Rotamer

Lore & Background

Rotamers arise from rotations about single bonds, with small energy barriers between them. For example, ethane and propane have three staggered conformers (local energy minima) and three eclipsed conformations (transition states). Butane is the simplest molecule where single-bond rotations yield two types of nonequivalent structures: anti and gauche conformers. The anti conformer is the most stable, with the gauche conformer having an energy separation of 0.9 kcal/mol due to strain energy.

Reader's Guide

Rotamers are significant in conformational analysis, which studies the energetics of bond rotation. This analysis can predict and explain product selectivity, mechanisms, and rates of reactions. Conformational analysis also plays an important role in rational, structure-based drug design. Rotamers are almost never separable in practice due to their small energy differences, but when interconversion is slow enough (half-life of 1000 seconds or longer), they are termed atropisomers. The study of rotamers helps understand molecular stability and reactivity, particularly in alkanes and more complex molecules.

Did You Know?

• Rotamers are a subset of conformers, which are local minima on the potential energy surface.
• Butane has three conformers: two gauche (enantiomeric) and one anti, with the anti being most stable.
• The energy barrier for eclipsed conformations in butane is about 5 kcal/mol when methyl groups eclipse, and about 3.5 kcal/mol when methyl groups eclipse hydrogens.
• When the half-life of interconversion of rotamers is 1000 seconds or longer, the species are called atropisomers.