Dispersive intermolecular force

Let s progress now to the situation with supercritical ethane. There are dangers to blindly employing solubility parameters with any gas-liquid pair. We can illustrate them by investigating supercritical ethane at 400 bar and 37°C. What does it do with the five liquids listed in table 5.2 The solubility parameter of ethane is about 6.0H. Ethane is miscible with hexane we previously related that fact in chapter 3, and we would predict that ethane and hexane would be miscible based upon solubility parameter considerations. Hildebrand solubility parameters can in fact be employed here because only dispersive intermolecular forces are in play and ethane has a liquid-like... 

An important feature of dispersion intermolecular forces (see Chapter 1,1) is their additivity interaction between two volumes of condensed phases separated by a gap is the result of summed attraction between all molecules making these volumes. For non-polar phases in the absence of non-dispersion forces the interaction energy, U h), is almost entirely determined by dispersion forces. The role of dispersion interactions is especially important in disperse systems in which each particle represents a microscopic volume of condensed... 

QM—semiempirical Good for predicting molecular geometry and energetics, predicting vibrational modes and transition structures (but does so less reliably than ab initio methods) Poor results for van der Waals and dispersion intermolecular forces (lack of diffuse basic functions)... 

Chapter 15 is devoted to the bonding nature in molecular and ionic crystals. We recall the dipole-dipole, dipole-induced and dispersion intermolecular forces. The van der Waals and hydrogen bonds are considered. We discuss intermolecular structure and strength of ice and the solid noble gases. The description of organic molecular crystals is presented. In conclusion we consider ionic crystals and calculate their interatomic bonding. 

Which of the following compound(s) exhibit only London dispersion intermolecular forces Which compound(s) exhibit hydrogen-bonding forces Considering only the compounds without hydrogen-bonding interactions, which compounds have dipole-dipole intermolecular forces ... 

Tho-e are a few problems often encountered in crystal structure analysis of mesogens. First, it is sometimes very difficult to obtain single crystals suitable for crystal structure anafysis, because the anisotropy of the molecules results in very thin plate- or needle-like crystals. Secondly, incompleteness of crystals such as disorder and large thermal motions of the molecules as well as structure modulation are occasionally found in crystals due to rather weak dispersive intermolecular forces. S(Mne of the problems are solved by using strong X-ray sources such as synchrotron radiation and/or very sensitive area detectors at low temperatures [2]. More seriously, relationships between the crystal and mesophase structures are not straightforward due to the first order phase transition with... 

The different kinds of intermolecular forces (dispersion, dipole-dipole, hydrogen bonding, etc. see Section VI-1) may not equally contribute to A-A, B-B, and A-B... 

Douketis C, Socles G, Marchetti S, Zen M and Thakkar A J 1982 Intermolecular forces via hybrid Hartree-Fock SCF plus damped dispersion (HFD) energy calculations. An improved spherical model J. Chem. Phys. 76 3057... 

Dispersive Interactions. For pairs of nonpolar polymers, the intermolecular forces are primarily of the dispersive type, and in such cases the energy of interaction between unlike segments is expected to be closely approximated by the geometric mean of the energies of interaction between the two like pairs (98). In this case, the Flory-Huggins interaction energy between this polymer pair can be expressed in terms of the solubiUty parameters 5 of the pure components. 

Hydrophobic Interaction. This is the tendency of hydrophobic groups, especially alkyl chains such as those present in synthetic fibers, and disperse dyes to associate together and escape from the aqueous environment. Hydrophobic bonding is considered (7) to be a combination of van der Waals forces and hydrogen bonding taking place simultaneously rather than being a completely new type of bond or intermolecular force. 

Intermolecular forces are generally less than lOkcal/mole. In polymers, in the absence of hydrogen bonding, the intermolecular force is primarily due to dispersion effects. 

Molecular interactions are the result of intermolecular forces which are all electrical in nature. It is possible that other forces may be present, such as gravitational and magnetic forces, but these are many orders of magnitude weaker than the electrical forces and play little or no part in solute retention. It must be emphasized that there are three, and only three, different basic types of intermolecular forces, dispersion forces, polar forces and ionic forces. All molecular interactions must be composites of these three basic molecular forces although, individually, they can vary widely in strength. In some instances, different terms have been introduced to describe one particular force which is based not on the type of force but on the strength of the force. Fundamentally, however, there are only three basic types of molecular force. 

