Intermolecular Forces and the Properties of Molecular Solids

Abstract

The notion of intermolecular potentials is based on separability at two different levels. The Born-Oppenheimer separation between electronic and nuclear motions prescribes the use of the electronic energy surface as the potential energy for the nuclear motions. The nuclear motions can be separated into internal molecular motions, i.e. molecular vibrations, and external motions, i.e. (relative) translations and rotations of whole molecules. The latter separation follows from the shape of the potential energy surface, which is determined by the nature of the interactions involved. Molecules are kept together by “chemical”, mainly covalent, bonds between the atoms, which, for neutral molecules, are considerably stronger than the intermolecular interactions. For molecular ions the intermolecular Coulomb interaction energies are equally large as the intramolecular covalent binding energies, but even in this case the steep distance dependence and strong directionality of the covalent bonds make the potential energy surface depend most sensitively on the internal molecular coordinates. So there is a clear separation between internal molecular coordinates and external ones. Molecules are recognizable by their electronic and vibrational spectra; the intermolecular interactions cause (slight) modifications of these spectra (line shifts, splittings, broadening). This separation becomes less distinct for larger molecules which are often flexible in some of their internal coordinates. The motions along those specific coordinates will be strongly influenced by intermolecular interactions and coupled to the overall motions of the molecules.