London energy dispersion

With this basis, one gets for the constant in the London dispersion energy formula E = — j... 

In the absence of electron donor-acceptor interactions, the London dispersive energy is the dominant contributor to the overall attractions of many molecules to their surroundings. Hence, understanding this type of intermolecular interaction and its dependency on chemical structure allows us to establish a baseline for chemical attractions. If molecules exhibit stronger attractions than expected from these interactions, then this implies the importance of other intermolecular forces. To see the superposition of these additional interactions and their effect on various partitioning phenomena below, we have to examine the role of dispersive forces in more detail,... 

By time averaging Ej t> nt>, the London dispersion energy is obtained. Recently a book on the theory of intermolecular forces that treats the subject in a comprehensive manner [153] has been published. 

Here a and b are occupied MO s of systems A and B. Equation (6,32) is easily expressible in terms of integrals over atomic basis functions and elements of the density matrix. In eqn. (5.31) two terms may be distinguished. The first one is due to single electron excitations of the type a r") and (b —->s"), where a and r", respectively, are occupied and virtual MO s in the system A, and b and s" are occupied and virtual MO s in the system B, Contribution of these terms corresponds to the classical polarization interaction energy, Ep, Two-electron excitations (a r", b — s"), i.e. simultaneous single excitations of either subsystem, may be taken as contributions to the second term - the classical London dispersion energy, Ep, If the Mjiller-Ples-set partitioning of the Hamiltonian is used, Ep may be expressed in... 

These jr-bonding effects are part of the theory of the HSAB Principle. We can also imagine that London dispersion energies between atoms or groups in an A B complex could stabilize it. Since these dispersion energies, or van der Waals energies, depend on the product of the polarizabilities of the two groups, soft-soft combinations would be stabilized in this way. The hydride ion is very polarizable, and its softness depends on this factor, presumably. 

Since, London dispersion energy is the potential energy between non-polar molecules, Equations (68) and (69) show that Vd is independent of temperature. On the other hand, the first ionization potentials of most substances do not differ very much from one another, so London s equation is more sensitive to the polarizability than it is to the ionization potential. We can rewrite Equations (68) and (69) ... 

A parameter with indices that are identical is just the pure-component parameter, i.e., fl = A and bii = b,. The fly with i j i a cross-interaction parameter, and is found only in mixtures containing i and j, but never in a pure substance. Since a expresses the effect of intermolecular attractive energy, for similar molecules i and j, fly is approximated by the geometric mean of and Ujj in analogy to the London dispersion energy. But the geometric mean becomes worse as the i and j molecules become more dissimilar. It is generally useful to represent fly by... 

We shall limit our consideration to systems with London (dispersion) energies only. For such systems London (1,2) derived the famous relation... 

A Short Survey of the Theory of the London Dispersion Energy... 

---