There have been many attempts to divide the overall solubility parameter into components corresponding to the several intermolecular forces. For example, a so-called three-dimensional solubility parameter concept is built on the assumption that the ced is an additive function of contributions from dispersion (d), polar (p), and H-bonding (h) forces. It follows that... 

When thinking about chemical reactivity, chemists usually focus their attention on bonds, the covalent interactions between atoms within individual molecules. Also important, hotvever, particularly in large biomolecules like proteins and nucleic acids, are a variety of interactions between molecules that strongly affect molecular properties. Collectively called either intermolecular forces, van der Waals forces, or noncovalent interactions, they are of several different types dipole-dipole forces, dispersion forces, and hydrogen bonds. 

Noncovalcnt interaction (Section 2.13) An interaction between molecules, commonly called intermolecular forces or van der VVaals forces. Hydrogen bonds, dipole-dipole forces, and dispersion forces are examples. 

In nonpolar molecules, dispersion is the only intermolecular force. 

Ihe boiling points of different molecular substances are directly related to the strength of the intermolecular forces involved. The stronger the intermolecular forces, the higher the boiling point of the substance. In the remainder of this section, we examine the nature of the three different types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. 

The most common type of intermolecular force, found in all molecular substances, is referred to as a dispersion force. It is basically electrical in nature, involving an attraction between temporary or induced dipoles in adjacent molecules. To understand the origin of dispersion forces, consider Figure 9.8. 

Strategy Determine whether the molecule is polar or nonpolar and identify the intermolecular forces present. Remember, dispersion forces are always present and increase with molar mass. 

We have now discussed three types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. You should bear in mind that all these forces are relatively weak compared with ordinary covalent bonds. Consider, for example, the situation in HzO. The total intermolecular attractive energy in ice is about 50 kj/mol. In contrast, to dissociate one mole of water vapor into atoms requires the absorption of928 kj of energy, that is, 2(OH bond energy). This explains why it is a lot easier to boil water than to decompose it into the elements. Even at a temperature of 1000°C and 1 atm, only about one H20 molecule in a billion decomposes to hydrogen and oxygen atoms. 

Most nonpolar substances have very small water solubilities. Petroleum, a mixture of hydrocarbons, spreads out in a thin film on the surface of a body of water rather than dissolving. The mole fraction of pentane, CsH12, in a saturated water solution is only 0.0001. These low solubilities are readily understood in terms of the structure of liquid water, which you will recall (Chapter 9) is strongly hydrogen-bonded. Dissimilar intermolecular forces between C5H12 (dispersion) and H2O (H bonds) lead to low solubility. 

The effect of molecular interactions on the distribution coefficient of a solute has already been mentioned in Chapter 1. Molecular interactions are the direct effect of intermolecular forces between the solute and solvent molecules and the nature of these molecular forces will now be discussed in some detail. There are basically four types of molecular forces that can control the distribution coefficient of a solute between two phases. They are chemical forces, ionic forces, polar forces and dispersive forces. Hydrogen bonding is another type of molecular force that has been proposed, but for simplicity in this discussion, hydrogen bonding will be considered as the result of very strong polar forces. These four types of molecular forces that can occur between the solute and the two phases are those that the analyst must modify by choice of the phase system to achieve the necessary separation. Consequently, each type of molecular force enjoins some discussion. 

Molecular solids are aggregates of molecules bound together by intermolecular forces. Substances that are gases under normal conditions form molecular solids when they condense at low temperature. Many larger molecules have sufficient dispersion forces to exist as solids at room temperature. One example is naphthalene (Cio Hg), a white solid that melts at 80 °C. Naphthalene has a planar structure like that of benzene (see Section 10-), with a cloud of ten delocalized n electrons that lie above and below the molecular plane. Naphthalene molecules are held in the solid state by strong dispersion forces among these highly polarizable n electrons. The molecules in... 

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