Intermolecular forces Internal energy

Intermetallics Intermolecular forces Internal antistatic agents Internal conversion Internal energy Internal placticizers... 

The cohesive energy coh of a substance in a condensed state is defined as the increase in internal energy AU per mole of substance if all the intermolecular forces are eliminated. 

It is beyond the scope of this review to cover in depth either valence theory or the theory of intermolecular forces and I shall only attempt to deal with some general principles of both which appear to be important for an understanding of potential energy surfaces. Before dealing separately with weak and strong interactions, there is one point they have in common and that is the increasing computational effect that is required as the number of internal coordinates increases. 

The solvation of a solute reflects the subtle balance between two opposite components. First, the interaction between solute and solvent molecules, which is a favorable contribution arising from the different intermolecular forces that can be formed depending on the chemical nature of both solute and solvent. Second, the interaction between solvent molecules, which is an unfavorable term due to the disruption of the internal structure of the bulk solvent caused by the presence of the solute. The key magnitude to characterize the transfer of solute from gas phase to solution is the free energy of solvation, AGsoi, which can be defined as the reversible work required to transfer the solute from the ideal gas phase to solution at a given temperature, pressure and chemical composition [1], This definition allows us to compute AGsoi as the difference in the reversible works necessary to build up the solute both in solution and in the gas phase. 

Since the surface tension is a manifestation of intermolecular forces, it may be expected to be related to other properties derived from intermolecular forces, such as internal pressure, compressibility and cohesion energy density. This is found to be so indeed. In the first place there exists a relationship between compressibility and surface tension. According to McGowan (1967) the correlation is ... 

U = molar internal energy Fm = molar volume T = absolute temperature). This small expansion does not necessarily disrupt all the intermolecular solvent-solvent interactions. The internal pressure results from the forces of attraction between solvent molecules exceeding the forces of repulsion, i.e. mainly dispersion and dipole-dipole interactions cf. Table 3-2). 

Berkowitz and Wahl9 have reviewed the experimental and theoretical estimates of the dissociation energy of molecular fluorine. The Raman and far-i.r. spectra of crystalline F2 show that in this state the element resembles 02 more closely than it does the other halogens.10 The intermolecular forces, in particular, are extremely weak, as exemplified by the small shifts of the internal frequencies from their gas-phase values, the absence of observable factor-group splitting of the fundamental and overtones, and the low value of the external (lattice) vibrations. 

Nevertheless, detailed experimental analysis of gases shows that, at distances large compared to the molecular dimensions, weak intermolecular forces exist, whereas at distances of the order of the molecular dimensions the molecules repel each other strongly. Moreover, collisions between complex molecules may in general also redistribute energy between the translational and internal energy forms. 

It should be noted that it is assumed that the intermolecular forces do not affect the internal degrees of freedom so that is independent of whether these forces are present or not. When they are absent (Zf = 0), the integral Z collapses to and equation (2.2.31) becomes the same as equation (2.2.23). The important task of the statistical thermodynamics of imperfect gases and liquids is to evaluate Z. This subject is discussed in detail later in this chapter. However, the nature of the intermolecular forces which give rise to the potential energy U is considered next. 

Most of the internal vibrations of a molecule in a lattice are rarely in any phase relationship with those of another molecule in the lattice. The energy of these vibrations is the same for all molecules and so independent of any phase. The dispersion curve is flat and the vibrations are localised on individual molecules. This is because the intermolecular forces of the lattice are making insufficient contribution to the potential energy stored during a molecular distortion to carry phase information. As the strength of the intramolecular forces falls the significance of the intermolecular forces rises, phase information is carried and the lowest vibrations of molecules are often dispersed. Under these circumstances the mode is distributed amongst the many molecules involved in the phase relationship and it is, therefore, extended in space and not localised. 

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