Intermolecular forces determining types

The type of model potential that is sufficient to predict a molecular crystal structure is very dependent on the shape of the molecule. If the shape has many well-defined protrusions and cavities, so that there is only one way that it can pack densely with a tight fitting of the protrusions of one molecule into the cavities of its neighbours, then any potential which represents this shape and has an attractive component will be acceptable. However, many shapes, such as discs or cylinders, can close pack in many ways, each generating a range of structures with differing tilt angles, etc. In these cases, the chemical nature of the atoms and resultant intermolecular forces determine which of the reasonably close-packed structures is adopted. 

A Figure 11.14 Flowchart for determining intermolecular forces. Multiple types of intermolecular forces can be at work in a given substance or mixture. In particular, dispersion forces occur in all substances. 

In addition to hydrogen bonding, secondary-bond forces lead to the aggregation of separate particles into solid and liquid phases they are not of great importance for stable chemical compounds. However, many physical properties such as surface tension and frictional properties, miscibility and solubility are determined to a large extent by intermolecular forces. Three types of forces acting between molecules are recognized, dipole, induction, and dispersion forces. Occasionally, the term van der Waals forces is applied to the dispersion forces alone. 

We now have three substances remaining methane, CH4, methyl fluoride, CH3F, and krypton difluoride, KrF2. We also have two types of intermolecular force remaining dipole-dipole forces and London forces. In order to match these substances and forces we must know which of the substances are polar and which are nonpolar. Polar substances utilize dipole-dipole forces, while nonpolar substances utilize London forces. To determine the polarity of each substance, we must draw a Lewis structure for the substance (Chapter 9) and use valence-shell electron pair repulsion (VSEPR) (Chapter 10). The Lewis structures for these substances are ... 

Organic compounds that have the same functional group often have similar physical properties, such as boiling points, melting points, and solubilities. Physical properties are largely determined by intermolecular forces, the forces of attraction and repulsion between particles. Three types of intermolecular forces are introduced below. You will examine these forces further in Chapter 4. 

Quantum-chemical ab initio calculations have become an alternative to experiments for determining accurately structures, vibrational frequencies and electronic properties as well as intermolecular forces and molecular reactivity.28-31 Two specific approximations were developed to solve the problems of surface chemistry periodic approximation, where quantum-chemical method employs a periodic structure of the calculated system and cluster approximation, where a model of solid phase of finite size is created as a cutoff from the system of solid phase (it produces unsaturated dangling bonds at the border of the cluster). Cluster approximation has been widely used for studying interactions of molecules with all types of solids and their surfaces.32 This approach is powerful in calculating the systems with deviations from the ideal periodic structure like doping and defects. 

Intermolecular forces are the interactions that exist between molecules. A functional group determines the type and strength of these interactions. 

Sample Problem 3.1 illustrates how to determine the relative strength of intermolecular forces for a group of compormds. Table 3.4 summarizes the four types of interactions that affect the properties of all compounds. 

In the solid state, molecules line up in a pattern forming a crystal lattice similar to that of an ionic solid, but with less attraction between particles. The structure of the crystal lattice depends on the shape of the molecule and the type of intermolecular force. Most information about molecules, including properties, molecular shape, bond length, and bond angle, has been determined by studying molecular solids. 

Viscosity Do you know the meaning of the phrase slow as molasses Have you ever tried to get ketchup to flow out of a bottle If so, you are already familiar with the concept of viscosity. Viscosity is a measure of the resistance of a liquid to flow. The particles in a liquid are close enough for attractive forces to slow their movement as they flow past one another. The viscosity of a liquid is determined by the type of intermolecular forces involved, the shape of the particles, and the temperature. 

Pure solvents Both UV-vis and fluorescent probes, which exhibit solvatochromic shifts, have been used to study clustering. Solvatochromic shifts are caused by the same types of solute-solvent intermolecular forces (i.e. dispersion, induction, and dipole-dipole forces) that influence solubilities, interacting over the same range. Consequently, the values of clustering determined spectroscopically are appreciate for considering the effect of clustering on solubilities. 

Since the equilibrium properties of the medium are involved in determining these effects they are called static medium effects. However, there is another type of relevant phenomenon which is related to the dynamic properties of a condensed phase. Since the movement of molecules with respect to one another is required for the reaction to take place, the local viscosity of the system can also influence the rate of reaction. This property is also related to local intermolecular forces. Effects which depend on local viscosity have also been studied experimentally and are known as dynamic medium effects. 

The following example shows that if we know the kind of species present, we can readily determine the types of intermolecular forces that exist between the species. 

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