Intermolecular forces dipolar attractions

Intermolecular forces van der Waals forces Dipolar attractions Hydrogen bonding... 

Like other measures of pressure, c has units of MPa. In theory, a liquid will break all solvent-solvent interactions on vaporization, and so c is a measure of the sum of all the attractive intermolecular forces acting in that liquid. Hydrogen-bonding and dipolar solvents therefore have high c values. Water has a large value for c, and fluorocarbons very low values (Table 1.5). 

The three intermolecular forces are dipolar attractions, van der Waals forces (also called London forces), and hydrogen bonding. The effects of dipoles (Section 13.5) are considered first, followed by discussions of van der Waals forces and hydrogen bonding. 

Intermolecular attractions include dipolar attractions, as well as van der Waals forces and hydrogen bonding. van der Waals forces are similar to dipolar attractions but result from instantaneous dissymmetry of charge, which may disappear the next instant. The more electrons in the molecule, the greater is the van der Waals force. However, van der Waals forces tend to be lower in magnitude than dipolar attractions. 

Dispersion Forces, van der Waals postulated that neutral molecules exert forces of attraction on each other that are caused by electrical interactions between three types of dipolar configurations. The attraction results from the orientation of dipoles that may be (1) two permanent dipoles, (2) dipole-induced dipole, or (3) induced dipole-induced dipole. Induced dipole-induced dipole forces between nonpolar molecules are also called London dispersion forces. Except for quite polar materials, the London dispersion forces are the more significant of the three. For molecules the force varies inversely with the sixth power of the intermolecular distance. 

Intermolecular forces, which include hydrogen bonding, ion-dipole, dipole-dipole, ion-induced dipole, dipole-induced dipole, and London dispersion forces, are all short-range in character. These types of chemical forces occur between molecules or between chains and layers in the solid state. As a result, they are typically several orders of magnitude weaker than a chemical bond. As indicated by the data in Table 13.1, the relative strengths of intermolecular forces relate to the distances over which they can act (r). Ion-dipole forces occur either when a cation attracts the nep tive end of a dipolar molecule or an anion attracts the positive... 

The fact that such nonpolar atoms can be liquefied and so-Udifled indicates that some kind of intermolecular forces are possible between atoms in these substances. London dispersion forces arise when a temporary (instantaneous) dipolar arrangement of charge develops as the electrons of an atom move around its nucleus. This instantaneous dipole can induce a similar dipole in a neighboring atom, leading to a momentary attraction. 

In this chapter, we will deal with the forces between neutral molecules due to dipolar interactions. This type of forces are called van der Waals forces to honor the contribution of Johannes Diderik van der Waals in the field of the equation of state for gases and liquids. In his famous doctoral thesis Over de Continuiteit van den Gas-en Vloeistojioestand (On the continuity of the gas and liquid state), he showed the necessity of taking into account the finite volumes of the gas molecules as well as the intermolecular forces to establish the relationship between the pressure, volume, and temperature of gases and liquids. Such intermolecular forces can easily be understood on the basis of electrostatics if at least one of the molecules carries a dipole moment. To explain why even nonpolar molecules are able to attract each other -which is obvious from the fact that gases of such molecules do condense to liquids when cooled to sufficiently low temperatures - is more complex and requires the application of quantum theory. 

The dispersion forces in acetone are nearly the same as those in 2-methylpropane, but the addition of dipolar forces makes the total amount of intermolecular attraction between acetone molecules substantially greater than the attraction between molecules of 2-methylpropane. Consequently, acetone boils at a considerably higher... 

Methyl ethyl ether is a gas at room temperature (boiling point = 8 °C), but 1-propanol, shown in Figure 11-13. is a liquid (boiling point = 97 °C). The compounds have the same molecular formula, C3 Hg O, and each has a chain of four inner atoms, C—O—C—C and O—C—C—C. Consequently, the electron clouds of these two molecules are about the same size, and their dispersion forces are comparable. Each molecule has an s p -hybridized oxygen atom with two polar single bonds, so their dipolar forces should be similar. The very different boiling points of 1-propanol and methyl ethyl ether make it clear that dispersion and dipolar forces do not reveal the entire story of intermolecular attractions. 

The immobilization of a large volume of liquid by a small quantity of gelator is achieved efficiently if the elementary assemblies are rodlike and have large aspect ratios. Such linear structures are determined by specific binding forces associated with the chemical constitution of the gelators. In nonaqueous liquids, the attractive forces are mainly the van der Waals type and can be supplemented by dipolar interactions, intermolecular hydrogen bonds, metal-coordination bonds, or electron transfers, etc. 

In atomic and simple molecular liquids, the Kerr effect, electric saturation, etc., are caused not only by non-linearities directly induced in the atoms and molecules by the external electric field (the Voigt effect), but are primarily due to Yvon-Kirkwood fluctuational-statistical processes. One should keep in mind, however, that the intrinsic electric polarizability of non-dipolar molecules is subject to modification by at least three factors (i) the effect of translational fluctuations, (//) intermolecular attractive or repulsive forces, and (in) non-linearities induced by the tenq>oraIly and spatially fluctuating electric fields of neighbouring molecules. Thus, the Yvonr-Kirkwood process discussed in Section 4 leads, in dipolar approximation, to the following variation of the electric polarizability tensor of a molecule immersed in a statistically noit-uniform medium ... 

The electric field of a dipole can induce the formation of a dipole in a second molecule, generating net attraction between dipolar and nonpolar molecules. Such forces are, however, weak, and important only at very short intermolecular distances. The energy of this interaction decreases with distance according to 1/r. ... 

Maier and SaupeS first pointed out the importance of attractive orientational interactions such as the induced dipolar forces. Later work included both the hard core repulsion and Van der Waals attraction in a mean field approximation. The total effective pseu-dopotiential is found by expansion of the intermolecular interaction in spherical harmonics for cylindrical rods. The anisotropic part therefore includes all interactions with the symmetry of the quadrupolar Legendre polynomial P2(Cos 0). 

The liquid crystalline (mesomorphic) state is produced by a combination of dispersive and attractive forces [5]. In this state of matter, intermolecular interactions are sufficiently weakened to force a substance into a liquid state but attractive dipolar interaction hold the molten molecules into a low dimensional lattice which lacks long range positional order. The weakening of the intermolecular interactions can be accomplished by heating the material which creates molecular motions or by the addition of a liquid component which partially solvates the material. Liquid crystalline phases formed by heating and solvation are termed thermotropics and lyotropics respectively and the remainder of our discussion will focus on thermotropic materials. 

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