Overlap repulsion

The destabilization is caused by the exchange of electrons between the occupied orbitals through the orbital overlap. The force is then termed exchange repulsion or overlap repulsion. The exchange repulsion is a major cause of the steric repulsion. There are many occupied orbitals in the sterically crowded space. 

We shall first consider how nonbonded interactions influence bond angles in molecules. Our approach will be illustrated by reference to the model systems difluoro-methane and 1,1-difluoroethylene. In these problems, we shall consider not only stabilizing orbital interactions but also overlap repulsion in order to demonstrate some interesting trends which obtain in these angle problems. 

In general, F2pz—F2pz overlap repulsion will be outweighed by ns-jr overlap repulsion, the net effect favoring angle shrinkage. 

As can be seen, the ng MO of the tram isomer is lower in energy than the ns MO of the cis isomer, a result consistent only with the presence of strong through space and through bond interaction of the two Cl atoms. This is an important result insofar as it indicates that four electron overlap repulsion is greater for tram than for cis 1,2-disubstituted ethylenes. 

The parent system to be considered is ethane itself. Here, the dominant interactions, or, better, the only type of interactions are uch ctch- The conformation which places vicinal C-H bonds anti with respect to each other, i.e. the staggered conformation, is the one which is predicted to be preferred. The same conclusions are reached by considering overlap repulsion and steric effects. 

The simplest type of substituted ethane is the CH3CH2X system. When X is more electronegative than H, the dominant interaction is oCH—o x which dictates a staggered conformation. However, the same conformation is also predicted on the basis of overlap repulsion and steric effects. Indeed, irrespective of the nature of X all such molecules exist in staggered form366 ... 

We believe that the criticism of Bingham is without a real basis. Accidentally, the final conclusion, i. e. overlap repulsions favoring crowded structures, is, in some cases, correct. 

In other papers408, the discussion of conjugative destabilization has been based on an incorrect assumption, L e that the destabilizing interaction of filled orbitals increases as their energy separation decreases, while, as we have seen in Eq. (5 ), four electron overlap repulsions depend not on the energy separation of the two interacting MO s but rather on the sum of them. Thus, situation A is less destabilizing than situation B (constant Sy). 

These various relationships between force and particle separation imply that the attractive force between particles will become infinite when they touch. In reality, other short-range forces will modify this relationship when r is very small, in particular the repulsion from overlap of atomic orbitals. The van der Waals attraction will then be balanced by this overlap repulsion. At these short distances (a few tenths of a nanometer), the van der Waals attraction will be strong enough to hold the particles fairly strongly together. This balance between van der Waals forces of attraction and overlap repulsion forces is shown schematically in Fig. 1.4, where the very steep repulsive interaction at atomic distances is due to the overlap repulsion. Hydration forces (see section 1.3.3) may also result in repulsion between surfaces at somewhat greater separations. 

Fig. 1.4 Resultant interaction energy between two particles with van der Waals attractive interactions and electron overlap repulsion interactions. 

Fig. 1.6 DLVO interactions showing the energetics of colloidal particles as a competition between electrostatic double-layer repulsion and van der Waals attractions. The primary minimum is due to strong short-range electron overlap repulsion (shown in Figure 1.4... 

The concepts which we need for understanding the structural trends within covalently bonded solids are most easily introduced by first considering the much simpler system of diatomic molecules. They are well described within the molecular orbital (MO) framework that is based on the overlapping of atomic wave functions. This picture, therefore, makes direct contact with the properties of the individual free atoms which we discussed in the previous chapter, in particular the atomic energy levels and angular character of the valence orbitals. We will see that ubiquitous quantum mechanical concepts such as the covalent bond, overlap repulsion, hybrid orbitals, and the relative degree of covalency versus ionicity all arise naturally from solutions of the one-electron Schrodinger equation for diatomic molecules such as H2, N2, and LiH. 

Fig. 3.2 The bonding and antibonding states for (a) the homonuclear and (b) the heteronuclear diatomic molecule. The shift in the energy levels due to overlap repulsion has not been shown.

Since the dominant contribution to is the overlap repulsion, we expect from eqn (3.19) that... 

Not only do double layers interact with double layers, the metal of one sphere also interacts with the metal of the second sphere. There is what is called the van der Waals attraction, which is essentially a dispersion interaction that depends on r-6, and the electron overlap repulsion, which varies as r-12. These interactions between the bulk... 

C) Charge-cloud overlap repulsions The existence of some type of repulsive force follows from the fact that all the other contributory effects lead to bonding and shortening of the H-bond. But these forces must be fairly strong, as can be seen from the distances involved. For example, in the 0X —H 02 system, we find 0X 02 distances as low as 2-5 A, yet the closest 0 0 distance found in crystals, where no H-bond exists, is about 3 0 A. Alternatively, the sum of the van... 

TABLE 10. Conventional strain energies (CSE), hybridizations, s-character, overlap values, overlap repulsions and geminal delocalizations of propane, cyclobutane, cyclopropane and their heterologues with X = NH, O, SiH2, PH, S from Reference 47 ... 

Destabilizing (antibonding) overlap repulsions between geminal bonds increase (i.e. IBP becomes more negative) in the order open-chain compound < four-membered ring < three-membered ring. 

Both ct and rc bonds are weaker in the second row than in the first one, and this fact alone would lead to the prediction that Si4H4 and P4 have a lesser tendency to distort than their first-row analogs. In addition, because of the rather long bond lengths, the overlap repulsions between the it bonds in the Kekule structures of square Si4H4 and P4 must not be very large. The striking result is that these two species are nearly indifferent to distortion, despite their expected antiaromatic character. 

Pauli repulsion The repulsion of two electrons with identical spins on two centers, (e.g., A B). This repulsion also appears in VB structures bearing three and four electrons, that is, A B, A B, A B. The Pauli repulsion is precisely the same as the overlap repulsion known from qualitative MO theory. (See Table 3.1.)... 

Rejection of protein adsorption to the outermost grafted surface is attributed to a steric hinderance due to the tethered chains. A grafted surface in contact with an aqueous medium, a good solvent of the chains, has been identified to have a diffuse structure [67]. Reversible deformation of tethered chains due to invasion of mobile protein molecules into the layer would lead to a repulsive force which is governed by the balance of entropic elasticity of the chains and osmotic pressure owing to the rise in the segment concentration. The overlapped repulsive force would prevent the direct contact of protein molecules with the substrate surface. 

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