are shown in Fig. 5.22. Skeletal bond lengths and bond orders of these conformers are compared in Table 5.18. As shown in Fig. 5.22, the H-bonded conformer is favored in this case by more than 26 kcalmol-1, which is indicative of a powerful intramolecular no aoH interaction (estimated second-order stabilization 104 kcalmol-1) that is sufficient to overcome the severe steric repulsion of the extremely short H- -O nonbonded distance ( 1.3 A).56... 

The chemist s knowledge of molecular geometry extends beyond typical values of bond distances. He will also be able to predict many bond angles fairly accurately. This is equivalent to specifying a 1-3 nonbonded interatomic distance. The chemist s sketch portrays cis and trans isomerization, syn and anti, and gauche conformations which specify either torsion angles, or indirectly, a 1-4 nonbonded distance. 

Small structural differences should, of course, always be treated cautiously in electron diffraction work. It is also to be realized, however, that the reliability of the determination of these small differences depends not only on the determination of the parameters directly involved in describing them but contributions from other, usually nonbond distances may play important role. This was clearly seen in the determination of the ring deformatin of p-xylene where the long C C distances were particularly instrumental ... 

---