Rotation about the Carbon-Carbon single bond is generallyregarded to be unrestricted. About 50 years ago, rotationalisomers were found to exhibit optical activity: substitutedbiphenyl derivatives. However, the author has investigatedother classes of compounds that have stable rotationalisomers at room temperature. These compounds can serve asmodels for studying very weak intermolecular interactions,that are difficult to observe. Even diastereotopic groupsshow different chemical reactivity.From the contents: The "Free Rotation Concept" - RotamerPopulations - Barriers to Rotation - Reactivity ofRotational Isomers.