conformational isomerism

thumb|300px|Rotation about single bond of butane to interconvert one conformation to another. The gauche conformation on the right is a conformer, while the eclipsed conformation on the left is a transition state between conformers. Above: Newman projection; below: depiction of spatial orientation.

In chemistry, rotamers are chemical species that differ from one another primarily due to rotations about one single bond. Various arrangements of atoms in a molecule that differ by rotation about single bonds can also be referred to as conformations. Conformations, which represent local minima on the potential energy surface, are called conformers. Conformers can differ from one another due to rotation of multiple bonds; rotamers are a subset of conformers. Conformers/rotamers usually differ little in their energies, so they are almost never separable in a practical sense. Rotations about single bonds are subject to small energy barriers. When the time scale for interconversion is long enough for isolation of individual rotamers (usually arbitrarily defined as a half-life of interconversion of 1000 seconds or longer), the species are termed atropisomers. The ring-flip of substituted cyclohexanes constitutes a common form of conformers.