<P> London dispersion forces are the weakest type of non-covalent interaction . They are also known as "induced dipole - induced dipole interactions" and present between all molecules, even those which inherently do not have permanent dipoles . They are caused by the temporary repulsion of electrons away from the electrons of a neighboring molecule, leading to a partially positive dipole on one molecule and a partially negative dipole on another molecule . Hexane is a good example of a molecule with no polarity or highly electronegative atoms, yet is a liquid at room temperature due mainly to London dispersion forces . In this example, when one hexane molecule approaches another, a temporary, weak partially negative dipole on the incoming hexane can polarize the electron cloud of another, causing a partially positive dipole on that hexane molecule . While these interactions are short - lived and very weak, they can be responsible for why certain non-polar molecules are liquids at room temperature . </P> <P> π - effects can be broken down into numerous categories, including π - π interactions, cation - π & anion - π interactions, and polar - π interactions . In general, π - effects are associated with the interactions of molecules with the π - systems of conjugated molecules such as benzene . </P> <P> π - π interactions are associated with the interaction between the π - orbitals of a molecular system . For a simple example, a benzene ring, with its fully conjugated π cloud, will interact in two major ways (and one minor way) with a neighboring benzene ring through a π - π interaction (see figure 3). The two major ways that benzene stacks are edge - to - face, with an enthalpy of ~ 2 kcal / mol, and displaced (or slip stacked), with an enthalpy of ~ 2.3 kcal / mol . Interestingly, the sandwich configuration is not nearly as stable of an interaction as the previously two mentioned due to high electrostatic repulsion of the electrons in the π orbitals . </P> <P> Cation - π interactions involve the positive charge of a cation interacting with the electrons in a π - system of a molecule . This interaction is surprisingly strong (as strong or stronger than H - bonding in some contexts), and has many potential applications in chemical sensors . For example, the sodium ion can easily sit atop the π cloud of a benzene molecule, with C symmetry (for more on point groups and molecular symmetry, see the Wikipedia page on point groups) (See figure 4). </P>

Which of the following types of bonding interactions is not classified as non-covalent