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Intermolecular Interactions

in the Gas Phase

Gary L. Bertrand

Department of Chemistry

University of Missouri-Rolla

Interactions between two or more molecules are called intermolecular interactions, while the interactions between the atoms within a molecule are called intramolecular interactions. Intermolecular interactions occur between all types of molecules or ions in all states of matter. They range from the strong, long-distance electrical attractions and repulsions between ions to the relatively weak dispersion forces which have not yet been completely explained. The various types of interactions are classified as (in order of decreasing strength of the interactions):

ion - ion
ion - dipole
dipole - dipole
ion - induced dipole
dipole - induced dipole
dispersion forces

Without these interactions, the condensed forms of matter (liquids and solids) would not exist except at extremely low temperatures. We will explore these various forces and interactions in the gas phase to understand why some materials vaporize at very low temperatures, and others persist as solids or liquids to extremely high temperatures.

Ion - Ion Interactions

The interactions between ions (ion - ion interactions) are the easiest to understand: like charges repel each other and opposite charges attract. These Coulombic forces operate over relatively long distances in the gas phase. The force depends on the product of the charges (Z₁, Z₂) divided by the square of the distance of separation (d²): F = - Z₁Z₂/d² Two oppositely-charged particles flying about in a vacuum will be attracted toward each other, and the force becomes stronger and stronger as they approach until eventually they will stick together and a considerable amount of energy will be required to separate them. They form an ion-pair, a new particle which has a positively-charged area and a negatively-charged area. There are fairly strong interactions between these ion pairs and free ions, so that these the clusters tend to grow, and they will eventually fall out of the gas phase as a liquid or solid (depending on the temperature).

Ion - Ion Interactions in the Gas Phase

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Dipole Moment

Let's go back to that first ion pair which was formed when the positive ion and the negative ion came together. If the electronegativities of the elements are sufficiently different (like an alkali metal and a halide), the charges on the paired ions will not change appreciably - there will be a full electron charge on the blue ion and a full positive charge on the red ion. The bond formed by the attraction of these opposite charges is called an ionic bond. If the difference in electronegativity is not so great, however, there will be some degree of sharing of the electrons between the two atoms. The result is the same whether two ions come together or two atoms come together:

Polar Molecule

The combination of atoms or ions is no longer a pair of ions, but rather a polar molecule which has a measureable dipole moment. The dipole moment (D) is defined as if there were a positive (+q) and a negative (-q) charge separated by a distance (r):
D = qr
If there is no difference in electronegativity between the atoms (as in a diatomic molecule such as O₂ or F₂) there is no difference in charge and no dipole moment. The bond is called a covalent bond, the molecule has no dipole moment, and the molecule is said to be non-polar. Bonds between different atoms have different degrees of ionicity depending on the difference in the electronegativities of the atoms. The degree of ionicity may range from zero (for a covalent bond between two atoms with the same electronegativity) to one (for an ionic bond in which one atom has the full charge of an electron and the other atom has the opposite charge). In some cases, two or more partially ionic bonds arranged symmetrically around a central atom may mutually cancel each other's polarity, resulting in a non-polar molecule. An example of this is seen in the carbon tetrachloride (CCl₄) molecule. There is a substantial difference between the electronegativities of carbon (2.55) and chlorine (3.16), but the four chlorine atoms are arranged symmetrically about the carbon atom in a tetrahedral configuration, and the molecule has zero dipole moment. Saturated hydrocarbons (C_nH_n+2) are non-polar molecules because of the small difference in the electronegativities of carbon and hydrogen plus the near symmetry about each carbon atom.

Non-polar Molecule

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Polar molecules can interact with ions:

Ion - Dipole Interactions

or with other polar molecules:

Dipole - Dipole Interactions

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The charges on ions and the charge separation in polar molecules explain the fairly strong interactions between them, with very strong ion - ion interactions, weaker ion - dipole interactions, and considerably weaker dipole - dipole interactions. Even in a non-polar molecule, however, the valence electrons are moving around and there will occasionally be instances when more are on one side of the molecule than on the other. This gives rise to fluctuating or instantaneous dipoles:

Fluctuating Dipole in a Non-polar Molecule

These instantaneous dipoles may be induced and stabilized as an ion or a polar molecule approaches the non-polar molecule.

Ion - Induced Dipole Interaction

Dipole - Induced Dipole Interaction

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Dispersion Forces

Interactions between ions, dipoles, and induced dipoles account for many properties of molecules - deviations from ideal gas behavior in the vapor state, and the condensation of gases to the liquid or solid states. In general, stronger interactions allow the solid and liquid states to persist to higher temperatures. However, non-polar molecules show similar behavior, indicating that there are some types of intermolecular interactions that cannot be attributed to simple electrical attractions. These interactions are generally called dispersion forces. Electrical forces operate when the molecules are several molecular diameters apart, and become stronger as the molecules or ions approach each other. Dispersion forces are very weak until the molecules or ions are almost touching each other, as in the liquid state. These forces appear to increase with the number of "contact points" with other molecules, so that long non-polar molecules such as n-octane (C₈H₁₈) may have stronger intermolecular interactions than very polar molecules such as water (H₂O), and the boiling point of n-octane is actually higher than that of water.

Dispersion Forces

It is possible that these forces arise from the fluctuating dipole of one molecule inducing an opposing dipole in the other molecule, giving an electrical attraction. It is also possible that these interactions are due to some sharing of electrons between the molecules in "intermolecular orbitals", similar to the "molecular orbitals" in which electrons from two atoms are shared to form a chemical bond. These dispersion forces are assumed to exist between all molecules and/or ions when they are sufficiently close to each other. The stronger farther-reaching electrical forces from ions and dipoles are considered to operate in addition to these forces.

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