Heitler-London dispersion forces

A nonpolar molecule is nonpolar only in a statistical sense. At any instant, the random motion of the electrons will lead to a dipole moment, but this fluctuates as the electrons move around and its average value is zero. Thus a hydrogen atom at any given instant has a dipole moment since the electron and proton are at different points in space as the electron moves around the proton, the direction and size of the dipole fluctuate, the average value being zero. When two neutral molecules are close together, the electrostatic interactions between them cause the electrons to correlate these motions so that the instantaneous dipole moments of the molecules lead to a net attraction. These attractive forces, due to electron correlation between adjacent molecules, are called Heitler-London dispersion forces, usually abbreviated to dispersion forces. 

It is customary to talk about van der Waals attractions between two atoms, say He He, in the Heitler-London (H.L.) description. The attraction near the minimum is due partly to the distortion of H.L. AO s (in the orbital average polarization effect), but especially at large intemuclear distances r, it arises from simultaneous excitations in both atoms (the usual London dispersion force). The two effects have been compared for Hg in the triplet state as a function of r. 

The foundations for our present understanding of intermolecular forces were laid in the first decades of this century. First, Keesotn Debye and Falckenhagen elucidated the role played by permanent electric moments and polarizabilities. After the advent of quantum mechanics, Heitler and London S identified the exchange forces which keep molecules apart, and London discovered the dispersion forces which explained such puzzling phenomena as the condensation of noble gases. 

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