Intermolecular force variation with distance

The dispersed species will be attraeted to each other through electric dipole interactions, which may be due to (1) two permanent dipoles, (2) dipole-induced dipole, or (3) indueed dipole-induced dipole. The latter forces between nonpolar moleeules are also called London dispersion forces. Except for quite polar materials, the London dispersion forces are the more signifieant of the three. Whereas for molecules the force varies inversely with the sixth power of the intermolecular distance, the nature of the variation with distance is somewhat different for dispersions. For dispersed droplets (or particles, ete.) the dispersion forces can be approximated by adding up die attraetions between all inter-droplet pairs of molecules. When added this way the dispersion force between two droplets decays less rapidly as a function of separation distance than is the case for individual molecules. For two spheres of radius a, separated by distance H, the attractive energy, can be approximated by. 

Reactions between neutrals include atom/radical + radical and atom/radical + molecule reactions. As discussed above, the intermolecular forces are shorter range than is the case with ion-molecule reactions, so that it is necessary to consider chemical interactions explicitly when modelling a reaction. After a section on experimental methods, the ideas behind transition state (TS) theory and its variational modification are discussed, together with theories of reactions where the TS switches, as the temperature increases, from A-B distances mainly controlled by the potential arising from electrostatic interaction to shorter distances where chemical forces are important. While the pressure in the ISM is too low for pressure dependent reactions, this topic is important in the conditions used to measure rate coefficients and in the chemistry of planetary atmospheres, including those of the exoplanets (see Chap. 5). This topic is discussed in Sect. 3.4.4, which also introduces the ideas that lie behind master equation models, which are widely used for such reactions. These models can also be used for reactions in which the adduct AB from an A + B reaction dissociates into several products, and these ideas are discussed in Sect. 3.4.5. Section 3.4 concludes with discussion of two examples of neutral + neutral reactions. 

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