In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds. Such isomers are generally referred to as conformational isomers or conformers and, specifically, rotations about single bonds are restricted by a rotational energy barrier which must be overcome to interconvert one conformer to another. Conformational isomerism arises when the rotation about a bond is relatively unhindered. That is, the barrier must be small enough for the interconversion to occur. Conformational isomers are thus distinct from the classes of stereoisomers where interconversion necessarily involves breaking and reforming of chemical bonds. The study of the energetics between different rotamers is referred to as conformational analysis and it is useful for understanding the stability of different isomers, for example, by taking into account the spatial orientation and through-space interactions of substituents. In addition, conformational analysis can be used to predict and explain product selectivity, mechanisms, the types of conformational isomers are related to the spatial orientations of the substituents between two vicinal atoms. The staggered conformation includes the gauche and anti conformations, depending on the orientations of the two substituents. The energy difference between gauche and anti is 0.9 kcal/mol associated with the energy of the gauche conformer. The anti conformer is, therefore, the most stable, the three eclipsed conformations with dihedral angles of 0°, 120°, and 240° are not considered to be rotamers, but are instead transition states of higher energy. While simple molecules can be described by these types of conformations, more specific examples of conformational isomerism are detailed elsewhere, Ring conformation Cyclohexane conformations with chair and boat conformers. Allylic strain – energetics related to rotation about the bond between sp2 and sp3 carbons. Atropisomerism – due to restricted rotation about a bond, a molecule can become chiral, folding of molecules, where some shapes are stable and functional, but others are not. Three isotherms are given in the depicting the equilibrium distribution of two conformers at different temperatures. At a free energy difference of 0 kcal/mol, this gives a constant of 1. The two have equal energy, neither is more stable, so neither predominates compared to the other. A negative difference in free energy means that a conformer interconverts to a more stable conformation. For example, the ΔG of butane from gauche to anti is −0.9 kcal/mol, therefore the equilibrium constant is 4.5, conversely, a positive difference in free energy means the conformer already is the more stable one, so the interconversion is an unfavorable equilibrium
Rotation about single bond of butane to interconvert one conformation to another. The staggered 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.
Free energy diagram of butane as a function of dihedral angle.
Equilibrium distribution of two conformers at different temperatures given the free energy of their interconversion.
Boltzmann distribution % of lowest energy conformation in a two component equilibrating system at various temperatures (°C, color) and energy difference in kcal/mol (x-axis)