Space-filling effect

Once a guest molecule has entered the internal surface, its rate of diffusion will be strongly dependent on the size and shape of the channels and cages of the internal pores compared to its own size and shape. In addition, the number density, the location and the size of the exchangeable cations associated with the internal framework will influence diffusion by both chemical (electronic) effects e.g., electrostatic and dispersion factors, and by steric (space filling) effects. 

This high smectic tendency is generated because of favomable lamellar packing afforded by the polarity of the lateral substituent combined with the space-filling effect. 

Concrete hardens as a result of chemical reaction between Portland cement and water. The hardened Portland cement is called cement paste. As the volume of the hydration products is greater than the volume of concrete grain, the accumulation of hydration products create a space filling effect. The paste contains two different forms of pores - capillary pores and gel pores. 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

Next, examine the Sn2 transition states as space-filling models. Are you able to identify unfavorable nonbondec (steric) interactions that are not present in the reactants If so, which Sn2 reaction is likely to be most affected b] steric interactions Least affected Rationalize you observations. Hint Compare CBr bond distances in thf Sn2 transition states. How do these change with increasec substitution at carbon What effect, if any, does this havf on crowding ... 

Examine space-filling models for the two conformers and identify any likely unfavorable nonbonded interactions. Based on steric effects, which conformer would you anticipate would be the more stable Compare energies of anti-1,2-ethanediol and gauche-1,2-ethanediol to see if you are correct. Is this the same ordering of conformer energies as seen for n-butane (see Chapter 5, Problem 3)7... 

Examine the spin density surface for BHT radical. Is the unpaired electron localized or delocalized Examine BHT radical as a space-filling model. What effect do the bulky tert-butyl groups have on the chemistry of the species (Hint BHT radical does not readily add to alkenes or abstract hydrogens from other molecules.)... 

A realistic model of a solution requires at least several hundred solvent molecules. To prevent the outer solvent molecules from boiling off into space, and minimizing surface effects, periodic boundary conditions are normally employed. The solvent molecules are placed in a suitable box, often (but not necessarily) having a cubic geometry (it has been shown that simulation results using any of the five types of space filling polyhedra are equivalent ). This box is then duplicated in all directions, i.e. the central box is suiTounded by 26 identical cubes, which again is surrounded by 98 boxes etc. If a... 

Space filling van der Waals models (A3) are useful for illustrating the actual shape and size of molecules. These models represent atoms as truncated balls. Their effective extent is determined by what is known as the van der Waals radius. This is calculated from the energetically most favorable distance between atoms that are not chemically bonded to one another. 

Figure 4. A nine-molecule space-filling model of the (100) sucrose crystal. Atoms of one of the molecules are labeled. The sub-layer of the screw-axis related molecules is completely shielded from possible solvent effects.

---