After this lesson, you should be able to...
In this lesson, we’re going to examine the interactions that exist between molecules. Since these interactions occur between different molecules, we call them intermolecular molecular interactions or intermolecular forces (IMFs for short). As we will see, IMFs are vital for understanding most of the properties that we observe for molecules, such as melting and boiling point. We will also see that nearly everything that happens in a living system is the result the specific patterns of IMFs that form between proteins, nucleic acids, and other biological molecules. A molecule’s structure determines the types and strengths of IMFs it can form; thus, IMFs are a key connection between structure and function.
All interactions between molecules are the result of electrostatic attraction. That is, IMFs exist due to the attractive forces between opposite charges. The force that exists between two charged species is described by Coulomb’s law. We previously saw Coulomb’s law expressed in terms of energy (Chapter 1), but now (Equation 1) it is instead expressed with electrostatic force (the only difference is that “r” is squared in the equation for force). As you can see, the force between two charged species is dependent on their charges and on distance. The more charge the objectives have, the greater the force. The closer they are to one another, the greater the force.
Equation 1:
The types of charges found within molecules vary depending on the structure of the molecule. The molecules with the most charge are ions, in which the charge of the ion (Q) determines the strength of the IMFs it makes. We also saw that partial charges exist in polar molecules. The magnitude of these partial charges, and thus the strength of IMFs formed by such molecules, depends on the dipole moment (μ) of the molecule. We’ve also seen that some molecules are nonpolar, and it turns out that such molecules are able to form IMFs too. Nonpolar molecules can form IMFs because all molecules are polarizable, meaning that imbalances of charge can exist transiently in the molecule due to temporary distortions of the electron cloud. The ease with which these electron clouds are distorted is described by the polarizability (α) of the molecule. Polarizability thus influences the strength of IMFs formed by nonpolar molecules. Large molecules (with high molecular weights) tend to be the most polarizable since they have more electrons and thus a larger electron cloud to distort. The transient dipoles that exist in polarizable molecules are known as instantaneous dipoles and induced dipoles (these two terms of used interchangeably).
Experimentally we can use physical properties like melting point and boiling point to evaluate the strength of IMFs in a substance. When a substance boils, molecules have enough energy to overcome all of the interactions that they form with other molecules and enter the gas phase. Having a high boiling point means that a lot of energy is required to break the IMFs in the substance; thus, the IMFs in the substance must be strong.