Water electron repulsion

C09-0115. The H—O—H bond angle in a water molecule is 104.5°. The H—S—H bond angle in hydrogen sulfide is only 92.2°. Explain these variations in bond angles, using orbital sizes and electron-electron repulsion arguments. Draw space-filling models to illustrate your explanation. 

AEba = —45 kJ mol 1 for the HO + SiH4 reaction and AEba = —43 kJ mol-1 in the reaction of hydrogen atom with water. The repulsion of the electron orbitals of the atoms forming the reaction center AER plays an important role in all the radical abstraction reactions. In the interaction of radicals with molecules the contribution of this repulsion ranges from 25 to 46 kJ mol-1. In reactions of molecules with hydrogen atoms the contribution is naturally smaller, varying from 8 to 16kJ mol-1. 

The extraordinally large pKa for 5 (13.5 in 10% water-dioxane at 25 °C) is compatible with this small cavity because of the instability of the resulting anion due to a large electronic repulsive force (2) the coloration takes place only when the specimen is in contact with lithium salts (3) the monodemethylation of tetramethoxycyclophanes 31 and 36 occurs exclusively in an aprotic solvent such as benzene even in the presence of excess LiAlH4 (no evidence for complexing lithium with 36 was obtained by both picrate salt extraction and NMR spec-... 

Molecular shape complementarity is critical to biomolecular recognition and specificity. Even if the molecules change conformation on binding and water molecules are trapped at the interface, bound complexes show high shape complementarity (31). This shape complementarity is dependent on van der Waals interactions between the binding molecules. Electron-electron repulsion prevents atomic overlap and intermolecular penetration. However, induced dipole effects as atoms approach lead to short-range attractive interactions. 

When the water molecule is rotated so that one hydrogen comes closer to the metal surface, there is more electron repulsion and there is a change in (as discussed above with a and the dispersive energy). Thus the sined contribution of oig and number of electrons are used in (3). 

The bond angle in water is a little smaller (104.5°) than the tetrahedral bond angle (109.5°) in methane, presumably because each lone pair feels only one nucleus, which makes the lone pair more diffuse than the bonding pair that feels two nuclei and is therefore relatively confined between them. Consequently, there is more electron repulsion between lone-pair electrons, causing the O—H bonds to squeeze closer together, thereby decreasing the bond angle. 

The three molecules of interest are methane (4A), ammonia (6A), and water (7A), shown first in the Lewis electron dot representations. Using the VSEPR model, these three molecules are drawn again using the wedge-dashed line notation. Methane (CH4, 4B) has no unshared electrons on carbon but there are electrons in the C-H covalent bonds. Assume that repulsion of the electrons in the bonds leads to a tetrahedral arrangement to minimize electronic repulsion. Ammonia (H3N, 6B) has a tetrahedral array around nitrogen if the electron pair is taken into account. If only the atoms are viewed, however, 6B has the pyramidal shape shown. Water (HOH, 7B) has two electron pairs that occupy the corners of a tetrahedral shape, as shown. 

In the treatment of Badiali et al. (1981) the jellium model of the metal electron system is used with the jellium edge being assumed to be a plane passing through the centers of surface atoms of the metal. Solvent molecules then lie in contact with the surface at a distance equal to the radii, T, of surface metal atoms and hence are separated from the jellium edge by a distance T. This is not an altogether realistic model and, in fact, does not take into account the "overspill effect associated with the wave function of the metal s electrons at the surface. Another problem is that the solvent is represented by an electron-repulsive dielectric continuum, little related to the properties of water dipoles which are involved at the Hg surface in aqueous systems that have mostly been experimentally studied in double-layer capacitance works. 

Figure 1-20 Bonding and electron repulsion in ammonia and water. The arcs indicate increased electron repulsion by the lone pairs located close to the central nucleus.

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