Molecular interactions London dispersion formula

In this way, for each H atom, the calculated dipole pseudospectra a, si] i = 1,2, , N of Table 4.2 can be used to obtain better and better values for the C6 London dispersion coefficient for the H—H interaction a molecular (two-centre) quantity C6 can be evaluated in terms of atomic (one-centre), nonobservable, quantities, a, (a alone is useless). The coupling between the different components of the polarizabilities occurs through the denominator in the London formula (4.19), so that we cannot sum over i or / to get the full, observable,12 aA oraB. 

Pyykko P, Wang C, Straka M, Vaara 1 (2007) A London-type formula for the dispersion interactions of endohedral A B systems. Phys Chem Chem Phys 9 2954-2958 Dunitz JD, Gavezzotti A (1999) Attractions and repulsions in molecular crystals what can be learned from the crystal structures of condensed ring aromatic hydrocarbons. Acc Chem Res 32 677-684... 

Both molecules have the same molecular formula, but they have different structures. The London (dispersion) forces between pentane molecules in the liquid or solid states are stronger because the linear molecules have a larger surface area for interaction. In contrast, its isomer, 2,2 dimethylpropane, is more compact (adopting a roughly spherical shape) owing to its extensive branching, and hence has a smaller surface area for interaction (Figure 4.73). 

Dispersion Forces. One of the basic molecular forces acting at the interface is the dispersion force involving the simultaneous excitation of both molecules. The perturbation responsible for dispersion forces is the instantaneous coulomb interaction between the electrons and nuclei in the two molecules. London derived an approximate formula for the dispersion energy for two spherical... 

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