Repulsion interatomic

There are forces other than bond stretching forces acting within a typical polyatomic molecule. They include bending forces and interatomic repulsions. Each force adds a dimension to the space. Although the concept of a surface in a many-dimensional space is rather abstract, its application is simple. Each dimension has a potential energy equation that can be solved easily and rapidly by computer. The sum of potential energies from all sources within the molecule is the potential energy of the molecule relative to some arbitrary reference point. A... 

Figure 4-15 A van der Waals Potential Energy Function. The Energy minimum is shallow and the interatomic repulsion energy is steep near the van der Waals radius.

The regular octahedral structure of SFg and the related structure of S2F]q (Fig. 15.20) call for little comment except to note the staggered (D4d) arrangement of the two sets of Feq in S2F10 and the unusually long S-S distance, both features presumably reflecting interatomic repulsion between the F atoms. SFg is also of... 

It has been suggested that the spike pressure is due not to molecular impacts in the usual sense but rather to interatomic repulsion forces (Ref 6, p 298 Ref 13, p 25a). 

In the bond valence model quantum effects are treated classically by including them in the interatomic repulsion described by eqn (3.1) or (3.2). There are, however, a number of cases where quantum effects are directly responsible for deviations from the higher symmetry that would otherwise be expected. Such electronically distorted structures were discussed in Chapter 8. 

The attack of a chlorine atom on a methane hydrogen is not expected to require a precisely oriented collision. Moreover, the interatomic repulsions should be considerably smaller than in the four-center mechanism discussed previously for the reaction of molecular chlorine with methane because only two centers have to come close together (Figure 4-8). The methyl radical resulting from the attack of atomic chlorine on a hydrogen of methane then can remove a chlorine atom from molecular chlorine and form chloromethane and a new chlorine atom ... 

Use of bond-dissociation energies gives a calculated AH0 of—26 kcal for this reaction, which is certainly large enough, by our rule of thumb, to predict that Kqq will be greater than 1. Attack of a methyl radical on molecular chlorine is expected to require a somewhat more oriented collision than for a chlorine atom reacting with methane (the chlorine molecule probably should be endwise, not sidewise, to the radical) but the interatomic repulsion probably should not b much different. 

Note that r/j is positive for large a and small b, corresponding to large interatomic attractive forces and small interatomic repulsive forces. For j > 0, (3T/3P)H > 0 ... 

Konig, E. (1971). The nephelauxitic effect calculation and accuracy of interatomic repulsion parameters in cubic high spin cP, and J systems. 

If two or more rings are present in one complex, they can interact with esch othei and certain conformations might be expected to be stabilized as a result of possible reductions in interatomic repulsions. For example, consider a square planar complex containing , two chelated rings of ethylenediamtne. From a purely statistical point of view we might expect to find three structures, which may be formulated M65, MAA, and M5A (which is identical to MA. The first two molecules lack a plane of symmetry, but M5A is a meso form. Corey and Bailar 8 were the first to show that the and MAA should predominate over the meso form since the latter has unfavorable H—H interactions of the axial-axial and equatorial-equatorial type between the two rings (Fig. 12.28). The enantiomeric M86 and MAA forms are expected to be about 4 kJ mor more stable than the meso isomer, other factors being equal. 

Random vs. Selected Structures. - A completely unbiased approach is that of studying randomly generated positions for the N nuclei. Of course, it shall be assured that the atoms are not so far apart that they do not interact with each other and, therefore, the random positions are usually confined to a finite volume, for instance a cube or a sphere. The volume of that could be slightly larger than the sum of the atomic volumes of the N nuclei. On the other hand, it shall also be assured that no two atoms are so close that they feel a very strong interatomic repulsion, so that such configurations will be excluded aforehand, too. 

The MNDO model is a very successful model, again with some documented limitations. MNDO produces spurious interatomic repulsions, generally... 

The properties of the metal phase have been successfully described by rather simple models, most notably the jelliiun model. In many theoretical treatments of the liquid/metal interface, the hquid electrolyte in contact with the metal has been described, to first order, as an external field, acting on the jellium model (see Ref. 13 and references therein). In many simulation studies, the reverse approach is taken. The focus is on the description of the liquid phase and the effect of the metal on the aqueous phase is approximated, to first order, by an external potential acting on the ions and molecules in the liquid phase. This is done within the framework of classical mechanics and classical statistical mechanics. The models for the interparticle interactions will consist of distributed point charges in combination with soft interatomic repulsions and dispersive attractions. Some of the models can also be considered chemical models they can be regarded as a first step towards electrochemical modeling, very much in the spirit of molecular modeling . 

The number of collisions for gases at standard temperature and pressure (STP 0°C and 1 atm) has been calculated to be more than 10 ° s. If all these collisions were effective, then reaction rates would be extremely fast. However, this is not true because only a small fraction of collisions are effective. The extra amount of energy required in a collision to overcome interatomic repulsive forces and produce a chemical reaction is known as the energy of activation. Its magnitude depends on properties of the reactants. 

By virtue of its tetrahedral symmetry, the HCH bonds in CH4 are all 109.5°. In this configuration, the H---H distances are all 1.78 A. The van der Waals radius of an atom is defined such that whenever the distance between two atoms equals the sum of their van der Waals radii, their interatomic repulsion is RT. Since the H-atom van der Waals radius is 1.2 A, any distance less than 2.4 A between H-atom centers will produce significant repulsion between the electron clouds of the two H atoms. There have been many theoretical attempts to quantify this repulsion its magnitude is given approximately by an empirical formula proposed by Huggins ... 

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