Intermolecular forces comparison

Substitution of fluorine for hydrogen in an organic compound has a profound influence on the compound s chemical and physical properties. Several factors that are characteristic of fluorine and that underHe the observed effects are the large electronegativity of fluorine, its small size, the low degree of polarizabiHty of the carbon—fluorine bond and the weak intermolecular forces. These effects are illustrated by the comparisons of properties of fluorocarbons to chlorocarbons and hydrocarbons in Tables 1 and 2. 

Surface Protection. The surface properties of fluorosihcones have been studied over a number of years. The CF group has the lowest known intermolecular force of polymer substituents. A study (6) of liquid and solid forms of fluorosihcones has included a comparison to fluorocarbon polymers. The low surface tensions for poly(3,3,3-trifluoropropyl)methylsiloxane and poly(3,3,4,4,5,5,6,6,6-nonafluorohexyl)methylsiloxane both resemble some of the lowest tensions for fluorocarbon polymers, eg, polytetrafluoroethylene. 

The degree of realism of these model structures can be assessed by comparison of computed properties with experimental ones. The cohesive energy is, by definition, the difference in energy per mole of substance between a parent chain in its bulk environment and the same parent chain in vacuo, i.e., when all intermolecular forces are eliminated. This difference is readily computed from the minimized... 

In molecular covalent compounds, intermolecular forces are very weak in comparison with intramolecular forces. For this reason, most covalent substances with a low molecular mass are gaseous at room temperature. Others, with higher molecular masses may be liquids or solids, though with relatively low melting and boiling points. 

Let s compare the intermolecular forces between I2 and Cl2. I2 has the greater molecular mass so the van der Waals forces between its molecules are greater in comparison with Cl2. Therefore at room temperature iodine is solid whereas chlorine is gas. 

Comment on the validity of the following statement Dispersion forces are weak in comparison to other intermolecular forces. ... 

The traditional method of dealing with irreversible processes is, of course, the use of the Boltzmann integro-differential equation and its various extensions. But this method leads to two serious difficulties. The first is that Boltzmann s equation is neither provable nor even meaningful except in the context of molecular encounters, i.e., under the assumption that the intermolecular forces are of such short range in comparison with molecular distances that a molecule spends only a negligible fraction of its time within the influence of others. This drastically restricts the field of applicability, confining the treatment to gases close to the ideal state. But even then the equation can only be established on the basis of an essential assumption of molecular probabilistic independence ( micromolecular chaos ).3... 

A comparison of the various kinds of intermolecular forces discussed in this section is shown in Table 10.5. 

There is nothing magical about fluorocarbons or, specifically, the -(CF2) F group [2]. The -(CF2)MF is similar to -(CH2) H in many ways. These include dipole moments and polarizabilities that are related to intermolecular forces and, hence, surface tension. Where they do differ is in size, specifically diameter, and a relative comparison for a typical hydrocarbon and similar fluorocarbon surfactant is shown in Figure 6.54. 

The collision integral for diffusion depends upon the choice of the intermolecular force law between colliding molecules and is a function of temperature. The characteristic length also depends upon the intermolecular force law selected. In comparison with the simple Eq. (6-2) for perfect gases, Eq. (6-3) takes into account the interactive forces between real molecules. But, while in the first case only two specific parameters are needed, the diffusion collision integral, Q0, is a complicated function of several parameters. 

You have learned that pure covalent compounds are not held together by ionic bonds in lattice structures. They do form liquids and solids at low temperatures, however. Something must hold the molecules together when a covalent compound is in its liquid or solid state. The forces that bond the atoms to each other within a molecule are called intramolecular forces. Covalent bonds are intramolecular forces. In comparison, the forces that bond molecules to each other are called intermolecular forces. 

At best, this approach provides a quantitative index to solvent polarity, from which absolute or relative values of rate or equilibrium constants for many reactions, as well as absorption maxima in various solvents, can be derived. Since they reflect the complete picture of all the intermolecular forces acting in solution, these empirical parameters constitute a more comprehensive measure of the polarity of a solvent than any other single physical constant. In applying these solvent polarity parameters, however, it is tacitly assumed that the contribution of intermolecular forces in the interaction between the solvent and the standard substrate is the same as in the interaction between the solvent and the substrate of interest. This is obviously true only for closely related solvent-sensitive processes. Therefore, an empirical solvent scale based on a particular reference process is not expected to be universal and useful for all kinds of reactions and absorptions. Any comparison of the effect of solvent on a process of interest with a solvent polarity parameter is, in fact, a comparison with a reference process. 

Most of the agrochemicals are relatively small molecules and the antibodies produced in animals may, by comparison, be fairly uniform with respect to complementarity. When antisera to haptens are diluted sufficiently so as to favor interaction with the most avid antibodies, the Scatchard plots often are indicative of fairly homogeneous populations of antibodies. Their affinity constants could reach as high as 1012 M 1. The intermolecular forces involved in the binding of antigens to antibody include hydrophobic, Van der Waals, electrostatic and hydrogen binding (28-31). 

Particularly VCD and ROA provide not only static structural information, but also a picture of intermolecular forces as well. A comparison to molecular dynamics modeling is an evident application for the near future. 

These spectra illustrate a number of features of INS spectra. In comparison with the infim-ed speetrum of benzene vapour, in solid benzene the out-of-plane C-H bending modes are shifted to higher wavenumbers (e.g. vn, 673 to 694 cm ). The shift is a consequence of intermolecular forces in the solid. The in-plane modes are not shifted. Out-of-plane C-H bending modes are shifted ftirther to higher wavenumbers for benzene in NaY (vn, 700 em" ). 

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