Ion-Ion, Dipole-Dipole, and Ion-Dipole Interactions and Hydrogen Bonding

Ionic and covalent bonds are both examples of an intramolecular force, meaning an attractive or repulsive force between atoms that makes up molecules or between ions that make up materials. Intramolecular forces are not the only forces that are present in a material. An intermolecular force is an attractive or repulsive force between a molecule and a nearby molecule, atom, or ion. Like intramolecular forces, intermolecular forces are electrostatic in nature. Intermolecular forces are generally much weaker than intramolecular forces, but they operate at longer ranges. Coulomb's law, i.e., $F\sim1/r^2$, describes forces between protons, electrons, or dipolar ends.

Intermolecular forces affect physical properties of matter, such as melting and boiling points. Substances with strong intermolecular forces tend to be solids at room temperature. Liquids have weaker intermolecular forces than solids. Ideal gases are defined as having no intermolecular forces. At normal temperatures and pressures, intermolecular forces are negligible in gases. At extreme temperatures or pressures, intermolecular forces start to become important.

A dipole is a molecule with partial positive and partial negative charges. Polar molecules contain permanent dipoles. The positive ends of a dipole attract the negative ends of adjacent dipoles on other molecules and vice versa. At the same time, the positive ends of a dipole repulse the positive ends of dipoles of nearby molecules, and the negative ends repulse the negative ends of nearby dipoles. An attractive or repulsive force between two polar molecules because of partial positive and partial negative charges of the molecules is called a dipole-dipole interaction. Dipole-dipole interactions grow stronger as the polarity of the molecules involved becomes larger.

Consider the methyl cyanide molecule (CH₃CN). This molecule is polar, with the nitrogen side having a partial negative charge and the hydrogen side having a partial positive charge. In a solid the molecules that make up methyl cyanide arrange themselves to maximize the attraction between these ends. In a liquid, the molecules are free to move and experience both attraction and repulsion. Methyl cyanide (CH₃CN) has a molecular weight comparable to propane (C₃H₈). Propane is nonpolar and has a boiling point of –42°C. Methyl cyanide is polar and has a boiling point of 82°C. The difference in boiling points is due mostly to the additional intermolecular interactions in methyl cyanide. A special kind of dipole-dipole interaction is a hydrogen bond, which is a weak intermolecular bond between a hydrogen atom of one molecule and a highly electronegative atom of a nearby molecule. The electronegative atom is usually fluorine, oxygen, or nitrogen. A neutral hydrogen atom has only one electron. When a hydrogen bond forms, hydrogen's electron is attracted to the electronegative side of a nearby molecule. This attraction leaves the hydrogen nucleus with a partial positive charge. At the same time, the electronegative part of the nearby molecule carries a negative charge. Hydrogen bonds are the strongest of intermolecular forces in small molecules. Water is an example of a compound that forms hydrogen bonds. Water has many unique properties, such as a very high specific heat capacity and high boiling point. Most of these properties can be traced to its strong hydrogen bonding. Ions and dipoles both carry a charge and can interact. An electrostatic interaction between ions, such as sodium and chlorine ions in sodium chloride, and polar ends of molecules with permanent dipoles, such as water, is called an ion-dipole interaction. Water dissolves salts through ion-dipole interactions.