Rotation about the Carbon-Carbon single bond is generally regarded to be unrestricted. About 50 years ago, rotational isomers were found to exhibit optical activity: substituted biphenyl derivatives. However, the author has investigated other classes of compounds that have stable rotational isomers at room temperature. These compounds can serve as models for studying very weak intermolecular interactions, that are difficult to observe. Even diastereotopic groups show different chemical reactivity. From the contents: The "Free Rotation Concept" - Rotamer Populations - Barriers to Rotation - Reactivity of Rotational Isomers.